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

On 12/7/18 1:06 AM, Richard Biener wrote:
On Thu, 6 Dec 2018, Martin Sebor wrote:

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

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,

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.

Please always _only_ use gimple_call_fntype when the decl
isn't visible.  As elsewhere the type of the function
pointer doesn't have any semantic meaning (it could be a
wrong one).

I don't understand what you're asking me to do differently here:

-  callee = gimple_call_fndecl (call);
-  if (!callee)
+  tree calltype;
+  if (tree callfn = gimple_call_fndecl (call))
+    calltype = TREE_TYPE (callfn);
+  else
+    calltype = gimple_call_fntype (call);

Can you show the change you're looking for?  (The change I had
made originally was in response to this same request you made
in Bugzilla, which Jakub then suggested may not be robust enough.)

Btw., it should be straightforward to ask "give me the attribute
if this is a call to an alloc_size-kind of a function?" (or one
with whatever other attribute of interest).  Since it appears
to be anything but straightforward, we should consider providing
an API to make it so.  Maybe something like:

  tree gimple_call_attribute (gcall *, tree attribute);




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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