Re: [PATCH] xfs_repair: don't let bplist index go negative in prefetch

[Eric Sandeen <sandeen@sandeen.net>]

On 5/15/14, 1:59 PM, Eric Sandeen wrote:
> After:
> 
> bbd3275 repair: don't unlock prefetch tree to read discontig buffers
> 
> Coverity spotted that it's possible for us to arrive at the loop
> below with num == 1, and then we decrement it to 0, and try to
> index bplist[num-1].
> 
> I think this was possible before the change, i.e. it's probably
> not a regression.
> 
> Fix this by not trying to shrink the window unless we have
> more than one buffer in the array.
> 
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---

FWIW, I'm not sure this can actually be hit; see below.

> 
> diff --git a/repair/prefetch.c b/repair/prefetch.c
> index 4595310..b6d4755 100644
> --- a/repair/prefetch.c
> +++ b/repair/prefetch.c
> @@ -505,7 +505,7 @@ pf_batch_read(
>  		first_off = LIBXFS_BBTOOFF64(XFS_BUF_ADDR(bplist[0]));
>  		last_off = LIBXFS_BBTOOFF64(XFS_BUF_ADDR(bplist[num-1])) +
>  			XFS_BUF_SIZE(bplist[num-1]);

Indexing bplist[num-1] after we do num-- is only a problem if num==1.

If num==1, then last_off - first_off == XFS_BUF_SIZE(bplist[0]) above.

> -		while (last_off - first_off > pf_max_bytes) {

so we can only go here if XFS_BUF_SIZE(bplist[0] > pf_max_bytes, and

        pf_max_bytes = sysconf(_SC_PAGE_SIZE) << 7;

for a 4k page that's 512k.

So unless XFS_BUF_SIZE(bplist[0]) > 512k, we won't run into trouble.

And I don't ... think that can happen, right?  So it's probably impossible
to hit; worth being defensive, but not critical.

That's my take anyhoo.

-Eric

> +		while (num > 1 && last_off - first_off > pf_max_bytes) {
>  			num--;
>  			last_off = LIBXFS_BBTOOFF64(XFS_BUF_ADDR(bplist[num-1])) +
>  				XFS_BUF_SIZE(bplist[num-1]);
> 
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