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[Qemu-devel] [PATCH 1/2] block/file-posix: Unaligned O_DIRECT block-stat

From: Max Reitz
Subject: [Qemu-devel] [PATCH 1/2] block/file-posix: Unaligned O_DIRECT block-status
Date: Tue, 14 May 2019 23:42:29 +0200

Currently, qemu crashes whenever someone queries the block status of an
unaligned image tail of an O_DIRECT image:
$ echo > foo
$ qemu-img map --image-opts driver=file,filename=foo,cache.direct=on
Offset          Length          Mapped to       File
qemu-img: block/io.c:2093: bdrv_co_block_status: Assertion `*pnum &&
QEMU_IS_ALIGNED(*pnum, align) && align > offset - aligned_offset'

This is because bdrv_co_block_status() checks that the result returned
by the driver's implementation is aligned to the request_alignment, but
file-posix can fail to do so, which is actually mentioned in a comment
there: "[...] possibly including a partial sector at EOF".

Fix this by rounding up those partial sectors.

There are two possible alternative fixes:
(1) We could refuse to open unaligned image files with O_DIRECT
    altogether.  That sounds reasonable until you realize that qcow2
    does necessarily not fill up its metadata clusters, and that nobody
    runs qemu-img create with O_DIRECT.  Therefore, unpreallocated qcow2
    files usually have an unaligned image tail.

(2) bdrv_co_block_status() could ignore unaligned tails.  It actually
    throws away everything past the EOF already, so that sounds
    Unfortunately, the block layer knows file lengths only with a
    granularity of BDRV_SECTOR_SIZE, so bdrv_co_block_status() usually
    would have to guess whether its file length information is inexact
    or whether the driver is broken.

Fixing what raw_co_block_status() returns is the safest thing to do.

There seems to be no other block driver that sets request_alignment and
does not make sure that it always returns aligned values.

Cc: address@hidden
Signed-off-by: Max Reitz <address@hidden>
 block/file-posix.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/block/file-posix.c b/block/file-posix.c
index e09e15bbf8..f489a5420c 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2488,6 +2488,9 @@ static int coroutine_fn 
raw_co_block_status(BlockDriverState *bs,
     off_t data = 0, hole = 0;
     int ret;
+    assert(QEMU_IS_ALIGNED(offset, bs->bl.request_alignment) &&
+           QEMU_IS_ALIGNED(bytes, bs->bl.request_alignment));
     ret = fd_open(bs);
     if (ret < 0) {
         return ret;
@@ -2513,6 +2516,20 @@ static int coroutine_fn 
raw_co_block_status(BlockDriverState *bs,
         /* On a data extent, compute bytes to the end of the extent,
          * possibly including a partial sector at EOF. */
         *pnum = MIN(bytes, hole - offset);
+        /*
+         * We are not allowed to return partial sectors, though, so
+         * round up if necessary.
+         */
+        if (!QEMU_IS_ALIGNED(*pnum, bs->bl.request_alignment)) {
+            int64_t file_length = raw_getlength(bs);
+            if (file_length > 0) {
+                /* Ignore errors, this is just a safeguard */
+                assert(hole == file_length);
+            }
+            *pnum = ROUND_UP(*pnum, bs->bl.request_alignment);
+        }
         ret = BDRV_BLOCK_DATA;
     } else {
         /* On a hole, compute bytes to the beginning of the next extent.  */

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