On Wed, Jan 25, 2017 at 06:20:54PM -0600, Eric Sandeen wrote:
> On 1/25/17 1:58 PM, Darrick J. Wong wrote:
> > In a bmapx call, bmv_count is the total size of the array, including the
> > zeroth element that userspace uses to supply the search key. The output
> > array starts at offset 1 so that we can set up the user for the next
> > invocationn. Therefore, we must ensure that cur_ext (which indexes the
> > output array) never exceeds bmv_count-1, not bmv_count. Failure to do
> > this causes heap corruption in bmapx callers such as xfs_io and
> > xfs_scrub when the formatter overflows the array. xfs/348 can reproduce
> > this problem.
>
> Ok, well, this worked until f86f40379 :) nexleft (number extents
> left) used to keep an accurate count and break when done. That
> loop condition is getting a bit insane, but *shrug* for now.
>
> It would be worth making the commit log more clear that the problem
> currently exists only for shared extents (right?)
>
>
> > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > ---
> > fs/xfs/xfs_bmap_util.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> > index b9abce5..883e55f 100644
> > --- a/fs/xfs/xfs_bmap_util.c
> > +++ b/fs/xfs/xfs_bmap_util.c
> > @@ -698,7 +698,7 @@ xfs_getbmap(
> > ASSERT(nmap <= subnex);
> >
> > for (i = 0; i < nmap && nexleft && bmv->bmv_length &&
> > - cur_ext < bmv->bmv_count; i++) {
> > + cur_ext < bmv->bmv_count - 1; i++) {
>
> ok, so the cur_ext vs. bmv_count test was added w/ the above commit.
>
> > out[cur_ext].bmv_oflags = 0;
> > if (map[i].br_state == XFS_EXT_UNWRITTEN)
> > out[cur_ext].bmv_oflags |= BMV_OF_PREALLOC;
> > @@ -769,7 +769,7 @@ xfs_getbmap(
> > cur_ext++;
> > }
> > } while (nmap && nexleft && bmv->bmv_length &&
> > - cur_ext < bmv->bmv_count);
> > + cur_ext < bmv->bmv_count - 1);
>
> at the bottom of the loop we have:
>
> if (inject_map.br_startblock != NULLFSBLOCK) {
> map[i] = inject_map;
> i--;
> } else
> nexleft--;
> bmv->bmv_entries++;
>
> nexleft used to be our "should we stop" counter, if we have extents
> left, keep going, if not, stop. It was initialized in a roundabout
> way from bmv->bmv_count - 1;, via the "nex" variable.
>
> So nexleft started out as the maximum times through the loop,
> but now you're resetting i, advancing the cur_ext anyway,
> going through the loop again, but not decrementing nexleft.
Ah yes, I was trying to gate the loop on the output index variable
cur_ext, but your suggestion of using the slots-remaining counter is
simpler. I will resubmit the patch.
--D
>
> If you revert the loop condition to:
>
> for (i = 0; i < nmap && nexleft && bmv->bmv_length; i++) {
>
> and set the end of the loop nextent handling to:
>
> nexleft--;
> if (inject_map.br_startblock != NULLFSBLOCK) {
> map[i] = inject_map;
> i--;
> }
>
> does that not fix it? i.e.:
>
>
>
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index b9abce5..95dd839 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -697,8 +697,7 @@
> goto out_free_map;
> ASSERT(nmap <= subnex);
>
> - for (i = 0; i < nmap && nexleft && bmv->bmv_length &&
> - cur_ext < bmv->bmv_count; i++) {
> + for (i = 0; i < nmap && nexleft && bmv->bmv_length; i++) {
> out[cur_ext].bmv_oflags = 0;
> if (map[i].br_state == XFS_EXT_UNWRITTEN)
> out[cur_ext].bmv_oflags |= BMV_OF_PREALLOC;
> @@ -760,11 +759,11 @@
> continue;
> }
>
> + nexleft--;
> if (inject_map.br_startblock != NULLFSBLOCK) {
> map[i] = inject_map;
> i--;
> - } else
> - nexleft--;
> + }
> bmv->bmv_entries++;
> cur_ext++;
> }
>
>
>
>
> > out_free_map:
> > kmem_free(map);
> > --
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