Re: [Qemu-devel] [PATCH v2] qemu-coroutine: segfault when restarting co_queue

[Bin Wu]

On 2015/2/9 22:48, Stefan Hajnoczi wrote:
> On Mon, Feb 09, 2015 at 02:50:39PM +0800, Bin Wu wrote:
>> From: Bin Wu <address@hidden>
>>
>> We tested VMs migration with their disk images by drive_mirror. With
>> migration, two VMs copyed large files between each other. During the
>> test, a segfault occured. The stack was as follow:
>>
>> (gdb) bt
>> qemu-coroutine-lock.c:66
>> to=0x7fa5a1798648) at qemu-coroutine.c:97
>> request=0x7fa28c2ffa10, reply=0x7fa28c2ffa30, qiov=0x0, offset=0) at
>> block/nbd-client.c:165
>> sector_num=8552704, nb_sectors=2040, qiov=0x7fa5a1757468, offset=0) at
>> block/nbd-client.c:262
>> sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468) at
>> block/nbd-client.c:296
>> nb_sectors=2048, qiov=0x7fa5a1757468) at block/nbd.c:291
>> req=0x7fa28c2ffbb0, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
>> flags=0) at block.c:3321
>> offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
>> block.c:3447
>> sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 
>> 0)) at
>> block.c:3471
>> nb_sectors=2048, qiov=0x7fa5a1757468) at block.c:3480
>> nb_sectors=2048, qiov=0x7fa5a1757468) at block/raw_bsd.c:62
>> req=0x7fa28c2ffe30, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
>> flags=0) at block.c:3321
>> offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
>> block.c:3447
>> sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 
>> 0)) at
>> block.c:3471
>> coroutine-ucontext.c:121
> 
> This backtrace is incomplete.  Where are the function names?  The
> parameter lists appear incomplete too.
> 

I put the stack in the git log, so some lines are missed:
(gdb) bt
#0  0x00007fa5a0c63fc5 in qemu_co_queue_run_restart (co=0x7fa5a1798648) at
qemu-coroutine-lock.c:66
#1  0x00007fa5a0c63bed in coroutine_swap (from=0x7fa5a178f160,
to=0x7fa5a1798648) at qemu-coroutine.c:97
#2  0x00007fa5a0c63dbf in qemu_coroutine_yield () at qemu-coroutine.c:140
#3  0x00007fa5a0c9e474 in nbd_co_receive_reply (s=0x7fa5a1a3cfd0,
request=0x7fa28c2ffa10, reply=0x7fa28c2ffa30, qiov=0x0, offset=0) at
block/nbd-client.c:165
#4  0x00007fa5a0c9e8b5 in nbd_co_writev_1 (client=0x7fa5a1a3cfd0,
sector_num=8552704, nb_sectors=2040, qiov=0x7fa5a1757468, offset=0) at
block/nbd-client.c:262
#5  0x00007fa5a0c9e9dd in nbd_client_session_co_writev (client=0x7fa5a1a3cfd0,
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468) at
block/nbd-client.c:296
#6  0x00007fa5a0c9dda1 in nbd_co_writev (bs=0x7fa5a198fcb0, sector_num=8552704,
nb_sectors=2048, qiov=0x7fa5a1757468) at block/nbd.c:291
#7  0x00007fa5a0c509a4 in bdrv_aligned_pwritev (bs=0x7fa5a198fcb0,
req=0x7fa28c2ffbb0, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
flags=0) at block.c:3321
#8  0x00007fa5a0c50f3f in bdrv_co_do_pwritev (bs=0x7fa5a198fcb0,
offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3447
#9  0x00007fa5a0c51007 in bdrv_co_do_writev (bs=0x7fa5a198fcb0,
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3471
#10 0x00007fa5a0c51074 in bdrv_co_writev (bs=0x7fa5a198fcb0, sector_num=8552704,
nb_sectors=2048, qiov=0x7fa5a1757468) at block.c:3480
#11 0x00007fa5a0c652ec in raw_co_writev (bs=0x7fa5a198c110, sector_num=8552704,
nb_sectors=2048, qiov=0x7fa5a1757468) at block/raw_bsd.c:62
#12 0x00007fa5a0c509a4 in bdrv_aligned_pwritev (bs=0x7fa5a198c110,
req=0x7fa28c2ffe30, offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468,
flags=0) at block.c:3321
#13 0x00007fa5a0c50f3f in bdrv_co_do_pwritev (bs=0x7fa5a198c110,
offset=4378984448, bytes=1048576, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3447
#14 0x00007fa5a0c51007 in bdrv_co_do_writev (bs=0x7fa5a198c110,
sector_num=8552704, nb_sectors=2048, qiov=0x7fa5a1757468, flags=(unknown: 0)) at
block.c:3471
#15 0x00007fa5a0c542b3 in bdrv_co_do_rw (opaque=0x7fa5a17a0000) at block.c:4706
#16 0x00007fa5a0c64e6e in coroutine_trampoline (i0=-1585909408, i1=32677) at
coroutine-ucontext.c:121
#17 0x00007fa59dc5aa50 in __correctly_grouped_prefixwc () from /lib64/libc.so.6
#18 0x0000000000000000 in ?? ()


>> After analyzing the stack and reviewing the code, we find the
>> qemu_co_queue_run_restart should not be put in the coroutine_swap function 
>> which
>> can be invoked by qemu_coroutine_enter or qemu_coroutine_yield. Only
>> qemu_coroutine_enter needs to restart the co_queue.
>>
>> The error scenario is as follow: coroutine C1 enters C2, C2 yields
>> back to C1, then C1 ternimates and the related coroutine memory
>> becomes invalid. After a while, the C2 coroutine is entered again.
>> At this point, C1 is used as a parameter passed to
>> qemu_co_queue_run_restart. Therefore, qemu_co_queue_run_restart
>> accesses an invalid memory and a segfault error ocurrs.
>>
>> The qemu_co_queue_run_restart function re-enters coroutines waiting
>> in the co_queue. However, this function should be only used int the
>> qemu_coroutine_enter context. Only in this context, when the current
>> coroutine gets execution control again(after the execution of
>> qemu_coroutine_switch), we can restart the target coutine because the
>> target coutine has yielded back to the current coroutine or it has
>> terminated.
>>
>> First we want to put qemu_co_queue_run_restart in qemu_coroutine_enter,
>> but we find we can not access the target coroutine if it terminates.
> 
> This example captures the scenario you describe:
> 
> diff --git a/qemu-coroutine.c b/qemu-coroutine.c
> index 525247b..883cbf5 100644
> --- a/qemu-coroutine.c
> +++ b/qemu-coroutine.c
> @@ -103,7 +103,10 @@ static void coroutine_swap(Coroutine *from, Coroutine 
> *to)
>  {
>      CoroutineAction ret;
>  
> +    fprintf(stderr, "> %s from %p to %p\n", __func__, from, to);
>      ret = qemu_coroutine_switch(from, to, COROUTINE_YIELD);
> +    fprintf(stderr, "< %s from %p to %p switch %s\n", __func__, from, to,
> +            ret == COROUTINE_YIELD ? "yield" : "terminate");
>  
>      qemu_co_queue_run_restart(to);
>  
> @@ -111,6 +114,7 @@ static void coroutine_swap(Coroutine *from, Coroutine *to)
>      case COROUTINE_YIELD:
>          return;
>      case COROUTINE_TERMINATE:
> +        fprintf(stderr, "coroutine_delete %p\n", to);
>          trace_qemu_coroutine_terminate(to);
>          coroutine_delete(to);
>          return;
> diff --git a/tests/test-coroutine.c b/tests/test-coroutine.c
> index 27d1b6f..d44c428 100644
> --- a/tests/test-coroutine.c
> +++ b/tests/test-coroutine.c
> @@ -13,6 +13,7 @@
>  
>  #include <glib.h>
>  #include "block/coroutine.h"
> +#include "block/coroutine_int.h"
>  
>  /*
>   * Check that qemu_in_coroutine() works
> @@ -122,6 +123,35 @@ static void test_yield(void)
>      g_assert_cmpint(i, ==, 5); /* coroutine must yield 5 times */
>  }
>  
> +static void coroutine_fn c2_fn(void *opaque)
> +{
> +    fprintf(stderr, "c2 Part 1\n");
> +    qemu_coroutine_yield();
> +    fprintf(stderr, "c2 Part 2\n");
> +}
> +
> +static void coroutine_fn c1_fn(void *opaque)
> +{
> +    Coroutine *c2 = opaque;
> +
> +    fprintf(stderr, "c1 Part 1\n");
> +    qemu_coroutine_enter(c2, NULL);
> +    fprintf(stderr, "c1 Part 2\n");
> +}
> +
> +static void test_co_queue(void)
> +{
> +    Coroutine *c1;
> +    Coroutine *c2;
> +
> +    c1 = qemu_coroutine_create(c1_fn);
> +    c2 = qemu_coroutine_create(c2_fn);
> +
> +    qemu_coroutine_enter(c1, c2);
> +    memset(c1, 0xff, sizeof(Coroutine));
> +    qemu_coroutine_enter(c2, NULL);
> +}
> +
>  /*
>   * Check that creation, enter, and return work
>   */
> @@ -343,6 +373,7 @@ static void perf_cost(void)
>  int main(int argc, char **argv)
>  {
>      g_test_init(&argc, &argv, NULL);
> +    g_test_add_func("/basic/co_queue", test_co_queue);
>      g_test_add_func("/basic/lifecycle", test_lifecycle);
>      g_test_add_func("/basic/yield", test_yield);
>      g_test_add_func("/basic/nesting", test_nesting);
> 
> Here is the output (with printfs in coroutine_swap):
> 
> -> coroutine_swap from MAIN to C1
> c1 Part 1
> -> coroutine_swap from C1 to C2
> c2 Part 1
> -> coroutine_swap from C2 to C1
> <- coroutine_swap from C1 to C2 switch yield
> c1 Part 2
> <- coroutine_swap from MAIN to C1 switch terminate
> coroutine_delete C1
> -> coroutine_swap from MAIN to C2
> <- coroutine_swap from C2 to C1 switch yield  !!!
> c2 Part 2
> <- coroutine_swap from MAIN to C2 switch terminate
> coroutine_delete C2
> 
> I have marked the problematic line with "!!!".  The to=C1 variable is
> used after C1 has been deleted.
> 
> The test crashes since it writes 0xff to C1 after it has terminated.

yes, this is exactly the error scenario I described. Thanks, stefan.

> 
>> Signed-off-by: Bin Wu <address@hidden>
>> ---
>>  qemu-coroutine.c | 16 ++++++++++------
>>  1 file changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/qemu-coroutine.c b/qemu-coroutine.c
>> index 525247b..cc0bdfa 100644
>> --- a/qemu-coroutine.c
>> +++ b/qemu-coroutine.c
>> @@ -99,29 +99,31 @@ static void coroutine_delete(Coroutine *co)
>>      qemu_coroutine_delete(co);
>>  }
>>  
>> -static void coroutine_swap(Coroutine *from, Coroutine *to)
>> +static CoroutineAction coroutine_swap(Coroutine *from, Coroutine *to)
>>  {
>>      CoroutineAction ret;
>>  
>>      ret = qemu_coroutine_switch(from, to, COROUTINE_YIELD);
>>  
>> -    qemu_co_queue_run_restart(to);
>> -
>>      switch (ret) {
>>      case COROUTINE_YIELD:
>> -        return;
>> +        break;
>>      case COROUTINE_TERMINATE:
>>          trace_qemu_coroutine_terminate(to);
>> +        qemu_co_queue_run_restart(to);
>>          coroutine_delete(to);
>> -        return;
>> +        break;
>>      default:
>>          abort();
>>      }
>> +
>> +    return ret;
>>  }
>>  
>>  void qemu_coroutine_enter(Coroutine *co, void *opaque)
>>  {
>>      Coroutine *self = qemu_coroutine_self();
>> +    CoroutineAction ret;
>>  
>>      trace_qemu_coroutine_enter(self, co, opaque);
>>  
>> @@ -132,7 +134,9 @@ void qemu_coroutine_enter(Coroutine *co, void *opaque)
>>  
>>      co->caller = self;
>>      co->entry_arg = opaque;
>> -    coroutine_swap(self, co);
>> +    ret = coroutine_swap(self, co);
>> +    if (ret == COROUTINE_YIELD)
>> +        qemu_co_queue_run_restart(co);
>>  }
> 
> Your fix looks correct although QEMU coding style requires {}.
> 
> I tried to think of a simpler solution that keeps a single
> qemu_co_queue_run_restart() call but was unable to find one.
> 
> Please send another revision with a test-coroutine.c test case so we can
> protect against regressions.

OK, I will send another version latter.

> 
> Thanks,
> Stefan
> 

-- 
Bin Wu