qemu segfaults when writing to very large qcow2 disk

[Richard W.M. Jones]

Create a ridiculously large qcow2 disk:

qemu-img create -f qcow2 test1.img $((2**63-513))

Attach it to a guest and try to use parted to partition it. This is easy with virt-rescue: you just do:

virt-rescue test1.img
><rescue> parted /dev/vda mklabel gpt
<-- bang! qemu segfaults here

The stack trace is:

Program received signal SIGSEGV, Segmentation fault.
0x0000000000434cac in get_cluster_table (bs=0x3193030, offset=
    9223372036854764544, new_l2_table=0x591e3c8, new_l2_offset=0x591e3c0,
    new_l2_index=0x591e408) at block/qcow2-cluster.c:506
506 l2_offset = s->l1_table[l1_index];
(gdb) bt
#0 0x0000000000434cac in get_cluster_table (bs=0x3193030, offset=
    9223372036854764544, new_l2_table=0x591e3c8, new_l2_offset=0x591e3c0,
    new_l2_index=0x591e408) at block/qcow2-cluster.c:506
#1 0x000000000043535b in qcow2_alloc_cluster_offset (bs=0x3193030, offset=
    9223372036854764544, n_start=106, n_end=126, num=0x591e4e8, m=0x591e470)
    at block/qcow2-cluster.c:719
#2 0x000000000043b8d4 in qcow2_co_writev (bs=0x3193030, sector_num=
    18014398509481962, remaining_sectors=20, qiov=0x4a81ee0)
    at block/qcow2.c:554
#3 0x0000000000428691 in bdrv_co_rw (opaque=0x38bad10) at block.c:2781
#4 0x000000000046e03a in coroutine_trampoline (i0=59487248, i1=0)
    at coroutine-ucontext.c:125
#5 0x00000034dc6471b0 in ?? () from /lib64/libc.so.6
#6 0x00007fff76cbb430 in ?? ()
#7 0x0000000000000000 in ?? ()

This is qemu from git (8f440cda08c6df574 from 2011-09-29)

[Richard Jones (rjones-redhat)]

Still happening in upstream qemu from git:

Program terminated with signal 11, Segmentation fault.
#0 0x00007f4f86c721a0 in get_cluster_table (bs=bs@entry=0x7f4f886e7880,
    offset=offset@entry=1152921504606834688,
    new_l2_table=new_l2_table@entry=0x7f4f8ad9a0b0,
    new_l2_index=new_l2_index@entry=0x7f4f8ad9a0ac)
    at block/qcow2-cluster.c:525
525 l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
Missing separate debuginfos, use: debuginfo-install SDL-1.2.15-3.fc18.x86_64 bluez-libs-4.101-6.fc18.x86_64 brlapi-0.5.6-12.fc18.x86_64 celt051-0.5.1.3-5.fc18.x86_64 ceph-devel-0.56.3-1.fc18.x86_64 ceph-libs-0.56.3-1.fc18.x86_64 cryptopp-5.6.1-8.fc18.x86_64 cyrus-sasl-lib-2.1.25-2.fc18.x86_64 leveldb-1.7.0-4.fc18.x86_64 libfdt-1.3.0-5.fc18.x86_64 libseccomp-1.0.1-0.fc18.x86_64 libselinux-2.1.12-7.3.fc18.x86_64 libusbx-1.0.14-1.fc18.x86_64 snappy-1.0.5-2.fc18.x86_64 spice-server-0.12.2-3.fc18.x86_64 usbredir-0.6-1.fc18.x86_64 xen-libs-4.2.1-9.fc18.x86_64
(gdb) bt
#0 0x00007f4f86c721a0 in get_cluster_table (bs=bs@entry=0x7f4f886e7880,
    offset=offset@entry=1152921504606834688, new_l2_table=new_l2_table@entry=
    0x7f4f8ad9a0b0, new_l2_index=new_l2_index@entry=0x7f4f8ad9a0ac)
    at block/qcow2-cluster.c:525
#1 0x00007f4f86c72fa3 in handle_copied (m=<optimized out>,
    bytes=<synthetic pointer>, host_offset=<synthetic pointer>, guest_offset=
    1152921504606834688, bs=0x7f4f886e7880) at block/qcow2-cluster.c:873
#2 qcow2_alloc_cluster_offset (bs=bs@entry=0x7f4f886e7880,
    offset=<optimized out>, offset@entry=1152921504606834688,
    n_start=n_start@entry=104, n_end=<optimized out>, num=num@entry=
    0x7f4f8ad9a14c, host_offset=host_offset@entry=0x7f4f8ad9a150, m=m@entry=
    0x7f4f8ad9a158) at block/qcow2-cluster.c:1217
#3 0x00007f4f86c773b3 in qcow2_co_writev (bs=0x7f4f886e7880, sector_num=
    2251799813685224, remaining_sectors=24, qiov=0x7f4f88d88f98)
    at block/qcow2.c:819
#4 0x00007f4f86c638d5 in bdrv_co_do_writev (bs=0x7f4f886e7880, sector_num=
    2251799813685224, nb_sectors=24, qiov=0x7f4f88d88f98, flags=flags@entry=
    (unknown: 0)) at block.c:2625
#5 0x00007f4f86c63a38 in bdrv_co_do_rw (opaque=0x7f4f88e16160) at block.c:4139
#6 0x00007f4f86c9a19a in coroutine_trampoline (i0=<optimized out>,
    i1=<optimized out>) at coroutine-ucontext.c:118
#7 0x00007f4f7fd776c0 in ?? () from /lib64/libc.so.6
#8 0x00007fff125e6620 in ?? ()
#9 0x0000000000000000 in ?? ()

[Richard Jones (rjones-redhat)]

Simple reproducer using only qemu tools:

$ qemu-img create -f qcow2 huge.qcow2 $((1024*1024))T
Formatting 'huge.qcow2', fmt=qcow2 size=1152921504606846976 encryption=off cluster_size=65536 lazy_refcounts=off
$ qemu-io /tmp/huge.qcow2 -c "write $((1024*1024*1024*1024*1024*1024 - 1024)) 512"
Segmentation fault

[Thomas Huth (th-huth)]

Fix has been included here:
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=2cf7cfa1cde6672b8a35b
... so I think it should be OK to close this ticket now.