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|
Hello,
I'd like to write a rule for either clang-tidy or static analyzer to help catch some potential errors in a project I'm working on.
My questions are:
a) is only one or the other will be able to do what I want to do?
b) if both are feasible which would have the simpler implementation?
The project involves writing an API that will run in a multi-threaded application and is responsible for serializing all access to a device structure. Therefore the first thing in every function in the API must be to call api_enter (which will among other things acquire a mutex on the device) and the last thing before returning must be to call api_exit. Also I want to enforce single exit point from every API function - or certainly if there are any return points that bypass the api_exit call.
So here is an example function with errors I want to catch highlighted.
int api_foo(device_t *dev) { int ret_val = 0;
bar();
// fn calls & decls before api_enter is ok- just don't access dev. dev->bla = 1; // NO! device access before api_enter() called api_enter(dev); // error if this call is not present exactly once
if (dev->bla) return; // NO! didn't call api_exit before rtn. Also two return points
if (dev->ma) { ret_val = 1;
goto cleanup; } tweak(dev);
cleanup: api_exit(dev); // error if this is not present exactly once
dev->bla = 1; //NO! device access after api_exit()
return ret_val;
}
I don't think it matters but the project is C compiled with gcc.
Also if both are feasible any other pointers, tips or good resources would be appreciated. E.g is there a totally different methodology I'm not considering - e.g. would using something like pycparser be a lot easier - though I'd prefer to keep it in clang as we plan to use tidy & static analyzer in any case for standard QA.
Thanks for reading, Billy.
_______________________________________________
cfe-dev mailing list
[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
|
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|
Hi,
It depends on how strict do you want the checking be and on the
details of the rule. If you're designing a new API from scratch and
stuck with gcc forever, i wouldn't mind using the gcc
__attribute__((cleanup())) for your purpose.
The rule you described should be reasonably easy to implement with
the Static Analyzer. The good side of it is that you get a lot of
semantic modeling for free. For instance, if the developer copies
`dev` into a local variable and then uses that local variable
outside of api_enter..api_exit, the tool will be able to handle
transparently, as it deals with values rather than with variables.
Also it will probably be the easiest tool for your problem. The
downside would be that it's not guaranteed to find all bugs; it'll
inevitably give up on complicated code with high cyclomatic
complexity :) So if you want strict/paranoid enforcement of rules,
the Static Analyzer is not the right tool. But if you want to simply
find some bugs for free, it's the right tool.
It sounds as if your problem is not inter-procedural. Let me
double-check this: would you have another api_enter..api_exit pair
in the body of your tweak() function? Or is just one
api_enter..api_exit enough? Or is it a bug to call api_enter twice
without an api_exit in between?
If you have to write api_enter..api_exit in *every* function that
deals with devices, then the problem is not inter-procedural, which
makes it much easier. In particular, you should be able to come up
with a purely syntactic analysis ("every function that accesses a
device_t must start with api_enter() and must end in exactly one
spot with api_exit()"). Such analysis should be easily do-able in
clang-tidy as long as you're satisfied with this level of
aggressiveness. In particular, you'll have to be willing to
sacrifice code like this:
void foo(device_t *dev) {
if (flip_a_coin()) {
api_enter(dev);
...
api_exit(dev);
}
}
But it may be perfectly fine if you seriously want to enforce a
strict structure on all your functions that deal with devices.
I think the truly-truly right tool for your problem would be to come
up with a custom analysis over Clang CFG. It would be harder to
implement, but it would allow you to express things like "every
execution path within a function that accesses `dev` must have a
api_enter before it and an api_exit after it; you are not allowed to
copy `dev` around". This would strictly enforce the rule. At the
same time it'll allow you to lift the requirement of exactly one
return point - you would still be able to ensure that all accesses
are covered. If you need to allow to copy `dev` around, it should
still be doable, but it will be significantly more difficult to
implement.
On 7/2/19 12:13 PM, Billy O'Mahony via
cfe-dev wrote:
Hello,
I'd like to write a rule for either clang-tidy or static
analyzer to help catch some potential errors in a project I'm
working on.
My questions are:
a) is only one or the other will be able to do what I
want to do?
b) if both are feasible which would have the simpler
implementation?
The project involves writing an API that will run in a
multi-threaded application and is responsible for serializing
all access to a device structure. Therefore the first thing in
every function in the API must be to call api_enter (which
will among other things acquire a mutex on the device) and the
last thing before returning must be to call api_exit. Also I
want to enforce single exit point from every API function - or
certainly if there are any return points that bypass the
api_exit call.
So here is an example function with errors I want to catch
highlighted.
int api_foo(device_t *dev) {
int ret_val = 0;
bar(); // fn calls & decls before api_enter is ok-
just don't access dev.
dev->bla = 1; // NO! device access before
api_enter() called
api_enter(dev); // error if this call is not present
exactly once
if (dev->bla)
return; // NO! didn't call api_exit before rtn.
Also two return points
if (dev->ma) {
ret_val = 1;
goto cleanup;
}
tweak(dev);
cleanup:
api_exit(dev); // error if this is not present exactly
once
dev->bla = 1; //NO! device access after api_exit()
return ret_val;
}
I don't think it matters but the project is C compiled with
gcc.
Also if both are feasible any other pointers, tips or good
resources would be appreciated. E.g is there a totally
different methodology I'm not considering - e.g. would using
something like pycparser be a lot easier - though I'd prefer
to keep it in clang as we plan to use tidy & static
analyzer in any case for standard QA.
Thanks for reading,
Billy.
_______________________________________________
cfe-dev mailing list
[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
_______________________________________________
cfe-dev mailing list
[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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|
|
In reply to this post by Nathan Ridge via cfe-dev
An idea to avoid a mutex that may block the
remaining threads.
This page shows std::atomic::operator++
http://www.cplusplus.com/reference/atomic/atomic/operatorplusplus/
Instead of writing directly to the previously mutexed device,
get the next number from the atomic increment and write to a
memory file with that number as a name. Each process will then
write a separate file with a different incremented name. Since
filenames are handled as strings we may want to, say, add a
large number to the increment in order to have time sequenced
files by name with all names having the same number of digits.
This is not the only way to get a file ordering but is an
example. Alternatively, on Linux we might use ls -1t *.
At this point there is no waiting for the multiple threads.
Now we need a thread to read the files in order to feed the
device. If we want to cut out the slack time between files for
finding, opening, closing, deleting, we can ping-pong between
two coordinated processes where the second has the next file
ready to go when the first process yields the device.
There are obvious issues with the above, for example, one being
that the throughput of the device is slower than the combined
throughput of the threads, another being wraparound on file
numbering.
The first issue is the throughput ratio between the device and
the number of threads. The ratio should be evenly balanced
around 1.0. This suggests that the number of threads can be
moved up and down to obtain the best balance, with perhaps some
sufficient volume of data pending to avoid any device slack
time.
Also with a little effort we can read, for sending to the
device, a file that is being written by a process in order to
avoid an initial device utilization delay, if that becomes
useful. But that should only be an issue for the first one or
two files.
Neil Nelson
On 7/2/19 1:13 PM,
Billy O'Mahony via cfe-dev wrote:
Hello,
I'd like to write a rule for either
clang-tidy or static analyzer to help catch some potential
errors in a project I'm working on.
My questions are:
a) is only one or the other will be
able to do what I want to do?
b) if both are feasible which would
have the simpler implementation?
The project involves writing an API that
will run in a multi-threaded application and is responsible
for serializing all access to a device structure. Therefore
the first thing in every function in the API must be to call
api_enter (which will among other things acquire a mutex on
the device) and the last thing before returning must be to
call api_exit. Also I want to enforce single exit point from
every API function - or certainly if there are any return
points that bypass the api_exit call.
So here is an example function with errors
I want to catch highlighted.
int api_foo(device_t *dev) {
int ret_val = 0;
bar(); // fn calls & decls before
api_enter is ok- just don't access dev.
dev->bla = 1; // NO! device access
before api_enter() called
api_enter(dev); // error if this call
is not present exactly once
if (dev->bla)
return; // NO! didn't call api_exit
before rtn. Also two return points
if (dev->ma) {
ret_val = 1;
goto cleanup;
}
tweak(dev);
cleanup:
api_exit(dev); // error if this is not
present exactly once
dev->bla = 1; //NO! device access
after api_exit()
return ret_val;
}
I don't think it matters but the project is
C compiled with gcc.
Also if both are feasible any other
pointers, tips or good resources would be appreciated. E.g
is there a totally different methodology I'm not considering
- e.g. would using something like pycparser be a lot easier
- though I'd prefer to keep it in clang as we plan to use
tidy & static analyzer in any case for standard QA.
Thanks for reading,
Billy.
_______________________________________________
cfe-dev mailing list
[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
_______________________________________________
cfe-dev mailing list
[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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|
|
In reply to this post by Nathan Ridge via cfe-dev
Hi Artem,
thanks for your well thought-out and useful reply. It sounds like clang-tidy will be the sweet spot between usefulness and effort to implement.
I have a few other responses down below.
Regards, Billy.
Hi,
It depends on how strict do you want the checking be and on the
details of the rule. If you're designing a new API from scratch and
stuck with gcc forever, i wouldn't mind using the gcc
__attribute__((cleanup())) for your purpose.
I didn't know about that gcc attrib. I need to read the gcc manual attrib section! I should've added that while we will be developing on gcc the code should be compilable on other toolchains/OSs also, so we are avoiding any gcc extensions (e.g. gcc has extensions for thread local storage but we are not using those for the same reason).
The rule you described should be reasonably easy to implement with
the Static Analyzer. The good side of it is that you get a lot of
semantic modeling for free. For instance, if the developer copies
`dev` into a local variable and then uses that local variable
outside of api_enter..api_exit, the tool will be able to handle
transparently, as it deals with values rather than with variables.
That is really cool.
Also it will probably be the easiest tool for your problem. The
downside would be that it's not guaranteed to find all bugs; it'll
inevitably give up on complicated code with high cyclomatic
complexity :) So if you want strict/paranoid enforcement of rules,
the Static Analyzer is not the right tool. But if you want to simply
find some bugs for free, it's the right tool.
It sounds as if your problem is not inter-procedural. Let me
double-check this: would you have another api_enter..api_exit pair
in the body of your tweak() function? Or is just one
api_enter..api_exit enough? Or is it a bug to call api_enter twice
without an api_exit in between?
Yes in this case tweak() could be another public fn of the api that would also have an enter/exit pair. But we are using recursive mutexes (a thread can acquire the same mutex N times (ie call api_enter) so long as it also releases N times) so that will be okay.
If you have to write api_enter..api_exit in *every* function that
deals with devices, then the problem is not inter-procedural, which
makes it much easier. In particular, you should be able to come up
with a purely syntactic analysis ("every function that accesses a
device_t must start with api_enter() and must end in exactly one
spot with api_exit()").
We will only insist on enter/exit in public API functions. Which are the only ones a client application can call. Internal private functions we won't have locking (as they can only be called ultimately from a public function so the device will be locked.) We are going to allow private fns to call back into the api via the public i/face. But we will have the public functions in specific files and have a specific naming prefix.
Such analysis should be easily do-able in
clang-tidy as long as you're satisfied with this level of
aggressiveness. In particular, you'll have to be willing to
sacrifice code like this:
void foo(device_t *dev) {
if (flip_a_coin()) {
api_enter(dev);
...
api_exit(dev);
}
}
But it may be perfectly fine if you seriously want to enforce a
strict structure on all your functions that deal with devices.
So is it the case that clang-tidy kind of passes info to the checker-extension in a syntactic code-parsing order. Whereas the static analyzer passes information to the checker in a simulated run-time order?
E.g in your foo() above my proposed checker gets fed 1) theres a function called foo, 2) theres an if with a call to flip_a_coin 3) in the true case there is a call to enter then exit 4) in the else there is nothing 5) there is a return (at which point my checker would need to be pretty smart and hold a lot of state to figure out something was wrong) . And to compare for the static analyzer it's more like 1) there is fn foo 2.1) there is a code path through foo with enter/exit 2.2) there is a code path with just return (at which point my reasonably simple checker would raise an error).
I think the truly-truly right tool for your problem would be to come
up with a custom analysis over Clang CFG.
.
By CFG you mean the Clang Static Analyzer?
It would be harder to
implement, but it would allow you to express things like "every
execution path within a function that accesses `dev` must have a
api_enter before it and an api_exit after it; you are not allowed to
copy `dev` around". This would strictly enforce the rule.
Yes that would be great. But I think just using clang-tidy from what you are saying would get us a long way. And there are heaps of simpler checks we would like to implement also.
At the
same time it'll allow you to lift the requirement of exactly one
return point - you would still be able to ensure that all accesses
are covered. If you need to allow to copy `dev` around, it should
still be doable, but it will be significantly more difficult to
implement
Does 'copy around' include passing to my private fns such as tweak()?. We don't need to copy dev anywhere within the public fns but we do need it to pass it to private fns.
On 7/2/19 12:13 PM, Billy O'Mahony via
cfe-dev wrote:
Hello,
I'd like to write a rule for either clang-tidy or static
analyzer to help catch some potential errors in a project I'm
working on.
My questions are:
a) is only one or the other will be able to do what I
want to do?
b) if both are feasible which would have the simpler
implementation?
The project involves writing an API that will run in a
multi-threaded application and is responsible for serializing
all access to a device structure. Therefore the first thing in
every function in the API must be to call api_enter (which
will among other things acquire a mutex on the device) and the
last thing before returning must be to call api_exit. Also I
want to enforce single exit point from every API function - or
certainly if there are any return points that bypass the
api_exit call.
So here is an example function with errors I want to catch
highlighted.
int api_foo(device_t *dev) {
int ret_val = 0;
bar(); // fn calls & decls before api_enter is ok-
just don't access dev.
dev->bla = 1; // NO! device access before
api_enter() called
api_enter(dev); // error if this call is not present
exactly once
if (dev->bla)
return; // NO! didn't call api_exit before rtn.
Also two return points
if (dev->ma) {
ret_val = 1;
goto cleanup;
}
tweak(dev);
cleanup:
api_exit(dev); // error if this is not present exactly
once
dev->bla = 1; //NO! device access after api_exit()
return ret_val;
}
I don't think it matters but the project is C compiled with
gcc.
Also if both are feasible any other pointers, tips or good
resources would be appreciated. E.g is there a totally
different methodology I'm not considering - e.g. would using
something like pycparser be a lot easier - though I'd prefer
to keep it in clang as we plan to use tidy & static
analyzer in any case for standard QA.
Thanks for reading,
Billy.
_______________________________________________
cfe-dev mailing list
[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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> It sounds like clang-tidy will be the sweet spot between
usefulness and effort to implement.
Yup, i approve. You can experiment with clang-query real quick and
it should give you an idea. If you find out that some of the useful
ASTMatchers are missing, feel free to add them - either locally or
upstreamed, as it's super easy.
> By CFG you mean the Clang Static Analyzer?
Clang CFG is https://clang.llvm.org/doxygen/classclang_1_1CFG.html .
It's a "source-level" control flow graph that consists of pointers
to AST statements, ordered in the control flow order (i.e., in order
of execution). A directed path in the CFG from statement A to
statement B would tell you that "B can potentially be executed after
A". This allows you to use your favorite graph algorithms to reason
about the program's behavior.
The Static Analyzer works over Clang CFG, but it's much more than
that: the Static Analyzer also understands the semantics of every
statement and can model their effects over the "symbolic" state of
the program. For example, in the following code:
void foo(bool x) {
if (x) {
}
if (x) {
}
}
the CFG would be a double-diamond: the control branches off at the
first statement, then joins back at the end, then branches off
again, then joins back. But the Static Analyzer would understand
that the if-conditions are the same, therefore these are simply two
unrelated execution paths, and the execution path in which the first
branch goes to true and the second branch goes to false is in fact
impossible. And if 'x' is overwritten in between, the Static
Analyzer would also know that.
Apart from the Static Analyzer, the CFG is used for some clang
warnings, such as -Wuninitialized (see AnalysisBasedWarnings.cpp).
There are some ready-made analyses for common data flow problems
implemented over Clang CFG (such as live variables analysis,
dominators analysis, etc.).
Clang CFG is not used during the actual compilation / code
generation. When Chris Lattner talks about MLIR and mentions that
Clang should have had a "CIL", he means that the Clang CFG (or
something similar) should have been used for compilation *as well
as* for analysis. This would, in particular, allow semantic
optimizations that require information that's already lost in LLVM
IR.
Because the CFG is not used for compilation, it's not necessarily
correct. There are a few known bugs in it, mostly around complicated
C++ and also around GNU extensions such as the *binary* operator ?:
or statement-expressions. But for plain C it should be nearly
perfect.
> Does 'copy around' include passing to my private fns such as
tweak()?
That's entirely up to you. What i was trying to say is that it if
you allow copying 'dev' into another variable, it will become much
harder to implement your analysis, so you'd much rather forbid the
user to do this. You might also allow the user to do this and suffer
from imprecision in your analysis. At the same time, because your
analysis is not inter-procedural, passing 'dev' into a function
should be fine. The function can still return it back "laundered" so
the user would be able to assign it into a variable behind your
back. But these are the usual trade-offs of static analysis, don't
be too scared by them - after all it all boils down to the halting
problem :)
On 7/4/19 3:40 AM, Billy O'Mahony
wrote:
Hi Artem,
thanks for your well thought-out and useful reply. It
sounds like clang-tidy will be the sweet spot between
usefulness and effort to implement.
I have a few other responses down below.
Regards,
Billy.
Hi,
It depends on how strict do you want the checking be and
on the details of the rule. If you're designing a new API
from scratch and stuck with gcc forever, i wouldn't mind
using the gcc __attribute__((cleanup())) for your purpose.
I didn't know about that gcc attrib. I need to read the
gcc manual attrib section! I should've added that while we
will be developing on gcc the code should be compilable on
other toolchains/OSs also, so we are avoiding any gcc
extensions (e.g. gcc has extensions for thread local storage
but we are not using those for the same reason).
The rule you described should be
reasonably easy to implement with the Static Analyzer. The
good side of it is that you get a lot of semantic modeling
for free. For instance, if the developer copies `dev` into
a local variable and then uses that local variable outside
of api_enter..api_exit, the tool will be able to handle
transparently, as it deals with values rather than with
variables.
That is really cool.
Also it will probably be the easiest
tool for your problem. The downside would be that it's not
guaranteed to find all bugs; it'll inevitably give up on
complicated code with high cyclomatic complexity :) So if
you want strict/paranoid enforcement of rules, the Static
Analyzer is not the right tool. But if you want to simply
find some bugs for free, it's the right tool.
It sounds as if your problem is not inter-procedural. Let
me double-check this: would you have another
api_enter..api_exit pair in the body of your tweak()
function? Or is just one api_enter..api_exit enough? Or is
it a bug to call api_enter twice without an api_exit in
between?
Yes in this case tweak() could be another public fn of
the api that would also have an enter/exit pair. But we are
using recursive mutexes (a thread can acquire the same mutex
N times (ie call api_enter) so long as it also releases N
times) so that will be okay.
If you have to write api_enter..api_exit in *every*
function that deals with devices, then the problem is not
inter-procedural, which makes it much easier. In
particular, you should be able to come up with a purely
syntactic analysis ("every function that accesses a
device_t must start with api_enter() and must end in
exactly one spot with api_exit()").
We will only insist on enter/exit in public API
functions. Which are the only ones a client application can
call. Internal private functions we won't have locking (as
they can only be called ultimately from a public function so
the device will be locked.) We are going to allow private
fns to call back into the api via the public i/face. But we
will have the public functions in specific files and have a
specific naming prefix.
Such analysis should be easily do-able in clang-tidy
as long as you're satisfied with this level of
aggressiveness. In particular, you'll have to be willing
to sacrifice code like this:
void foo(device_t *dev) {
if (flip_a_coin()) {
api_enter(dev);
...
api_exit(dev);
}
}
But it may be perfectly fine if you seriously want to
enforce a strict structure on all your functions that deal
with devices.
So is it the case that clang-tidy kind of passes info to
the checker-extension in a syntactic code-parsing order.
Whereas the static analyzer passes information to the
checker in a simulated run-time order?
E.g in your foo() above my proposed checker gets fed 1)
theres a function called foo, 2) theres an if with a call to
flip_a_coin 3) in the true case there is a call to enter
then exit 4) in the else there is nothing 5) there is a
return (at which point my checker would need to be pretty
smart and hold a lot of state to figure out something was
wrong) . And to compare for the static analyzer it's more
like 1) there is fn foo 2.1) there is a code path through
foo with enter/exit 2.2) there is a code path with just
return (at which point my reasonably simple checker would
raise an error).
I think the truly-truly right tool for your problem would
be to come up with a custom analysis over Clang CFG.
.
By CFG you mean the Clang Static Analyzer?
It would be harder to implement, but it would allow
you to express things like "every execution path within a
function that accesses `dev` must have a api_enter before
it and an api_exit after it; you are not allowed to copy
`dev` around". This would strictly enforce the rule.
Yes that would be great. But I think just using
clang-tidy from what you are saying would get us a long way.
And there are heaps of simpler checks we would like to
implement also.
At the same time it'll allow you to lift the
requirement of exactly one return point - you would still
be able to ensure that all accesses are covered. If you
need to allow to copy `dev` around, it should still be
doable, but it will be significantly more difficult to
implement
Does 'copy around' include passing to my private fns
such as tweak()?. We don't need to copy dev anywhere within
the public fns but we do need it to pass it to private fns.
On
7/2/19 12:13 PM, Billy O'Mahony via cfe-dev wrote:
Hello,
I'd like to write a rule for either clang-tidy or
static analyzer to help catch some potential errors
in a project I'm working on.
My questions are:
a) is only one or the other will be able to
do what I want to do?
b) if both are feasible which would have the
simpler implementation?
The project involves writing an API that will run
in a multi-threaded application and is responsible
for serializing all access to a device structure.
Therefore the first thing in every function in the
API must be to call api_enter (which will among
other things acquire a mutex on the device) and the
last thing before returning must be to call
api_exit. Also I want to enforce single exit point
from every API function - or certainly if there are
any return points that bypass the api_exit call.
So here is an example function with errors I want
to catch highlighted.
int api_foo(device_t *dev) {
int ret_val = 0;
bar(); // fn calls & decls before
api_enter is ok- just don't access dev.
dev->bla = 1; // NO! device access before
api_enter() called
api_enter(dev); // error if this call is
not present exactly once
if (dev->bla)
return; // NO! didn't call api_exit
before rtn. Also two return points
if (dev->ma) {
ret_val = 1;
goto cleanup;
}
tweak(dev);
cleanup:
api_exit(dev); // error if this is not
present exactly once
dev->bla = 1; //NO! device access after
api_exit()
return ret_val;
}
I don't think it matters but the project is C
compiled with gcc.
Also if both are feasible any other pointers,
tips or good resources would be appreciated. E.g is
there a totally different methodology I'm not
considering - e.g. would using something like
pycparser be a lot easier - though I'd prefer to
keep it in clang as we plan to use tidy & static
analyzer in any case for standard QA.
Thanks for reading,
Billy.
_______________________________________________
cfe-dev mailing list
[hidden email]
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
_______________________________________________
cfe-dev mailing list
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Hi Artem,
super. Good to know I'm on the right path. And thanks for the extra info - plenty of clang goodness to be looking into there.
Cheers, Billy.
> It sounds like clang-tidy will be the sweet spot between
usefulness and effort to implement.
Yup, i approve. You can experiment with clang-query real quick and
it should give you an idea. If you find out that some of the useful
ASTMatchers are missing, feel free to add them - either locally or
upstreamed, as it's super easy.
> By CFG you mean the Clang Static Analyzer?
Clang CFG is https://clang.llvm.org/doxygen/classclang_1_1CFG.html .
It's a "source-level" control flow graph that consists of pointers
to AST statements, ordered in the control flow order (i.e., in order
of execution). A directed path in the CFG from statement A to
statement B would tell you that "B can potentially be executed after
A". This allows you to use your favorite graph algorithms to reason
about the program's behavior.
The Static Analyzer works over Clang CFG, but it's much more than
that: the Static Analyzer also understands the semantics of every
statement and can model their effects over the "symbolic" state of
the program. For example, in the following code:
void foo(bool x) {
if (x) {
}
if (x) {
}
}
the CFG would be a double-diamond: the control branches off at the
first statement, then joins back at the end, then branches off
again, then joins back. But the Static Analyzer would understand
that the if-conditions are the same, therefore these are simply two
unrelated execution paths, and the execution path in which the first
branch goes to true and the second branch goes to false is in fact
impossible. And if 'x' is overwritten in between, the Static
Analyzer would also know that.
Apart from the Static Analyzer, the CFG is used for some clang
warnings, such as -Wuninitialized (see AnalysisBasedWarnings.cpp).
There are some ready-made analyses for common data flow problems
implemented over Clang CFG (such as live variables analysis,
dominators analysis, etc.).
Clang CFG is not used during the actual compilation / code
generation. When Chris Lattner talks about MLIR and mentions that
Clang should have had a "CIL", he means that the Clang CFG (or
something similar) should have been used for compilation *as well
as* for analysis. This would, in particular, allow semantic
optimizations that require information that's already lost in LLVM
IR.
Because the CFG is not used for compilation, it's not necessarily
correct. There are a few known bugs in it, mostly around complicated
C++ and also around GNU extensions such as the *binary* operator ?:
or statement-expressions. But for plain C it should be nearly
perfect.
> Does 'copy around' include passing to my private fns such as
tweak()?
That's entirely up to you. What i was trying to say is that it if
you allow copying 'dev' into another variable, it will become much
harder to implement your analysis, so you'd much rather forbid the
user to do this. You might also allow the user to do this and suffer
from imprecision in your analysis. At the same time, because your
analysis is not inter-procedural, passing 'dev' into a function
should be fine. The function can still return it back "laundered" so
the user would be able to assign it into a variable behind your
back. But these are the usual trade-offs of static analysis, don't
be too scared by them - after all it all boils down to the halting
problem :)
On 7/4/19 3:40 AM, Billy O'Mahony
wrote:
Hi Artem,
thanks for your well thought-out and useful reply. It
sounds like clang-tidy will be the sweet spot between
usefulness and effort to implement.
I have a few other responses down below.
Regards,
Billy.
Hi,
It depends on how strict do you want the checking be and
on the details of the rule. If you're designing a new API
from scratch and stuck with gcc forever, i wouldn't mind
using the gcc __attribute__((cleanup())) for your purpose.
I didn't know about that gcc attrib. I need to read the
gcc manual attrib section! I should've added that while we
will be developing on gcc the code should be compilable on
other toolchains/OSs also, so we are avoiding any gcc
extensions (e.g. gcc has extensions for thread local storage
but we are not using those for the same reason).
The rule you described should be
reasonably easy to implement with the Static Analyzer. The
good side of it is that you get a lot of semantic modeling
for free. For instance, if the developer copies `dev` into
a local variable and then uses that local variable outside
of api_enter..api_exit, the tool will be able to handle
transparently, as it deals with values rather than with
variables.
That is really cool.
Also it will probably be the easiest
tool for your problem. The downside would be that it's not
guaranteed to find all bugs; it'll inevitably give up on
complicated code with high cyclomatic complexity :) So if
you want strict/paranoid enforcement of rules, the Static
Analyzer is not the right tool. But if you want to simply
find some bugs for free, it's the right tool.
It sounds as if your problem is not inter-procedural. Let
me double-check this: would you have another
api_enter..api_exit pair in the body of your tweak()
function? Or is just one api_enter..api_exit enough? Or is
it a bug to call api_enter twice without an api_exit in
between?
Yes in this case tweak() could be another public fn of
the api that would also have an enter/exit pair. But we are
using recursive mutexes (a thread can acquire the same mutex
N times (ie call api_enter) so long as it also releases N
times) so that will be okay.
If you have to write api_enter..api_exit in *every*
function that deals with devices, then the problem is not
inter-procedural, which makes it much easier. In
particular, you should be able to come up with a purely
syntactic analysis ("every function that accesses a
device_t must start with api_enter() and must end in
exactly one spot with api_exit()").
We will only insist on enter/exit in public API
functions. Which are the only ones a client application can
call. Internal private functions we won't have locking (as
they can only be called ultimately from a public function so
the device will be locked.) We are going to allow private
fns to call back into the api via the public i/face. But we
will have the public functions in specific files and have a
specific naming prefix.
Such analysis should be easily do-able in clang-tidy
as long as you're satisfied with this level of
aggressiveness. In particular, you'll have to be willing
to sacrifice code like this:
void foo(device_t *dev) {
if (flip_a_coin()) {
api_enter(dev);
...
api_exit(dev);
}
}
But it may be perfectly fine if you seriously want to
enforce a strict structure on all your functions that deal
with devices.
So is it the case that clang-tidy kind of passes info to
the checker-extension in a syntactic code-parsing order.
Whereas the static analyzer passes information to the
checker in a simulated run-time order?
E.g in your foo() above my proposed checker gets fed 1)
theres a function called foo, 2) theres an if with a call to
flip_a_coin 3) in the true case there is a call to enter
then exit 4) in the else there is nothing 5) there is a
return (at which point my checker would need to be pretty
smart and hold a lot of state to figure out something was
wrong) . And to compare for the static analyzer it's more
like 1) there is fn foo 2.1) there is a code path through
foo with enter/exit 2.2) there is a code path with just
return (at which point my reasonably simple checker would
raise an error).
I think the truly-truly right tool for your problem would
be to come up with a custom analysis over Clang CFG.
.
By CFG you mean the Clang Static Analyzer?
It would be harder to implement, but it would allow
you to express things like "every execution path within a
function that accesses `dev` must have a api_enter before
it and an api_exit after it; you are not allowed to copy
`dev` around". This would strictly enforce the rule.
Yes that would be great. But I think just using
clang-tidy from what you are saying would get us a long way.
And there are heaps of simpler checks we would like to
implement also.
At the same time it'll allow you to lift the
requirement of exactly one return point - you would still
be able to ensure that all accesses are covered. If you
need to allow to copy `dev` around, it should still be
doable, but it will be significantly more difficult to
implement
Does 'copy around' include passing to my private fns
such as tweak()?. We don't need to copy dev anywhere within
the public fns but we do need it to pass it to private fns.
On
7/2/19 12:13 PM, Billy O'Mahony via cfe-dev wrote:
Hello,
I'd like to write a rule for either clang-tidy or
static analyzer to help catch some potential errors
in a project I'm working on.
My questions are:
a) is only one or the other will be able to
do what I want to do?
b) if both are feasible which would have the
simpler implementation?
The project involves writing an API that will run
in a multi-threaded application and is responsible
for serializing all access to a device structure.
Therefore the first thing in every function in the
API must be to call api_enter (which will among
other things acquire a mutex on the device) and the
last thing before returning must be to call
api_exit. Also I want to enforce single exit point
from every API function - or certainly if there are
any return points that bypass the api_exit call.
So here is an example function with errors I want
to catch highlighted.
int api_foo(device_t *dev) {
int ret_val = 0;
bar(); // fn calls & decls before
api_enter is ok- just don't access dev.
dev->bla = 1; // NO! device access before
api_enter() called
api_enter(dev); // error if this call is
not present exactly once
if (dev->bla)
return; // NO! didn't call api_exit
before rtn. Also two return points
if (dev->ma) {
ret_val = 1;
goto cleanup;
}
tweak(dev);
cleanup:
api_exit(dev); // error if this is not
present exactly once
dev->bla = 1; //NO! device access after
api_exit()
return ret_val;
}
I don't think it matters but the project is C
compiled with gcc.
Also if both are feasible any other pointers,
tips or good resources would be appreciated. E.g is
there a totally different methodology I'm not
considering - e.g. would using something like
pycparser be a lot easier - though I'd prefer to
keep it in clang as we plan to use tidy & static
analyzer in any case for standard QA.
Thanks for reading,
Billy.
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