Re: [PATCH] xfs: avoid null *src in memcpy call in xlog_write

Dave Chinner
2015-10-07T21:24:28Z

On Wed, Oct 07, 2015 at 11:33:43AM -0500, Eric Sandeen wrote:
> The gcc undefined behavior sanitizer caught this; surely
> any sane memcpy implementation will no-op if size == 0,
> but behavior with a *src of NULL is technically undefined
> (declared nonnull), so avoid it here.
>
> We are actually in this situation frequently via
> xlog_commit_record(), because:
>
> struct xfs_log_iovec reg = {
> .i_addr = NULL,
> .i_len = 0,
> .i_type = XLOG_REG_TYPE_COMMIT,
> };
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 4012523..8897fd1 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2424,7 +2424,10 @@ xlog_write(
>
> /* copy region */
> ASSERT(copy_len >= 0);
> - memcpy(ptr, reg->i_addr + copy_off, copy_len);
> + ASSERT(reg->i_addr + copy_off > 0 || copy_len == 0);
> + /* size == 0 is ok, but *src == NULL is undefined */
> + if (reg->i_addr + copy_off)
> + memcpy(ptr, reg->i_addr + copy_off, copy_len);
> xlog_write_adv_cnt(&ptr, &len, &log_offset, copy_len);

The comment doesn't explain anything about why copylen might be zero
or reg->i_addr might be null. If copylen is zero, we should really
skip the copy, not rely on some magic pointer arithmetic to tell us
it's ok to copy...

/*
* Copy region.
*
* Unmount records just log an opheader, so can have
* empty payloads with no data region to copy. Hence we only
* copy the payload if the vector says it has data to copy.
*/
ASSERT(copy_len >= 0);
if (copy_len > 0) {
memcpy(ptr, reg->i_addr + copy_off, copy_len);
xlog_write_adv_cnt(&ptr, &len, &log_offset, copy_len);
}

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com