Re: [Qemu-devel] [PATCH] ide: fix double free

[ChenLiang <chenliang88@huawei.com>]

On 2014/7/2 20:19, Paolo Bonzini wrote:

> Il 02/07/2014 13:57, ChenLiang ha scritto:
>>>> Hmm, dbs->in_cancel will be true always. Although this will avoid freeing dbs by dma_comlete.
>>>> But it maybe a mistake.
>>>
>>> This was on purpose; I'm doing the free myself in dma_aio_cancel, so I wanted to avoid the qemu_aio_release from dma_complete.  This was in case of a recursive call to dma_complete.  But I don't see how that recursive call could happen outside the "if (dbs->acb)"; and inside the "if" the protection is there already.
>>>
>>> Can you gather the backtraces for _both_ calls to qemu_aio_release, rather than just the second?
>>
>> (gdb) bt
>> #0  qemu_aio_release (p=0x7f44788d1290) at block.c:4260
>> #1  0x00007f4477494e5e in dma_complete (dbs=0x7f44788d1290, ret=0) at dma-helpers.c:135
>> #2  0x00007f44774952c2 in dma_aio_cancel (acb=0x7f44788d1290) at dma-helpers.c:195
>> #3  0x00007f447744825b in bdrv_aio_cancel (acb=0x7f44788d1290) at block.c:3848
>> #4  0x00007f4477513911 in ide_bus_reset (bus=0x7f44785f1bd8) at hw/ide/core.c:1957
>> #5  0x00007f4477516b3c in piix3_reset (opaque=0x7f44785f1530) at hw/ide/piix.c:113
>> #6  0x00007f4477647b9f in qemu_devices_reset () at vl.c:2131
>> #7  0x00007f4477647c0f in qemu_system_reset (report=true) at vl.c:2140
>> #8  0x00007f4477648127 in main_loop_should_exit () at vl.c:2274
>> #9  0x00007f447764823a in main_loop () at vl.c:2323
>> #10 0x00007f447764f6da in main (argc=57, argv=0x7fff5d194378, envp=0x7fff5d194548) at vl.c:4803
> 
> And the second is
> 
> #7  0x00007f3cb525de5e in dma_complete (dbs=0x7f3cb63f3220, ret=0) at dma-helpers.c:135
> #8  0x00007f3cb525df3d in dma_bdrv_cb (opaque=0x7f3cb63f3220, ret=0) at dma-helpers.c:152
> #9  0x00007f3cb5212102 in bdrv_co_em_bh (opaque=0x7f3cb6398980) at block.c:4127
> #10 0x00007f3cb51f6cef in aio_bh_poll (ctx=0x7f3cb622a8f0) at async.c:70
> #11 0x00007f3cb51f695a in aio_poll (ctx=0x7f3cb622a8f0, blocking=false) at aio-posix.c:185
> #12 0x00007f3cb51f7056 in aio_ctx_dispatch (source=0x7f3cb622a8f0, callback=0x0, user_data=0x0)
>     at async.c:167
> #13 0x00007f3cb48b969a in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
> 
> This explains why my patch "fixes" the bug.  It turns a double free
> into a dangling pointer: the second call now sees in_cancel == true
> and skips the free.
> 
> The second call should have happened within dma_aio_cancel's call to
> bdrv_aio_cancel.  This is the real bug.
> 
> What is your version of QEMU?  I cannot see any where bdrv_co_em_bh is
> at line 4127 or bdrv_aio_cancel is at line 3848.  Can you reproduce it
> with qemu.git master?
> 
> Paolo
> 
> .
> 


qemu master branch bt:

Program received signal SIGABRT, Aborted.
0x00007fd548355b55 in raise () from /lib64/libc.so.6
(gdb) bt
#0  0x00007fd548355b55 in raise () from /lib64/libc.so.6
#1  0x00007fd548357131 in abort () from /lib64/libc.so.6
#2  0x00007fd548393e0f in __libc_message () from /lib64/libc.so.6
#3  0x00007fd548399618 in malloc_printerr () from /lib64/libc.so.6
#4  0x00007fd54b15e80e in free_and_trace (mem=0x7fd54beb2230) at vl.c:2815
#5  0x00007fd54b3453cd in qemu_aio_release (p=0x7fd54beb2230) at block.c:4813
#6  0x00007fd54b15717d in dma_complete (dbs=0x7fd54beb2230, ret=0) at dma-helpers.c:132
#7  0x00007fd54b157253 in dma_bdrv_cb (opaque=0x7fd54beb2230, ret=0) at dma-helpers.c:148
#8  0x00007fd54b344db8 in bdrv_co_em_bh (opaque=0x7fd54bea4b30) at block.c:4676
#9  0x00007fd54b335a72 in aio_bh_poll (ctx=0x7fd54bcec990) at async.c:81
#10 0x00007fd54b34b1b4 in aio_poll (ctx=0x7fd54bcec990, blocking=false) at aio-posix.c:188
#11 0x00007fd54b335ee0 in aio_ctx_dispatch (source=0x7fd54bcec990, callback=0x0, user_data=0x0) at async.c:211
#12 0x00007fd549e3669a in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
#13 0x00007fd54b348c45 in glib_pollfds_poll () at main-loop.c:190
#14 0x00007fd54b348d3d in os_host_main_loop_wait (timeout=0) at main-loop.c:235
#15 0x00007fd54b348e2f in main_loop_wait (nonblocking=0) at main-loop.c:484
#16 0x00007fd54b15b0f8 in main_loop () at vl.c:2007
#17 0x00007fd54b162a35 in main (argc=57, argv=0x7fff152720a8, envp=0x7fff15272278) at vl.c:4526

(gdb) bt
#0  qemu_aio_release (p=0x7f86420ebec0) at block.c:4811
#1  0x00007f86412b617d in dma_complete (dbs=0x7f86420ebec0, ret=0) at dma-helpers.c:132
#2  0x00007f86412b65ab in dma_aio_cancel (acb=0x7f86420ebec0) at dma-helpers.c:192
#3  0x00007f86414a3996 in bdrv_aio_cancel (acb=0x7f86420ebec0) at block.c:4559
#4  0x00007f86413906af in ide_bus_reset (bus=0x7f8641fe3a20) at hw/ide/core.c:2056
#5  0x00007f86413967d6 in piix3_reset (opaque=0x7f8641fe32a0) at hw/ide/piix.c:114
#6  0x00007f86412b9a37 in qemu_devices_reset () at vl.c:1829
#7  0x00007f86412b9aef in qemu_system_reset (report=true) at vl.c:1842
#8  0x00007f86412b9fe2 in main_loop_should_exit () at vl.c:1971
#9  0x00007f86412ba100 in main_loop () at vl.c:2011
#10 0x00007f86412c1a35 in main (argc=57, argv=0x7fff2e827d38, envp=0x7fff2e827f08) at vl.c:4526


BTW, is it better to rename dbs->in_cancel to dbs->canceled ?

Best regards
Chenliang