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

[Dave Chinner <david@fromorbit.com>]

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

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