netfs: gcc-12: temporarily disable '-Wattribute-warning' for now

netfs: gcc-12: temporarily disable '-Wattribute-warning' for now

This is a pure band-aid so that I can continue merging stuff from people
while some of the gcc-12 fallout gets sorted out.

In particular, gcc-12 is very unhappy about the kinds of pointer
arithmetic tricks that netfs does, and that makes the fortify checks
trigger in afs and ceph:

  In function ‘fortify_memset_chk’,
      inlined from ‘netfs_i_context_init’ at include/linux/netfs.h:327:2,
      inlined from ‘afs_set_netfs_context’ at fs/afs/inode.c:61:2,
      inlined from ‘afs_root_iget’ at fs/afs/inode.c:543:2:
  include/linux/fortify-string.h:258:25: warning: call to ‘__write_overflow_field’ declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
    258 |                         __write_overflow_field(p_size_field, size);
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and the reason is that netfs_i_context_init() is passed a 'struct inode'
pointer, and then it does

        struct netfs_i_context *ctx = netfs_i_context(inode);

        memset(ctx, 0, sizeof(*ctx));

where that netfs_i_context() function just does pointer arithmetic on
the inode pointer, knowing that the netfs_i_context is laid out
immediately after it in memory.

This is all truly disgusting, since the whole "netfs_i_context is laid
out immediately after it in memory" is not actually remotely true in
general, but is just made to be that way for afs and ceph.

See for example fs/cifs/cifsglob.h:

  struct cifsInodeInfo {
        struct {
                /* These must be contiguous */
                struct inode    vfs_inode;      /* the VFS's inode record */
                struct netfs_i_context netfs_ctx; /* Netfslib context */
        };
	[...]

and realize that this is all entirely wrong, and the pointer arithmetic
that netfs_i_context() is doing is also very very wrong and wouldn't
give the right answer if netfs_ctx had different alignment rules from a
'struct inode', for example).

Anyway, that's just a long-winded way to say "the gcc-12 warning is
actually quite reasonable, and our code happens to work but is pretty
disgusting".

This is getting fixed properly, but for now I made the mistake of
thinking "the week right after the merge window tends to be calm for me
as people take a breather" and I did a sustem upgrade.  And I got gcc-12
as a result, so to continue merging fixes from people and not have the
end result drown in warnings, I am fixing all these gcc-12 issues I hit.

Including with these kinds of temporary fixes.

Cc: Kees Cook <keescook@chromium.org>
Cc: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/all/AEEBCF5D-8402-441D-940B-105AA718C71F@chromium.org/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>