Re: [Xen-devel] question about SIGSEGV in datacopier_readable in libxl_aoutil.c

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-09-03 at 15:01 +0800, Chunyan Liu wrote:
> Hi, List,

Adding Ian J, who knows this bit of the code.

Which version of Xen (and therefore libxl) are you using?

> I'm trying to add migration APIs to libvirt libxl driver. In testing
> HVM migration, on source side, when executing libxl_domain_suspend,
> often meet SIGSEGV in libxl_aoutil.c: datacopier_readable, the
> malloc() function place:
>       if (!buf || buf->used >= sizeof(buf->buf)) {
>             buf = malloc(sizeof(*buf));
> 
> I doubt the heap is corrupted someway but couldn't confirm the root
> cause. And I tried valgrind to find some clue, following is the info
> right before the SIGSEGV.
> #valgrind --leak-check=full /usr/sbin/libvirtd -l -d

Did you use a Xen aware version of valgrind?
http://blog.xen.org/index.php/2013/01/18/using-valgrind-to-debug-xen-toolstacks/

> [snip]
> 
> ==7510== Syscall param read(buf) points to unaddressable byte(s)
> ==7510==    at 0x8ECC76D: ??? (syscall-template.S:82)
> ==7510==    by 0x14AB3070: datacopier_readable (unistd.h:45)
> ==7510==    by 0x14AB833C: afterpoll_internal (libxl_event.c:995)
> ==7510==    by 0x14AB8F16: eventloop_iteration (libxl_event.c:1440)
> ==7510==    by 0x14AB9439: libxl__ao_inprogress (libxl_event.c:1685)
> ==7510==    by 0x14A9ABF7: libxl_domain_suspend (libxl.c:785)
> ==7510==    by 0x148404B3: libxlDomainMigratePerform3 (libxl_driver.c:5100)
> ==7510==    by 0x5390CFA: virDomainMigratePerform3 (libvirt.c:7050)
> ==7510==    by 0x12C262: remoteDispatchDomainMigratePerform3Helper 
> (remote.c:3507)
> ==7510==    by 0x53EACBE: virNetServerProgramDispatch 
> (virnetserverprogram.c:435)
> ==7510==    by 0x53EBCBD: virNetServerProcessMsg (virnetserver.c:165)
> ==7510==    by 0x53EC912: virNetServerHandleJob (virnetserver.c:186)
> ==7510==  Address 0x18a409ec is 0 bytes after a block of size 28 alloc'd
> ==7510==    at 0x4C26FFB: calloc (in 
> /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
> ==7510==    by 0x14AAECB6: libxl__zalloc (libxl_internal.c:83)
> ==7510==    by 0x14AB33B0: libxl__datacopier_prefixdata (libxl_aoutils.c:92)
> ==7510==    by 0x14AA6CDC: libxl__domain_save_device_model (libxl_dom.c:1447)
> ==7510==    by 0x14AA8097: libxl__xc_domain_save_done (libxl_dom.c:1382)
> ==7510==    by 0x14AB4268: helper_done (libxl_save_callout.c:332)
> ==7510==    by 0x14AB4CA2: helper_exited (libxl_save_callout.c:317)
> ==7510==    by 0x14ABB274: childproc_reaped (libxl_fork.c:264)
> ==7510==    by 0x14ABB97A: libxl__fork_selfpipe_woken (libxl_fork.c:300)
> ==7510==    by 0x14AB83A0: afterpoll_internal (libxl_event.c:1008)
> ==7510==    by 0x14AB8F16: eventloop_iteration (libxl_event.c:1440)
> ==7510==    by 0x14AB9439: libxl__ao_inprogress (libxl_event.c:1685)
> ==7510==
> --7510-- VALGRIND INTERNAL ERROR: Valgrind received a signal 11 (SIGSEGV) - 
> exiting
> --7510-- si_code=80;  Faulting address: 0x0;  sp: 0x406ad5da0
> 
> I couldn't find a clear problem in the code, but after trying to change the 
> code a little, it turned to be working.
> Following is the change.
> 
> --- a/tools/libxl/libxl_aoutils.c
> +++ b/tools/libxl/libxl_aoutils.c
> @@ -89,7 +89,8 @@ void libxl__datacopier_prefixdata(libxl_
> 
>      assert(len < dc->maxsz - dc->used);
> 
> -    buf = libxl__zalloc(NOGC, sizeof(*buf) - sizeof(buf->buf) + len);
> +//    buf = libxl__zalloc(NOGC, sizeof(*buf) - sizeof(buf->buf) + len);
> +    buf = libxl__zalloc(NOGC, sizeof(libxl__datacopier_buf));
>      buf->used = len;
>      memcpy(buf->buf, data, len);
> 
> @@ -141,10 +142,11 @@ static void datacopier_readable(libxl__e
>          libxl__datacopier_buf *buf =
>              LIBXL_TAILQ_LAST(&dc->bufs, libxl__datacopier_bufs);
>          if (!buf || buf->used >= sizeof(buf->buf)) {
> -            buf = malloc(sizeof(*buf));
> -            if (!buf) libxl__alloc_failed(CTX, __func__, 1, sizeof(*buf));
> -            buf->used = 0;
> -            LIBXL_TAILQ_INSERT_TAIL(&dc->bufs, buf, entry);
> +            libxl__datacopier_buf *newbuf = 
> malloc(sizeof(libxl__datacopier_buf));
> +            if (!newbuf) libxl__alloc_failed(CTX, __func__, 1, 
> sizeof(libxl__datacopier_buf));
> +            newbuf->used = 0;
> +            LIBXL_TAILQ_INSERT_TAIL(&dc->bufs, newbuf, entry);
> +            buf = newbuf;
>          }
>          int r = read(ev->fd,
>                       buf->buf + buf->used,
> 
> 
> Could anybody familiar with this part of code take a look at it?
> 
> 
> Thanks,
> Chunyan
> 
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