Re: [Qemu-devel] [PATCH] qcow2: avoid memcpy(dst, NULL, len)

[Kevin Wolf <kwolf@redhat.com>]

Am 13.09.2016 um 11:15 hat Stefan Hajnoczi geschrieben:
> Section "7.1.4 Use of library functions" in the C99 standard says:
> 
>   If an argument to a function has an invalid value (such as [...]
>   a null pointer [...]) [...] the behavior is undefined.
> 
> Additionally the "searching and sorting" functions are specified as
> requiring valid pointer values as described in 7.1.4.
> 
> This patch fixes the following static analyzer errors:

"sanitizer", not "static analyzer"

>   block/qcow2.c:1807:41: runtime error: null pointer passed as argument 2, which is declared to never be null
>   block/qcow2-cluster.c:86:26: runtime error: null pointer passed as argument 2, which is declared to never be null
> 
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/qcow2-cluster.c | 4 +++-
>  block/qcow2.c         | 5 ++++-
>  2 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index f941835..ab0dcdc 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -83,7 +83,9 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
>      }
>      memset(new_l1_table, 0, align_offset(new_l1_size2, 512));
>  
> -    memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));
> +    if (s->l1_table) {
> +        memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));
> +    }

We could add 'else assert(s->l1_size == 0)' to make the intentions
clearer.  Or actually, maybe the simpler way would be to do
'if (s->l1_size)', then it's obvious that the skipped memcpy() wouldn't
do anything anyway.

>      /* write new table (align to cluster) */
>      BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
> diff --git a/block/qcow2.c b/block/qcow2.c
> index c079aa8..758a997 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1804,7 +1804,10 @@ static size_t header_ext_add(char *buf, uint32_t magic, const void *s,
>          .magic  = cpu_to_be32(magic),
>          .len    = cpu_to_be32(len),
>      };
> -    memcpy(buf + sizeof(QCowExtension), s, len);
> +
> +    if (s) {
> +        memcpy(buf + sizeof(QCowExtension), s, len);
> +    }

Same thing here.

With or without the change:

Reviewed-by: Kevin Wolf <kwolf@redhat.com>