<div dir="ltr"><div>Hi Artem,</div><div><br></div><div>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.</div><div><br></div><div>Cheers,</div><div>Billy.<br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, 6 Jul 2019 at 04:05, Artem Dergachev <<a href="mailto:noqnoqneo@gmail.com">noqnoqneo@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">
> It sounds like clang-tidy will be the sweet spot between
usefulness and effort to implement.<br>
<br>
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.<br>
<br>
<br>
> By CFG you mean the Clang Static Analyzer? <br>
<br>
Clang CFG is <a class="gmail-m_-4326018427190480361moz-txt-link-freetext" href="https://clang.llvm.org/doxygen/classclang_1_1CFG.html" target="_blank">https://clang.llvm.org/doxygen/classclang_1_1CFG.html</a> .<br>
<br>
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.<br>
<br>
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:<br>
<br>
void foo(bool x) {<br>
if (x) {<br>
}<br>
if (x) {<br>
}<br>
}<br>
<br>
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.<br>
<br>
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.).<br>
<br>
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.<br>
<br>
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.<br>
<br>
<br>
> Does 'copy around' include passing to my private fns such as
tweak()?<br>
<br>
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 :)<br>
<br>
<div class="gmail-m_-4326018427190480361moz-cite-prefix">On 7/4/19 3:40 AM, Billy O'Mahony
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi Artem,</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>I have a few other responses down below.</div>
<div><br>
</div>
<div>Regards,</div>
<div>Billy.<br>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, 3 Jul 2019 at 23:23,
Artem Dergachev <<a href="mailto:noqnoqneo@gmail.com" target="_blank">noqnoqneo@gmail.com</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF"> Hi,<br>
<br>
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.<br>
<br>
</div>
</blockquote>
<div>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). <br>
</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF"> 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.</div>
</blockquote>
<div>That is really cool.</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF"> 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.<br>
<br>
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?<br>
</div>
</blockquote>
<div>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. <br>
</div>
<div><br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF"> <br>
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()").</div>
</blockquote>
<div>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.<br>
</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div> 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:<br>
<br>
void foo(device_t *dev) {<br>
if (flip_a_coin()) {<br>
api_enter(dev);<br>
...<br>
api_exit(dev);<br>
}<br>
}<br>
<br>
But it may be perfectly fine if you seriously want to
enforce a strict structure on all your functions that deal
with devices.</div>
</blockquote>
<div><br>
</div>
<div>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?</div>
<div><br>
</div>
<div>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).<br>
</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div> <br>
I think the truly-truly right tool for your problem would
be to come up with a custom analysis over Clang CFG.</div>
</blockquote>
<div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF">.<br>
</div>
</blockquote>
<div>By CFG you mean the Clang Static Analyzer? </div>
<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div> 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.</div>
</blockquote>
<div>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.<br>
</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div> 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<br>
</div>
</blockquote>
<div>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.<br>
</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF"> <br>
<div class="gmail-m_-4326018427190480361gmail-m_2793145245148744982moz-cite-prefix">On
7/2/19 12:13 PM, Billy O'Mahony via cfe-dev wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hello,</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>My questions are: <br>
</div>
<div> a) is only one or the other will be able to
do what I want to do?<br>
</div>
<div> b) if both are feasible which would have the
simpler implementation?</div>
<div><br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>So here is an example function with errors I want
to catch highlighted.</div>
<div><br>
</div>
<div>int api_foo(device_t *dev) {</div>
<div> int ret_val = 0;<br>
</div>
<div><br>
</div>
<div> bar(); // fn calls & decls before
api_enter is ok- just don't access dev.</div>
<div> dev->bla = 1; // NO! device access before
api_enter() called</div>
<div> api_enter(dev); // error if this call is
not present exactly once</div>
<div><br>
</div>
<div> if (dev->bla)</div>
<div> return; // NO! didn't call api_exit
before rtn. Also two return points</div>
<div><br>
</div>
<div> if (dev->ma) {</div>
<div> ret_val = 1;<br>
</div>
<div> goto cleanup;</div>
<div> }</div>
<div> tweak(dev);<br>
</div>
<div><br>
</div>
<div>cleanup:</div>
<div> api_exit(dev); // error if this is not
present exactly once<br>
</div>
<div> dev->bla = 1; //NO! device access after
api_exit()<br>
</div>
<div> return ret_val;<br>
</div>
<div>}</div>
<div><br>
</div>
<div>I don't think it matters but the project is C
compiled with gcc.<br>
</div>
<div><br>
</div>
<div>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.<br>
</div>
<div><br>
</div>
<div>Thanks for reading,</div>
<div>Billy.<br>
</div>
</div>
<br>
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</blockquote>
<br>
</div>
</blockquote>
</div>
</div>
</blockquote>
<br>
</div>
</blockquote></div></div>