bcachefs: Refactor memcpy into direct assignment

The memcpy() in bch2_bkey_append_ptr() is operating on an embedded fake
flexible array which looks to the compiler like it has 0 size. This
causes W=1 builds to emit warnings due to -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;
         |                                 ^

Avoid making any structure changes and just replace the u64 copy into a
direct assignment, side-stepping the entire problem.

Cc: Kent Overstreet <[email protected]>
Cc: Brian Foster <[email protected]>
Cc: [email protected]
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Kent Overstreet <[email protected]>

Author: kees

fs/bcachefs/extents.h

@@ -633,6 +633,8 @@ void bch2_bkey_extent_entry_drop(struct bkey_i *, union bch_extent_entry *);
 
 static inline void bch2_bkey_append_ptr(struct bkey_i *k, struct bch_extent_ptr ptr)
 {
+	struct bch_extent_ptr *dest;
+
 	EBUG_ON(bch2_bkey_has_device(bkey_i_to_s(k), ptr.dev));
 
 	switch (k->k.type) {
@@ -642,12 +644,8 @@ static inline void bch2_bkey_append_ptr(struct bkey_i *k, struct bch_extent_ptr
 		EBUG_ON(bkey_val_u64s(&k->k) >= BKEY_EXTENT_VAL_U64s_MAX);
 
 		ptr.type = 1 << BCH_EXTENT_ENTRY_ptr;
-
-		unsafe_memcpy((void *) &k->v + bkey_val_bytes(&k->k),
-			      &ptr,
-			      sizeof(ptr),
-			      "Our memcpy target is relative to a zero size array ,"
-			      "compiler bounds checking doesn't work here");
+		dest = (struct bch_extent_ptr *)((void *) &k->v + bkey_val_bytes(&k->k));
+		*dest = ptr;
 		k->k.u64s++;
 		break;
 	default: