[PING #4][PATCH] avoid warning on constant strncpy until next statement is reachable (PR 87028)

[Martin Sebor <msebor@gmail.com>]

Ping: https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01818.html

Please let me know if there is something I need to change here
to make the fix acceptable or if I should stop trying.

On 10/31/2018 10:33 AM, Martin Sebor wrote:
> Ping: https://gcc.gnu.org/ml/gcc-patches/2018-08/msg01818.html
>
> On 10/20/2018 06:01 PM, Martin Sebor wrote:
>> On 10/16/2018 03:21 PM, Jeff Law wrote:
>>> On 10/4/18 9:51 AM, Martin Sebor wrote:
>>>> On 10/04/2018 08:58 AM, Jeff Law wrote:
>>>>> On 8/27/18 9:42 AM, Richard Biener wrote:
>>>>>> On Mon, Aug 27, 2018 at 5:32 PM Jeff Law <law@redhat.com> wrote:
>>>>>>>
>>>>>>> On 08/27/2018 02:29 AM, Richard Biener wrote:
>>>>>>>> On Sun, Aug 26, 2018 at 7:26 AM Jeff Law <law@redhat.com> wrote:
>>>>>>>>>
>>>>>>>>> On 08/24/2018 09:58 AM, Martin Sebor wrote:
>>>>>>>>>> The warning suppression for -Wstringop-truncation looks for
>>>>>>>>>> the next statement after a truncating strncpy to see if it
>>>>>>>>>> adds a terminating nul.  This only works when the next
>>>>>>>>>> statement can be reached using the Gimple statement iterator
>>>>>>>>>> which isn't until after gimplification.  As a result, strncpy
>>>>>>>>>> calls that truncate their constant argument that are being
>>>>>>>>>> folded to memcpy this early get diagnosed even if they are
>>>>>>>>>> followed by the nul assignment:
>>>>>>>>>>
>>>>>>>>>>   const char s[] = "12345";
>>>>>>>>>>   char d[3];
>>>>>>>>>>
>>>>>>>>>>   void f (void)
>>>>>>>>>>   {
>>>>>>>>>>     strncpy (d, s, sizeof d - 1);   // -Wstringop-truncation
>>>>>>>>>>     d[sizeof d - 1] = 0;
>>>>>>>>>>   }
>>>>>>>>>>
>>>>>>>>>> To avoid the warning I propose to defer folding strncpy to
>>>>>>>>>> memcpy until the pointer to the basic block the strnpy call
>>>>>>>>>> is in can be used to try to reach the next statement (this
>>>>>>>>>> happens as early as ccp1).  I'm aware of the preference to
>>>>>>>>>> fold things early but in the case of strncpy (a relatively
>>>>>>>>>> rarely used function that is often misused), getting
>>>>>>>>>> the warning right while folding a bit later but still fairly
>>>>>>>>>> early on seems like a reasonable compromise.  I fear that
>>>>>>>>>> otherwise, the false positives will drive users to adopt
>>>>>>>>>> other unsafe solutions (like memcpy) where these kinds of
>>>>>>>>>> bugs cannot be as readily detected.
>>>>>>>>>>
>>>>>>>>>> Tested on x86_64-linux.
>>>>>>>>>>
>>>>>>>>>> Martin
>>>>>>>>>>
>>>>>>>>>> PS There still are outstanding cases where the warning can
>>>>>>>>>> be avoided.  I xfailed them in the test for now but will
>>>>>>>>>> still try to get them to work for GCC 9.
>>>>>>>>>>
>>>>>>>>>> gcc-87028.diff
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> PR tree-optimization/87028 - false positive -Wstringop-truncation
>>>>>>>>>> strncpy with global variable source string
>>>>>>>>>> gcc/ChangeLog:
>>>>>>>>>>
>>>>>>>>>>       PR tree-optimization/87028
>>>>>>>>>>       * gimple-fold.c (gimple_fold_builtin_strncpy): Avoid
>>>>>>>>>> folding when
>>>>>>>>>>       statement doesn't belong to a basic block.
>>>>>>>>>>       * tree-ssa-strlen.c (maybe_diag_stxncpy_trunc): Handle
>>>>>>>>>> MEM_REF on
>>>>>>>>>>       the left hand side of assignment.
>>>>>>>>>>
>>>>>>>>>> gcc/testsuite/ChangeLog:
>>>>>>>>>>
>>>>>>>>>>       PR tree-optimization/87028
>>>>>>>>>>       * c-c++-common/Wstringop-truncation.c: Remove xfails.
>>>>>>>>>>       * gcc.dg/Wstringop-truncation-5.c: New test.
>>>>>>>>>>
>>>>>>>>>> diff --git a/gcc/gimple-fold.c b/gcc/gimple-fold.c
>>>>>>>>>> index 07341eb..284c2fb 100644
>>>>>>>>>> --- a/gcc/gimple-fold.c
>>>>>>>>>> +++ b/gcc/gimple-fold.c
>>>>>>>>>> @@ -1702,6 +1702,11 @@ gimple_fold_builtin_strncpy
>>>>>>>>>> (gimple_stmt_iterator *gsi,
>>>>>>>>>>    if (tree_int_cst_lt (ssize, len))
>>>>>>>>>>      return false;
>>>>>>>>>>
>>>>>>>>>> +  /* Defer warning (and folding) until the next statement in the
>>>>>>>>>> basic
>>>>>>>>>> +     block is reachable.  */
>>>>>>>>>> +  if (!gimple_bb (stmt))
>>>>>>>>>> +    return false;
>>>>>>>>> I think you want cfun->cfg as the test here.  They should be
>>>>>>>>> equivalent
>>>>>>>>> in practice.
>>>>>>>>
>>>>>>>> Please do not add 'cfun' references.  Note that the next stmt is
>>>>>>>> also accessible
>>>>>>>> when there is no CFG.  I guess the issue is that we fold this
>>>>>>>> during
>>>>>>>> gimplification where the next stmt is not yet "there" (but still in
>>>>>>>> GENERIC)?
>>>>>>> That was my assumption.  I almost suggested peeking at gsi_next and
>>>>>>> avoiding in that case.
>>>>>>
>>>>>> So I'd rather add guards to maybe_fold_stmt in the gimplifier then.
>>>>> So I think the concern with adding the guards to maybe_fold_stmt is
>>>>> the
>>>>> possibility of further fallout.
>>>>>
>>>>> I guess they could be written to target this case specifically to
>>>>> minimize fallout, but that feels like we're doing the same thing
>>>>> (band-aid) just in a different place.
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>>>
>>>>>>>> We generally do not want to have unfolded stmts in the IL when we
>>>>>>>> can avoid that
>>>>>>>> which is why we fold most stmts during gimplification.  We also do
>>>>>>>> that because
>>>>>>>> we now do less folding on GENERIC.
>>>>>>> But an unfolded call in the IL should always be safe and we've got
>>>>>>> plenty of opportunities to fold it later.
>>>>>>
>>>>>> Well - we do.  The very first one is forwprop though which means
>>>>>> we'll miss to
>>>>>> re-write some memcpy parts into SSA:
>>>>>>
>>>>>>           NEXT_PASS (pass_ccp, false /* nonzero_p */);
>>>>>>           /* After CCP we rewrite no longer addressed locals into SSA
>>>>>>              form if possible.  */
>>>>>>           NEXT_PASS (pass_forwprop);
>>>>>>
>>>>>> likewise early object-size will be confused by memcpy calls that just
>>>>>> exist
>>>>>> to avoid TBAA issues (another of our recommendations besides using
>>>>>> unions).
>>>>>>
>>>>>> We do fold mem* early for a reason ;)
>>>>>>
>>>>>> "We can always do warnings earlier" would be a similar true sentence.
>>>>> I'm not disagreeing at all.  There's a natural tension between the
>>>>> benefits of folding early to enable more optimizations downstream and
>>>>> leaving the IL in a state where we can give actionable warnings.
>>>>
>>>> Similar trade-offs between folding early and losing information
>>>> as a result also impact high-level optimizations.
>>>>
>>>> For instance, folding the strlen argument below
>>>>
>>>>   void f3 (struct A* p)
>>>>   {
>>>>     __builtin_strcpy (p->a, "123");
>>>>
>>>>     if (__builtin_strlen (p->a + 1) != 2)   // not folded
>>>>       __builtin_abort ();
>>>>   }
>>>>
>>>> into
>>>>
>>>>   _2 = &MEM[(void *)p_4(D) + 2B];
>>>>
>>>> early on defeats the strlen optimization because there is no
>>>> mechanism to determine what member (void *)p_4(D) + 2B refers
>>>> to (this is bug 86955).
>>>>
>>>> Another example is folding of strlen calls with no-nconstant
>>>> offsets into constant strings like here:
>>>>
>>>>   const char a[] = "123";
>>>>
>>>>   void f (int i)
>>>>   {
>>>>     if (__builtin_strlen (&a[i]) > 3)
>>>>       __builtin_abort ();
>>>>   }
>>>>
>>>> into sizeof a - 1 - i, which then prevents the result from
>>>> being folded to false  (bug 86434), not to mention the code
>>>> it emits for out-of-bounds indices.
>>>>
>>>> There are a number of other similar examples in Bugzilla
>>>> that I've filed as I discovered then during testing my
>>>> warnings (e.g., 86572).
>>>>
>>>> In my mind, transforming library calls into "lossy" low-level
>>>> primitives like MEM_REF would be better done only after higher
>>>> level optimizations have had a chance to analyze them.  Ditto
>>>> for other similar transformations (like to other library calls).
>>>> Having more accurate information helps both optimization and
>>>> warnings.  It also makes the warnings more meaningful.
>>>> Printing "memcpy overflows a buffer" when the source code
>>>> has a call to strncpy is less than ideal.
>>>>
>>>>> Similarly there's a natural tension between warning early vs warning
>>>>> late.  Code that triggers the warning may ultimately be proved
>>>>> unreachable, or we may discover simplifications that either
>>>>> suppress or
>>>>> expose a warning.
>>>>>
>>>>> There is no easy answer here.  But I think we can legitimately ask
>>>>> questions.  ie, does folding strnlen here really improve things
>>>>> downstream in ways that are measurable?  Does the false positive
>>>>> really
>>>>> impact the utility of the warning?  etc.
>>>>>
>>>>> I'd hazard a guess that Martin is particularly sensitive to false
>>>>> positives based on feedback he's received from our developer community
>>>>> as well as downstream consumers of his work.
>>>>
>>>> Yes.  The kernel folks in particular have done a lot of work
>>>> cleaning up their code in an effort to adopt the warning and
>>>> attribute nonstring.  They have been keeping me in the loop
>>>> on their progress (and feeding me back test cases with false
>>>> positives and negatives they run into).
>>> I can't recall seeing further guidance from Richi WRT putting the checks
>>> earlier (maybe_fold_stmt).
>>>
>>> If the point here is to avoid false positives by not folding strncpy,
>>> particularly in cases where we don't see the NUL in the copy, but it
>>> appears in a subsequent store, then let's be fairly selective (so as not
>>> to muck up things on the optimization side more than is necessary).
>>>
>>> ISTM we can do this by refactoring the warning bits so they're reusable
>>> at different points in the pipeline.  Those bits would always return a
>>> boolean indicating if the given statement might generate a warning or
>>> not.
>>>
>>> When called early, they would not actually issue any warning.  They
>>> would merely do the best analysis they can and return a status
>>> indicating whether or not the statement would generate a warning given
>>> current context.  The goal here is to leave statements that might
>>> generate a warning as-is in the IL.
>>>
>>> When called late (assuming there is a point where we can walk the IL and
>>> issue the appropriate warnings), the routine would actually issue the
>>> warning.
>>>
>>> The kind of structure could potentially work for other builtins where we
>>> may need to look at subsequent statements to avoid false positives, but
>>> early folding hides cases by transforming the call into an undesirable
>>> form.
>>>
>>> Note that for cases where a call looks problematical early because we
>>> can't see statement which stores the terminator, but where the
>>> terminator statement ultimately becomes visible, we still get folding,
>>> it just happens later in the pipeline.
>>>
>>> Thoughts?
>>
>> The warning only triggers when the bound is less than or equal
>> to the length of the constant source string (i.e, when strncpy
>> truncates).  So IIUC, your suggestion would defer folding only
>> such strncpy calls and let gimple_fold_builtin_strncpy fold
>> those with a constant bound that's greater than the length of
>> the constant source string.  That would be fine with me, but
>> since strncpy calls with a bound that's greater than the length
>> of the source are pointless I don't think they are important
>> enough to worry about folding super early.  The constant ones
>> that serve any purpose (and that are presumably important to
>> optimize) are those that truncate.
>>
>> That said, when optimization isn't enabled, I don't think users
>> expect calls to library functions to be transformed to calls to
>> other  functions, or inlined.  Yet that's just what GCC does.
>> For example, besides triggering the warning, the following:
>>
>>   char a[4];
>>
>>   void f (char *s)
>>   {
>>     __builtin_strncpy (a, "1234", sizeof a);
>>     a[3] = 0;
>>   }
>>
>> is transformed, even at -O0, into:
>>
>>   f (char * s)
>>   {
>>     <bb 2> :
>>     MEM[(char * {ref-all})&a] = MEM[(char * {ref-all})"1234"];
>>     a[3] = 0;
>>     return;
>>   }
>>
>> That doesn't seem right.  GCC should avoid these transformations
>> at -O0, and one way to do that is to defer folding until the CFG
>> is constructed.  The patch does it for strncpy but a more general
>> solution would do that for all calls, e.g., in maybe_fold_stmt
>> as Richard suggested (and I subsequently tested).
>>
>> Martin
>