r281277 - [Sema] Fix PR30346: relax __builtin_object_size checks.

[Joerg Sonnenberger via cfe-commits]

On Mon, Sep 12, 2016 at 11:50:36PM -0000, George Burgess IV via cfe-commits wrote:
> Author: gbiv
> Date: Mon Sep 12 18:50:35 2016
> New Revision: 281277
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=281277&view=rev
> Log:
> [Sema] Fix PR30346: relax __builtin_object_size checks.
> 
> This patch makes us act more conservatively when trying to determine
> the objectsize for an array at the end of an object. This is in
> response to code like the following:
> 
> ```
> struct sockaddr {
>   /* snip */
>   char sa_data[14];
> };
> 
> void foo(const char *s) {
>   size_t slen = strlen(s) + 1;
>   size_t added_len = slen <= 14 ? 0 : slen - 14;
>   struct sockaddr *sa = malloc(sizeof(struct sockaddr) + added_len);
>   strcpy(sa->sa_data, s);
>   // ...
> }
> ```
> 
> `__builtin_object_size(sa->sa_data, 1)` would return 14, when there
> could be more than 14 bytes at `sa->sa_data`.
> 
> Code like this is apparently not uncommon. FreeBSD's manual even
> explicitly mentions this pattern:
> https://www.freebsd.org/doc/en/books/developers-handbook/sockets-essential-functions.html
> (section 7.5.1.1.2).
> 
> In light of this, we now just give up on any array at the end of an
> object if we can't find the object's initial allocation.
> 
> I lack numbers for how much more conservative we actually become as a
> result of this change, so I chose the fix that would make us as
> compatible with GCC as possible. If we want to be more aggressive, I'm
> happy to consider some kind of whitelist or something instead.

IMO this should be restricted to code that explicitly disables C/C++
aliasing rules.

Joerg