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

[Brian Foster]

On Tue, Oct 10, 2023 at 04:56:12PM -0700, Kees Cook wrote:
> 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>
> ---
>  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..f5e8cb43697b 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, bytes);
> +	};
>  };

Hi Kees,

I'm curious if this is something that could be buried in bch_val given
it's already kind of a fake structure..? If not, my only nitty comment
is that memcpy(k->bytes[], ...) makes it kind of read like we're copying
in opaque key data rather than value data, so perhaps a slightly more
descriptive field name would be helpful. But regardless I'd wait until
Kent has a chance to comment before changing anything..

Brian

>  
>  #define KEY(_inode, _offset, _size)					\
> diff --git a/fs/bcachefs/extents.h b/fs/bcachefs/extents.h
> index 7ee8d031bb6c..6248e17bbac5 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->bytes[bkey_val_bytes(&k->k)],
>  		       &ptr,
>  		       sizeof(ptr));
>  		k->k.u64s++;
> -- 
> 2.34.1
>