| Message ID | 20210511135257.878743-3-bfoster@redhat.com (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Headers | show |
| Series | xfs: fix buffer use after free on unpin abort | expand |
On Tue, May 11, 2021 at 09:52:57AM -0400, Brian Foster wrote: > This code goes back to a time when transaction commits wrote > directly to iclogs. The associated log items were pinned, written to > the log, and then "uncommitted" if some part of the log write had > failed. This uncommit sequence called an ->iop_unpin_remove() > handler that was eventually folded into ->iop_unpin() via the remove > parameter. The log subsystem has since changed significantly in that > transactions commit to the CIL instead of direct to iclogs, though > log items must still be aborted in the event of an eventual log I/O > error. However, the context for a log item abort is now asynchronous > from transaction commit, which means the committing transaction has > been freed by this point in time and the transaction uncommit > sequence of events is no longer relevant. > > Further, since stale buffers remain locked at transaction commit > through unpin, we can be certain that the buffer is not associated > with any transaction when the unpin callback executes. Remove this > unused hunk of code and replace it with an assertion that the buffer > is disassociated from transaction context. > > Signed-off-by: Brian Foster <bfoster@redhat.com> <nod> my brain kinda hurts now, but I have a vague recollection of wondering how you could get a stale buffer that was also being removed and not being able to figure out how one might stumble into this chunk of code. :) Reviewed-by: Darrick J. Wong <djwong@kernel.org> --D > --- > fs/xfs/xfs_buf_item.c | 20 +------------------- > 1 file changed, 1 insertion(+), 19 deletions(-) > > diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c > index 7ff31788512b..634abf30b5bc 100644 > --- a/fs/xfs/xfs_buf_item.c > +++ b/fs/xfs/xfs_buf_item.c > @@ -517,28 +517,10 @@ xfs_buf_item_unpin( > ASSERT(xfs_buf_islocked(bp)); > ASSERT(bp->b_flags & XBF_STALE); > ASSERT(bip->__bli_format.blf_flags & XFS_BLF_CANCEL); > + ASSERT(list_empty(&lip->li_trans) && !bp->b_transp); > > trace_xfs_buf_item_unpin_stale(bip); > > - if (remove) { > - /* > - * If we are in a transaction context, we have to > - * remove the log item from the transaction as we are > - * about to release our reference to the buffer. If we > - * don't, the unlock that occurs later in > - * xfs_trans_uncommit() will try to reference the > - * buffer which we no longer have a hold on. > - */ > - if (!list_empty(&lip->li_trans)) > - xfs_trans_del_item(lip); > - > - /* > - * Since the transaction no longer refers to the buffer, > - * the buffer should no longer refer to the transaction. > - */ > - bp->b_transp = NULL; > - } > - > /* > * If we get called here because of an IO error, we may or may > * not have the item on the AIL. xfs_trans_ail_delete() will > -- > 2.26.3 >
> + ASSERT(list_empty(&lip->li_trans) && !bp->b_transp);
Nit: Two separate ASSERTS are generally better than one with two
conditions and a "&&", so that when the assert triggers it shows which
condition caused it.
On Wed, May 12, 2021 at 01:00:25PM +0100, Christoph Hellwig wrote: > > + ASSERT(list_empty(&lip->li_trans) && !bp->b_transp); > > Nit: Two separate ASSERTS are generally better than one with two > conditions and a "&&", so that when the assert triggers it shows which > condition caused it. > In this case both checks pretty much mean the same thing so I don't see much added value, but I don't mind splitting it.. Brian
diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c index 7ff31788512b..634abf30b5bc 100644 --- a/fs/xfs/xfs_buf_item.c +++ b/fs/xfs/xfs_buf_item.c @@ -517,28 +517,10 @@ xfs_buf_item_unpin( ASSERT(xfs_buf_islocked(bp)); ASSERT(bp->b_flags & XBF_STALE); ASSERT(bip->__bli_format.blf_flags & XFS_BLF_CANCEL); + ASSERT(list_empty(&lip->li_trans) && !bp->b_transp); trace_xfs_buf_item_unpin_stale(bip); - if (remove) { - /* - * If we are in a transaction context, we have to - * remove the log item from the transaction as we are - * about to release our reference to the buffer. If we - * don't, the unlock that occurs later in - * xfs_trans_uncommit() will try to reference the - * buffer which we no longer have a hold on. - */ - if (!list_empty(&lip->li_trans)) - xfs_trans_del_item(lip); - - /* - * Since the transaction no longer refers to the buffer, - * the buffer should no longer refer to the transaction. - */ - bp->b_transp = NULL; - } - /* * If we get called here because of an IO error, we may or may * not have the item on the AIL. xfs_trans_ail_delete() will
This code goes back to a time when transaction commits wrote directly to iclogs. The associated log items were pinned, written to the log, and then "uncommitted" if some part of the log write had failed. This uncommit sequence called an ->iop_unpin_remove() handler that was eventually folded into ->iop_unpin() via the remove parameter. The log subsystem has since changed significantly in that transactions commit to the CIL instead of direct to iclogs, though log items must still be aborted in the event of an eventual log I/O error. However, the context for a log item abort is now asynchronous from transaction commit, which means the committing transaction has been freed by this point in time and the transaction uncommit sequence of events is no longer relevant. Further, since stale buffers remain locked at transaction commit through unpin, we can be certain that the buffer is not associated with any transaction when the unpin callback executes. Remove this unused hunk of code and replace it with an assertion that the buffer is disassociated from transaction context. Signed-off-by: Brian Foster <bfoster@redhat.com> --- fs/xfs/xfs_buf_item.c | 20 +------------------- 1 file changed, 1 insertion(+), 19 deletions(-)