question about SIGSEGV in datacopier_readable in libxl_aoutil.c

[Chunyan Liu <cyliu@suse.com>]

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Hi, List,

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
[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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