[PATCH] handle function pointers in __builtin_object_size (PR 88372)

Martin Sebor msebor@gmail.com
Thu Dec 6 23:01:00 GMT 2018


On 12/6/18 2:26 PM, Jakub Jelinek wrote:
> On Thu, Dec 06, 2018 at 01:21:58PM -0700, Martin Sebor wrote:
>> Bug 88372 - alloc_size attribute is ignored on function pointers
>> points out that even though the alloc_size attribute is accepted
>> on function pointers it doesn't have any effect on Object Size
>> Checking.  The reporter, who is implementing the feature in Clang,
>> wants to know if by exposing it under the same name they won't be
>> causing incompatibilities with GCC.
>>
>> I don't think it's intentional that GCC doesn't take advantage of
>> the attribute for Object Size Checking, and certainly not to detect
>> the same kinds of issues as with other allocation functions (such
>> as excessive or negative size arguments).  Rather, it's almost
>> certainly an oversight since GCC does make use of function pointer
>> attributes in other contexts (e.g., attributes alloc_align and
>> noreturn).
>>
>> As an oversight, I think it's fair to consider it a bug rather
>> than a request for an enhancement.  Since not handling
>> the attribute in Object Size Checking has adverse security
>> implications, I also think this bug should be addressed in GCC
>> 9.  With that, I submit the attached patch to resolve both
>> aspects of the problem.
> 
> This is because alloc_object_size has been written before we had attributes
> like alloc_size.  The only thing I'm unsure about is whether we should
> prefer gimple_call_fntype or TREE_TYPE (gimple_call_fndecl ()) if it is a
> direct call or if we should try to look for alloc_size attribute on both
> of those if they are different types.  E.g. if somebody does
> 
> #include <stdlib.h>
> 
> typedef void *(*allocfn) (size_t);
> 
> static inline void *
> foo (allocfn fn, size_t sz)
> {
>    return fn (sz);
> }
> 
> static inline void *
> bar (size_t sz)
> {
>    return foo (malloc, sz);
> }
> 
> then I think this patch would no longer treat it as malloc.
> 
> As this is security relevant, I'd probably look for alloc_size
> attribute in both gimple_call_fntype and, if gimple_call_fndecl is non-NULL,
> its TREE_TYPE.

Thanks for the test case!  I wondered if using fntype would
always work but couldn't think of when it wouldn't.  I've
adjusted the function to use both and added the test case.

While thinking about this it occurred to me that alloc_size
is only documented as a function attribute but not one that
applies to pointers or types.  I added documentation for
these uses to the Common Type and Common Variable sections.

Martin

PS Other function attributes that also apply to types and
variables are only documented in the function section.  They
should also be mentioned in the other sections.  Which, if
done in the established style, will result in duplicating
a lot of text in three places.  I think that suggests that
we might want to think about structuring these sections of
the manual differently to avoid the duplication.
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