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[Qemu-devel] [PATCH] block/mirror: check backing in bdrv_mirror_top_refr


From: Vladimir Sementsov-Ogievskiy
Subject: [Qemu-devel] [PATCH] block/mirror: check backing in bdrv_mirror_top_refresh_filename
Date: Thu, 28 Sep 2017 15:03:00 +0300

Backing may be zero after failed bdrv_attach_child in
bdrv_set_backing_hd, which leads to SIGSEGV.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
---

Hi all.

We have faced into this SIGSEGV because of image locking:
trying to call qemu-img commit on locked image leads to the following:

    Program terminated with signal 11, Segmentation fault.
    #0  bdrv_mirror_top_refresh_filename (bs=0x5616df9a7400, 
opts=0x5616df268400)
        at block/mirror.c:1203
    1203        bdrv_refresh_filename(bs->backing->bs);
    
    (gdb) bt
    #0  bdrv_mirror_top_refresh_filename (bs=0x5616df9a7400, 
opts=0x5616df268400)
        at block/mirror.c:1203
    #1  0x00005616ddc3d35f in bdrv_refresh_filename (bs=0x5616df9a7400) at 
block.c:4739
    #2  0x00005616ddc3d672 in bdrv_set_backing_hd (address@hidden, 
        address@hidden, address@hidden)
        at block.c:2035
    #3  0x00005616ddc3dee3 in bdrv_append (address@hidden, 
        address@hidden, address@hidden)
        at block.c:3168
    #4  0x00005616ddc84e5f in mirror_start_job (
        address@hidden "commit", address@hidden, 
        address@hidden, address@hidden, 
        address@hidden, address@hidden, granularity=65536, 
        address@hidden, buf_size=16777216, address@hidden, 
        address@hidden, 
        address@hidden, 
        address@hidden, 
        address@hidden, address@hidden <common_block_job_cb>, 
        address@hidden, address@hidden, 
        address@hidden <commit_active_job_driver>, 
        address@hidden, address@hidden, 
        address@hidden, 
        address@hidden) at block/mirror.c:1314
    #5  0x00005616ddc87580 in commit_active_start (
        address@hidden "commit", address@hidden, 
        address@hidden, address@hidden, 
        address@hidden, address@hidden, 
        address@hidden, 
        address@hidden <common_block_job_cb>, 
        address@hidden, address@hidden, 
        address@hidden) at block/mirror.c:1463
    #6  0x00005616ddc33a68 in img_commit (argc=<optimized out>, argv=<optimized 
out>)
        at qemu-img.c:1013
    #7  0x00005616ddc2fa79 in main (argc=4, argv=0x7ffff7896e00) at 
qemu-img.c:4548


    (gdb) p bs->backing
    $2 = (BdrvChild *) 0x0
    (gdb) fr 2
    #2  0x00005616ddc3d672 in bdrv_set_backing_hd (address@hidden, 
        address@hidden, address@hidden)
        at block.c:2035
    2035        bdrv_refresh_filename(bs);
    (gdb) p *errp
    $4 = (Error *) 0x5616df1c2660
    (gdb) p **errp
    $5 = {msg = 0x5616df2554e0 "Failed to get \"write\" lock", 
      err_class = ERROR_CLASS_GENERIC_ERROR, 
      src = 0x5616ddd267fe "block/file-posix.c", 
      func = 0x5616ddd26fe0 <__func__.27999> "raw_check_lock_bytes", line = 
682, 
      hint = 0x5616df1fe520}


 block/mirror.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 6f5cb9f26c..351b80ca2c 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1073,6 +1073,11 @@ static int coroutine_fn 
bdrv_mirror_top_pdiscard(BlockDriverState *bs,
 
 static void bdrv_mirror_top_refresh_filename(BlockDriverState *bs, QDict *opts)
 {
+    if (bs->backing == NULL) {
+        /* we can be here after failed bdrv_attach_child in
+         * bdrv_set_backing_hd */
+        return;
+    }
     bdrv_refresh_filename(bs->backing->bs);
     pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
             bs->backing->bs->filename);
-- 
2.11.1




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