Re: [PATCH] gimple-ssa-warn-access: Fix up asan_test.C -Wdangling-pointer regression [PR104103]

[Martin Sebor <msebor@gmail.com>]

On 1/18/22 16:56, Jakub Jelinek wrote:
> Hi!
> 
> As reported in the PR or as I've seen since the weekend, asan_test.C fails
> because of many warnings like:
> /home/jakub/src/gcc/gcc/testsuite/g++.dg/asan/asan_test.cc:1157:10: error: using a dangling pointer to an unnamed temporary [-Werror=dangling-pointer=]
> /home/jakub/src/gcc/gcc/testsuite/g++.dg/asan/asan_test.cc:1157:10: error: using a dangling pointer to an unnamed temporary [-Werror=dangling-pointer=]
> /home/jakub/src/gcc/gcc/testsuite/g++.dg/asan/asan_test.cc:1162:27: error: using a dangling pointer to an unnamed temporary [-Werror=dangling-pointer=]
> ...
> (lots of them).
> There are no dangling pointers though, the warning pass sees:
>    some_automatic_var ={v} {CLOBBER};
>    .ASAN_MARK (POISON, &some_automatic_var, 8);
> and warns on that (both on user vars and on e.g. TARGET_EXPR temporaries).
> There is nothing wrong on that, .ASAN_MARK is compiler instrumentation,
> which doesn't even touch the variable in any way nor make it escaped.
> What it instead does is change bytes in the shadow memory corresponding
> to the variable to reflect that the variable is out of scope and make
> sure that access to it would be diagnosed at runtime.
> So, for all purposes of the -Wdangling-pointer and -Wuse-after-free
> warnings, we should ignore this internal call.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, fixes asan_test.C
> FAIL (so no new test has been added), ok for trunk?

This is in line with what's done for -Wmaybe-uninitialized so it makes
sense to do it here as well.  -Wmaybe-uninitialized also exempts calls
to sanitizer built-ins from checking.  I don't know if they might come
up here but if it can't be ruled out, moving the code from
tree-ssa-uninit.cc into a utility helper and calling it from both
places might be a good idea.

Martin

> 
> 2022-01-18  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR middle-end/104103
> 	* gimple-ssa-warn-access.cc (pass_waccess::check_call): Don't check
> 	.ASAN_MARK calls.
> 
> --- gcc/gimple-ssa-warn-access.cc.jj	2022-01-16 20:55:46.783932110 +0100
> +++ gcc/gimple-ssa-warn-access.cc	2022-01-18 20:56:13.697780325 +0100
> @@ -4232,6 +4232,11 @@ pass_waccess::check_call (gcall *stmt)
>     if (gimple_call_builtin_p (stmt, BUILT_IN_NORMAL))
>       check_builtin (stmt);
>   
> +  /* .ASAN_MARK doesn't access any vars, only modifies shadow memory.  */
> +  if (gimple_call_internal_p (stmt)
> +      && gimple_call_internal_fn (stmt) == IFN_ASAN_MARK)
> +    return;
> +
>     if (!m_early_checks_p)
>       if (tree callee = gimple_call_fndecl (stmt))
>         {
> 
> 	Jakub
>