Re: [PATCH] Improve checks in c_strlen (PR 87053)

[Jeff Law <law@redhat.com>]

On 08/23/2018 03:27 AM, Bernd Edlinger wrote:
> On 08/22/18 18:28, Martin Sebor wrote:
>> On 08/22/2018 08:41 AM, Bernd Edlinger wrote:
>>> Hi!
>>>
>>>
>>> This patch adds some more checks to c_getstr to fix PR middle-end/87053
>>> wrong code bug.
>>>
>>> Unfortunately this patch alone is not sufficient to fix the problem,
>>> but also the patch for PR 86714 that hardens c_getstr is necessary
>>> to prevent the wrong folding.
>>>
>>>
>>> Bootstrapped and reg-tested on top of my PR 86711/86714 patch.
>>> Is it OK for trunk?
>>
>> This case is also the subject of the patch I submitted back in
>> July for 86711/86714 and 86552.  With it, GCC avoid folding
>> the strlen call early and warns for the missing nul:
>>
>> warning: ‘__builtin_strlen’ argument missing terminating nul [-Wstringop-overflow=]
>>     if (__builtin_strlen (u.z) != 7)
>>         ^~~~~~~~~~~~~~~~
>>
>> The patch doesn't doesn't prevent all such strings from being
>> folded and it eventually lets fold_builtin_strlen() do its thing:
>>
>>        /* To avoid warning multiple times about unterminated
>>           arrays only warn if its length has been determined
>>           and is being folded to a constant.  */
>>        if (nonstr)
>>          warn_string_no_nul (loc, NULL_TREE, fndecl, nonstr);
>>
>>        return fold_convert_loc (loc, type, len);
>>
>> Handling this case is a matter of avoiding the folding here as
>> well and moving the warning later.
>>
>> Since my patch is still in the review queue and does much more
>> than just prevent folding of non-nul terminated arrays it should
>> be reviewed first.
>>
> 
> Hmmm, now you made me curious.
> 
> So I tried to install your patch (I did this on r263508
> since it does not apply to trunk, one thing I noted is
> that part 4 and part 3 seem to create gcc/testsuite/gcc.dg/warn-strcpy-no-nul.c
> I did not check if they are identical or not).
> 
> So I tried the test case from this PR on the compiler built with your patch:
> 
> $ cat cat pr87053.c
> /* PR middle-end/87053 */
> 
> const union
> { struct {
>      char x[4];
>      char y[4];
>    };
>    struct {
>      char z[8];
>    };
> } u = {{"1234", "567"}};
> 
> int main ()
> {
>    if (__builtin_strlen (u.z) != 7)
>      __builtin_abort ();
> }
> $ gcc -S pr87053.c
> pr87053.c: In function 'main':
> pr87053.c:15:7: warning: '__builtin_strlen' argument missing terminating nul [-Wstringop-overflow=]
> 15 |   if (__builtin_strlen (u.z) != 7)
>     |       ^~~~~~~~~~~~~~~~
> pr87053.c:11:3: note: referenced argument declared here
> 11 | } u = {{"1234", "567"}};
>     |   ^
> $ cat pr87053.s
> 	.file	"pr87053.c"
> 	.text
> 	.globl	u
> 	.section	.rodata
> 	.align 8
> 	.type	u, @object
> 	.size	u, 8
> u:
> 	.ascii	"1234"
> 	.string	"567"
> 	.text
> 	.globl	main
> 	.type	main, @function
> main:
> .LFB0:
> 	.cfi_startproc
> 	pushq	%rbp
> 	.cfi_def_cfa_offset 16
> 	.cfi_offset 6, -16
> 	movq	%rsp, %rbp
> 	.cfi_def_cfa_register 6
> 	call	abort
> 	.cfi_endproc
> .LFE0:
> 	.size	main, .-main
> 	.ident	"GCC: (GNU) 9.0.0 20180813 (experimental)"
> 	.section	.note.GNU-stack,"",@progbits
> 
> 
> So we get a warning, and still wrong code.
> 
> That is the reason why I think this patch of yours adds
> confusion by trying to fix everything in one step.
> 
> And I would like you to think of ways how to solve
> a problem step by step.
> 
> And at this time, sorry, we should restore correctness issues.
> And fix wrong-code issues.
> If possible without breaking existing warnings, yes.
> But no new warnings, sorry again.
Just a note, Martin's most fix for 86711/86714 fixes codegen issues
without breaking existing warnings or adding new warnings.  The new
warnings were broken out into follow-up patches.

jeff
> Bernd.
>