Don't add access size hints to fortifiable functions

In the context of a function definition, the size hints imply that the
size of an object pointed to by one parameter is another parameter.
This doesn't make sense for the fortified versions of the functions
since that's the bit it's trying to validate.

This is harmless with __builtin_object_size since it has fairly simple
semantics when it comes to objects passed as function parameters.
With __builtin_dynamic_object_size we could (as my patchset for gcc[1]
already does) use the access attribute to determine the object size in
the general case but it misleads the fortified functions.

Basically the problem occurs when access attributes are present on
regular functions that have inline fortified definitions to generate
_chk variants; the attributes get inherited by these definitions,
causing problems when analyzing them.  For example with poll(fds, nfds,
timeout), nfds is hinted using the __attr_access as being the size of
fds.

Now, when analyzing the inline function definition in bits/poll2.h, the
compiler sees that nfds is the size of fds and tries to use that
information in the function body.  In _FORTIFY_SOURCE=3 case, where the
object size could be a non-constant expression, this information results
in the conclusion that nfds is the size of fds, which defeats the
purpose of the implementation because we're trying to check here if nfds
does indeed represent the size of fds.  Hence for this case, it is best
to not have the access attribute.

With the attributes gone, the expression evaluation should get delayed
until the function is actually inlined into its destinations.

Disable the access attribute for fortified function inline functions
when building at _FORTIFY_SOURCE=3 to make this work better.  The
access attributes remain for the _chk variants since they can be used
by the compiler to warn when the caller is passing invalid arguments.

[1] https://gcc.gnu.org/pipermail/gcc-patches/2021-October/581125.html

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/stdlib/stdlib.h b/stdlib/stdlib.h
index 0481c12355..74c00eee73 100644
--- a/stdlib/stdlib.h
+++ b/stdlib/stdlib.h
@@ -943,7 +943,8 @@ extern size_t mbstowcs (wchar_t *__restrict  __pwcs,
 extern size_t wcstombs (char *__restrict __s,
 			const wchar_t *__restrict __pwcs, size_t __n)
      __THROW
-  __attr_access ((__write_only__, 1, 3)) __attr_access ((__read_only__, 2));
+  __fortified_attr_access (__write_only__, 1, 3)
+  __attr_access ((__read_only__, 2));
 
 #ifdef __USE_MISC
 /* Determine whether the string value of RESPONSE matches the affirmation
@@ -997,7 +998,7 @@ extern char *ptsname (int __fd) __THROW __wur;
    terminal associated with the master FD is open on in BUF.
    Return 0 on success, otherwise an error number.  */
 extern int ptsname_r (int __fd, char *__buf, size_t __buflen)
-     __THROW __nonnull ((2)) __attr_access ((__write_only__, 2, 3));
+     __THROW __nonnull ((2)) __fortified_attr_access (__write_only__, 2, 3);
 
 /* Open a master pseudo terminal and return its file descriptor.  */
 extern int getpt (void);