[PATCH AUTOSEL 4.19 18/18] netfs: gcc-12: temporarily disable '-Wattribute-warning' for now

Sasha Levin
2022-06-14T02:27:44Z

From: Linus Torvalds <torvalds@linux-foundation.org>

[ Upstream commit 507160f46c55913955d272ebf559d63809a8e560 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/afs/inode.c | 3 +++
fs/ceph/inode.c | 3 +++
2 files changed, 6 insertions(+)

diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index a12ae3ef8fb4..4ed71605941d 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -24,6 +24,9 @@
#include <linux/iversion.h>
#include "internal.h"

+// Temporary: netfs does disgusting things with inode pointers
+#pragma GCC diagnostic ignored "-Wattribute-warning"
+
static const struct inode_operations afs_symlink_inode_operations = {
.get_link = page_get_link,
.listxattr = afs_listxattr,
diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
index 5f041fede7aa..93ae3a912d8f 100644
--- a/fs/ceph/inode.c
+++ b/fs/ceph/inode.c
@@ -19,6 +19,9 @@
#include "cache.h"
#include <linux/ceph/decode.h>

+// Temporary: netfs does disgusting things with inode pointers
+#pragma GCC diagnostic ignored "-Wattribute-warning"
+
/*
* Ceph inode operations
*
--
2.35.1