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Re: [PATCH] xfs: reset buffer pointers before freeing them

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: reset buffer pointers before freeing them
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 12 Apr 2011 23:45:43 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20110412110028.GA31057@dastard>
References: <1302491405-2290-1-git-send-email-david@xxxxxxxxxxxxx> <20110412000939.GC29358@xxxxxxxxxxxxx> <20110412110028.GA31057@dastard>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue, Apr 12, 2011 at 09:00:28PM +1000, Dave Chinner wrote:
> On Mon, Apr 11, 2011 at 08:09:39PM -0400, Christoph Hellwig wrote:
> > >                   error = xlog_bread_noalign(log, ealign, sectbb, bp);
> > > -                 if (error)
> > > -                         break;
> > >  
> > > -                 error = XFS_BUF_SET_PTR(bp, offset, bufblks);
> > > +                 /* must reset buffer pointer even on error */
> > > +                 error2 = XFS_BUF_SET_PTR(bp, offset, bufblks);
> > 
> > This seems to be incorrect both in the original and your version.
> > From all that I can see XFS_BUF_SET_PTR wants a byte count while bufblks
> > is a sector count, e.g. this should be:
> > 
> >                     error2 = XFS_BUF_SET_PTR(bp, offset, BBTOB(bufblks);
> 
> Yes, you are right. I missed that.
> 
> > While at I think this mess with the buffer virtual mapping, read into it
> > and set it back mess needs to go into a single helper instead of beeing
> > duplicated three times.  By having named arguments with proper types
> > bugs like the existing one above should also be much more obvious to
> > spot.
> 
> Ok, I'll try to come up with something sane.

Something like this?


xfs: reset buffer pointers before freeing them

From: Dave Chinner <dchinner@xxxxxxxxxx>

When we free a vmapped buffer, we need to ensure the vmap address
and length we free is the same as when it was allocated. In various
places in the log code we change the memory the buffer is pointing
to before issuing IO, but we never reset the buffer to point back to
it's original memory (or no memory, if that is the case for the
buffer).

As a result, when we free the buffer it points to memory that is
owned by something else and attempts to unmap and free it. Because
the range does not match any known mapped range, it can trigger
BUG_ON() traps in the vmap code, and potentially corrupt the vmap
area tracking.

Fix this by always resetting these buffers to their original state
before freeing them.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/linux-2.6/xfs_buf.c |   21 ++++++++++++
 fs/xfs/linux-2.6/xfs_buf.h |    1 +
 fs/xfs/xfs_log.c           |    8 ++++-
 fs/xfs/xfs_log_recover.c   |   79 +++++++++++++++++++++++---------------------
 4 files changed, 70 insertions(+), 39 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index 9ef9ed2..ecf0db6 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -709,6 +709,27 @@ xfs_buf_get_empty(
        return bp;
 }
 
+/*
+ * Return a buffer allocated as an empty buffer and associated to external
+ * memory via xfs_buf_associate_memory() back to it's empty state.
+ */
+void
+xfs_buf_set_empty(
+       struct xfs_buf          *bp,
+       size_t                  len)
+{
+       if (bp->b_pages)
+               _xfs_buf_free_pages(bp);
+
+       bp->b_pages = NULL;
+       bp->b_page_count = 0;
+       bp->b_addr = NULL;
+       bp->b_file_offset = 0;
+       bp->b_buffer_length = bp->b_count_desired = len;
+       bp->b_bn = XFS_BUF_DADDR_NULL;
+       bp->b_flags &= ~XBF_MAPPED;
+}
+
 static inline struct page *
 mem_to_page(
        void                    *addr)
diff --git a/fs/xfs/linux-2.6/xfs_buf.h b/fs/xfs/linux-2.6/xfs_buf.h
index a9a1c45..50a7d5f 100644
--- a/fs/xfs/linux-2.6/xfs_buf.h
+++ b/fs/xfs/linux-2.6/xfs_buf.h
@@ -178,6 +178,7 @@ extern xfs_buf_t *xfs_buf_read(xfs_buftarg_t *, xfs_off_t, 
size_t,
                                xfs_buf_flags_t);
 
 extern xfs_buf_t *xfs_buf_get_empty(size_t, xfs_buftarg_t *);
+extern void xfs_buf_set_empty(struct xfs_buf *bp, size_t len);
 extern xfs_buf_t *xfs_buf_get_uncached(struct xfs_buftarg *, size_t, int);
 extern int xfs_buf_associate_memory(xfs_buf_t *, void *, size_t);
 extern void xfs_buf_hold(xfs_buf_t *);
diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index b612ce4..3850a91 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1449,6 +1449,13 @@ xlog_dealloc_log(xlog_t *log)
 
        xlog_cil_destroy(log);
 
+       /*
+        * always need to ensure that the extra buffer does not point to memory
+        * owned by another log buffer before we free it.
+        */
+       xfs_buf_set_empty(log->l_xbuf, log->l_iclog_size);
+       xfs_buf_free(log->l_xbuf);
+
        iclog = log->l_iclog;
        for (i=0; i<log->l_iclog_bufs; i++) {
                xfs_buf_free(iclog->ic_bp);
@@ -1458,7 +1465,6 @@ xlog_dealloc_log(xlog_t *log)
        }
        spinlock_destroy(&log->l_icloglock);
 
-       xfs_buf_free(log->l_xbuf);
        log->l_mp->m_log = NULL;
        kmem_free(log);
 }      /* xlog_dealloc_log */
diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 5cc464a..fe7fbc8 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -205,6 +205,36 @@ xlog_bread(
 }
 
 /*
+ * Read at an offset into the buffer. Returns with the buffer in it's original
+ * state regardless of the result of the read.
+ */
+STATIC int
+xlog_bread_offset(
+       xlog_t          *log,
+       int             buf_offset,     /* offset into buffer */
+       int             buf_blks,       /* original buffer size */
+       xfs_daddr_t     blk_no,         /* block to read from */
+       int             nbblks,         /* blocks to read */
+       xfs_buf_t       *bp,
+       xfs_caddr_t     offset)
+{
+       xfs_caddr_t     orig_offset = XFS_BUF_PTR(bp);
+       int             error, error2;
+
+       error = XFS_BUF_SET_PTR(bp, offset + BBTOB(buf_offset), BBTOB(nbblks));
+       if (error)
+               return error;
+
+       error = xlog_bread_noalign(log, blk_no, nbblks, bp);
+
+       /* must reset buffer pointer even on error */
+       error2 = XFS_BUF_SET_PTR(bp, orig_offset, BBTOB(buf_blks));
+       if (error)
+               return error;
+       return error2;
+}
+
+/*
  * Write out the buffer at the given block for the given number of blocks.
  * The buffer is kept locked across the write and is returned locked.
  * This can only be used for synchronous log writes.
@@ -1229,20 +1259,12 @@ xlog_write_log_records(
                 */
                ealign = round_down(end_block, sectbb);
                if (j == 0 && (start_block + endcount > ealign)) {
-                       offset = XFS_BUF_PTR(bp);
-                       balign = BBTOB(ealign - start_block);
-                       error = XFS_BUF_SET_PTR(bp, offset + balign,
-                                               BBTOB(sectbb));
+                       balign = ealign - start_block;
+                       error = xlog_bread_offset(log, balign, bufblks, ealign,
+                                               sectbb, bp, XFS_BUF_PTR(bp));
                        if (error)
                                break;
 
-                       error = xlog_bread_noalign(log, ealign, sectbb, bp);
-                       if (error)
-                               break;
-
-                       error = XFS_BUF_SET_PTR(bp, offset, bufblks);
-                       if (error)
-                               break;
                }
 
                offset = xlog_align(log, start_block, endcount, bp);
@@ -3322,7 +3344,7 @@ xlog_do_recovery_pass(
        xfs_caddr_t             offset;
        xfs_buf_t               *hbp, *dbp;
        int                     error = 0, h_size;
-       int                     bblks, split_bblks;
+       int                     bblks, split_bblks, wrapped_bblks;
        int                     hblks, split_hblks, wrapped_hblks;
        struct hlist_head       rhash[XLOG_RHASH_SIZE];
 
@@ -3448,19 +3470,9 @@ xlog_do_recovery_pass(
                                 *   - order is important.
                                 */
                                wrapped_hblks = hblks - split_hblks;
-                               error = XFS_BUF_SET_PTR(hbp,
-                                               offset + BBTOB(split_hblks),
-                                               BBTOB(hblks - split_hblks));
-                               if (error)
-                                       goto bread_err2;
-
-                               error = xlog_bread_noalign(log, 0,
-                                                          wrapped_hblks, hbp);
-                               if (error)
-                                       goto bread_err2;
-
-                               error = XFS_BUF_SET_PTR(hbp, offset,
-                                                       BBTOB(hblks));
+                               error = xlog_bread_offset(log, split_hblks,
+                                                       hblks, 0, wrapped_hblks,
+                                                       hbp, offset);
                                if (error)
                                        goto bread_err2;
                        }
@@ -3511,19 +3523,10 @@ xlog_do_recovery_pass(
                                 *   _first_, then the log start (LR header end)
                                 *   - order is important.
                                 */
-                               error = XFS_BUF_SET_PTR(dbp,
-                                               offset + BBTOB(split_bblks),
-                                               BBTOB(bblks - split_bblks));
-                               if (error)
-                                       goto bread_err2;
-
-                               error = xlog_bread_noalign(log, wrapped_hblks,
-                                               bblks - split_bblks,
-                                               dbp);
-                               if (error)
-                                       goto bread_err2;
-
-                               error = XFS_BUF_SET_PTR(dbp, offset, h_size);
+                               wrapped_bblks = bblks - split_bblks;
+                               error = xlog_bread_offset(log, split_bblks,
+                                               BTOBB(h_size), 0,
+                                               wrapped_bblks, dbp, offset);
                                if (error)
                                        goto bread_err2;
                        }

Cheers,

Dave
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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