Re: [PATCH] bcache: Silence memcpy() run-time false positive warnings

[Coly Li]

> 2023年1月6日 14:02，Kees Cook <keescook@xxxxxxxxxxxx> 写道：
> 
> struct bkey has internal padding in a union, but it isn't always named
> the same (e.g. key ## _pad, key_p, etc). This makes it extremely hard
> for the compiler to reason about the available size of copies done
> against such keys. Use unsafe_memcpy() for now, to silence the many
> run-time false positive warnings:
> 

The keys is embedded in multiple data structures as a generalized model with some helpers, bkey_bytes() is one of them.

It is not surprised for such feeling when people read the code at first glance. And thank you for improving it :-)


>  memcpy: detected field-spanning write (size 264) of single field "&i->j" at drivers/md/bcache/journal.c:152 (size 240)
>  memcpy: detected field-spanning write (size 24) of single field "&b->key" at drivers/md/bcache/btree.c:939 (size 16)
>  emcpy: detected field-spanning write (size 24) of single field "&temp.key" at drivers/md/bcache/extents.c:428 (size 16)
> 

How does the above information can be founded? Should I use llvm and enable FORTIFY_SOURCE?

I don’t say the bkey and bkey_bytes() stuffs are elegant, but why the compiler cannot find such situation? IMHO it is quite similar to something like “struct foo *bar[0]” at the end of a data structure.


> Reported-by: Thorsten Leemhuis <linux@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/19200730-a3ba-6f4f-bb81-71339bdbbf73@xxxxxxxxxxxxx/
> Cc: Coly Li <colyli@xxxxxxx>
> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> Cc: linux-bcache@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

The code comments as justification is informative. Thanks for this.

Acked-by: Coly Li <colyli@xxxxxxx>


Coly Li


> ---
> drivers/md/bcache/bcache_ondisk.h | 3 ++-
> drivers/md/bcache/journal.c       | 3 ++-
> 2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/bcache/bcache_ondisk.h b/drivers/md/bcache/bcache_ondisk.h
> index d086a0ce4bc2..6620a7f8fffc 100644
> --- a/drivers/md/bcache/bcache_ondisk.h
> +++ b/drivers/md/bcache/bcache_ondisk.h
> @@ -106,7 +106,8 @@ static inline unsigned long bkey_bytes(const struct bkey *k)
> return bkey_u64s(k) * sizeof(__u64);
> }
> 
> -#define bkey_copy(_dest, _src) memcpy(_dest, _src, bkey_bytes(_src))
> +#define bkey_copy(_dest, _src) unsafe_memcpy(_dest, _src, bkey_bytes(_src), \
> + /* bkey is always padded */)
> 
> static inline void bkey_copy_key(struct bkey *dest, const struct bkey *src)
> {
> diff --git a/drivers/md/bcache/journal.c b/drivers/md/bcache/journal.c
> index e5da469a4235..c182c21de2e8 100644
> --- a/drivers/md/bcache/journal.c
> +++ b/drivers/md/bcache/journal.c
> @@ -149,7 +149,8 @@ reread: left = ca->sb.bucket_size - offset;
>    bytes, GFP_KERNEL);
> if (!i)
> return -ENOMEM;
> - memcpy(&i->j, j, bytes);
> + unsafe_memcpy(&i->j, j, bytes,
> + /* "bytes" was calculated by set_bytes() above */);
> /* Add to the location after 'where' points to */
> list_add(&i->list, where);
> ret = 1;
> -- 
> 2.34.1
>