[PATCH v2] bcachefs: Refactor bkey_i to use a flexible array

[Kees Cook]

The memcpy() in bch2_bkey_append_ptr() is operating on an embedded
fake flexible array. Instead, make it explicit, and convert the memcpy
to target the flexible array instead. Fixes the W=1 warning seen for
-Wstringop-overflow:

   In file included from include/linux/string.h:254,
                    from include/linux/bitmap.h:11,
                    from include/linux/cpumask.h:12,
                    from include/linux/smp.h:13,
                    from include/linux/lockdep.h:14,
                    from include/linux/radix-tree.h:14,
                    from include/linux/backing-dev-defs.h:6,
                    from fs/bcachefs/bcachefs.h:182:
   fs/bcachefs/extents.c: In function 'bch2_bkey_append_ptr':
   include/linux/fortify-string.h:57:33: warning: writing 8 bytes into a region of size 0 [-Wstringop-overflow=]
      57 | #define __underlying_memcpy     __builtin_memcpy
         |                                 ^
   include/linux/fortify-string.h:648:9: note: in expansion of macro '__underlying_memcpy'
     648 |         __underlying_##op(p, q, __fortify_size);                        \
         |         ^~~~~~~~~~~~~
   include/linux/fortify-string.h:693:26: note: in expansion of macro '__fortify_memcpy_chk'
     693 | #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
         |                          ^~~~~~~~~~~~~~~~~~~~
   fs/bcachefs/extents.c:235:17: note: in expansion of macro 'memcpy'
     235 |                 memcpy((void *) &k->v + bkey_val_bytes(&k->k),
         |                 ^~~~~~
   fs/bcachefs/bcachefs_format.h:287:33: note: destination object 'v' of size 0
     287 |                 struct bch_val  v;
         |                                 ^

Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: Brian Foster <bfoster@xxxxxxxxxx>
Cc: linux-bcachefs@xxxxxxxxxxxxxxx
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202309192314.VBsjiIm5-lkp@xxxxxxxxx/
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2 - Change flex array name to "v_bytes" (bfoster)
v1 - https://lore.kernel.org/r/20231010235609.work.594-kees@xxxxxxxxxx
---
 fs/bcachefs/bcachefs_format.h | 5 ++++-
 fs/bcachefs/extents.h         | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/bcachefs/bcachefs_format.h b/fs/bcachefs/bcachefs_format.h
index f0d130440baa..cb1af3799b59 100644
--- a/fs/bcachefs/bcachefs_format.h
+++ b/fs/bcachefs/bcachefs_format.h
@@ -300,7 +300,10 @@ struct bkey_i {
 	__u64			_data[0];
 
 	struct bkey	k;
-	struct bch_val	v;
+	union {
+		struct bch_val	v;
+		DECLARE_FLEX_ARRAY(__u8, v_bytes);
+	};
 };
 
 #define KEY(_inode, _offset, _size)					\
diff --git a/fs/bcachefs/extents.h b/fs/bcachefs/extents.h
index 7ee8d031bb6c..896fcfca4f21 100644
--- a/fs/bcachefs/extents.h
+++ b/fs/bcachefs/extents.h
@@ -642,7 +642,7 @@ static inline void bch2_bkey_append_ptr(struct bkey_i *k, struct bch_extent_ptr
 
 		ptr.type = 1 << BCH_EXTENT_ENTRY_ptr;
 
-		memcpy((void *) &k->v + bkey_val_bytes(&k->k),
+		memcpy(&k->v_bytes[bkey_val_bytes(&k->k)],
 		       &ptr,
 		       sizeof(ptr));
 		k->k.u64s++;
-- 
2.34.1