Problem about dump-core

[Wen Congyang <wency@cn.fujitsu.com>]

Hi, everyone:

The command 'xl dump-core' will fail after migration. The guest is HVM guest(without pv driver).
I use the newest staging branch to test. Both source and dest dom0 use the same kernel.

I use gdb to run 'xl dump-core' on dest dom0::
# gdb --args xl dump-core 1 vmcore
GNU gdb (GDB) Red Hat Enterprise Linux (7.2-56.el6)
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/sbin/xl...done.
(gdb) b main
Breakpoint 1 at 0x406ad8: file xl.c, line 298.
(gdb) b xc_core.c:482
No source file named xc_core.c.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb) r
Starting program: /usr/sbin/xl dump-core 1 vmcore
[Thread debugging using libthread_db enabled]

Breakpoint 1, main (argc=4, argv=0x7fffffffe3d8) at xl.c:298
298	    void *config_data = 0;
Missing separate debuginfos, use: debuginfo-install bzip2-libs-1.0.5-7.el6_0.x86_64 glibc-2.12-1.80.el6.x86_64 libuuid-2.17.2-12.7.el6.x86_64 yajl-1.0.7-3.el6.x86_64 zlib-1.2.3-27.el6.x86_64
(gdb) b xc_core.c:482
Breakpoint 2 at 0x7ffff794559d: file xc_core.c, line 482.
(gdb) c
Continuing.

Breakpoint 2, xc_domain_dumpcore_via_callback (xch=0x6262d0, domid=1, args=0x7fffffffe140, dump_rtn=0x7ffff79450c0 <local_file_dump>) at xc_core.c:482
482	    live_shinfo = xc_map_foreign_range(xch, domid, PAGE_SIZE,
(gdb) p live_shinfo 
$1 = (shared_info_any_t *) 0x0
(gdb) n
484	    if ( !live_shinfo && !info.hvm )
(gdb) p live_shinfo 
$2 = (shared_info_any_t *) 0x7ffff7ffb000
(gdb) p *live_shinfo 
Cannot access memory at address 0x7ffff7ffb000                  <==================== We cannot access live_shinfo
(gdb) b 763
Breakpoint 3 at 0x7ffff7946588: file xc_core.c, line 763.
(gdb) c
Continuing.

Breakpoint 3, xc_domain_dumpcore_via_callback (xch=0x6262d0, domid=1, args=0x7fffffffe140, dump_rtn=0x7ffff79450c0 <local_file_dump>) at xc_core.c:763
763	        sts = dump_rtn(xch, args, (char*)live_shinfo, PAGE_SIZE);
(gdb) s
local_file_dump (xch=0x6262d0, args=0x7fffffffe140, buffer=0x7ffff7ffb000 <Address 0x7ffff7ffb000 out of bounds>, length=4096) at xc_core.c:931
931	    if ( write_exact(da->fd, buffer, length) == -1 )
(gdb) s
write_exact (fd=14, data=0x7ffff7ffb000, size=4096) at xc_private.c:848
848	    while ( offset < size )
(gdb) n
850	        len = write(fd, (const char *)data + offset, size - offset);      <=============== We write live_shinfo to the core file, and fail
(gdb) p data
$3 = (const void *) 0x7ffff7ffb000
(gdb) p *data
Attempt to dereference a generic pointer.
(gdb) n
851	        if ( (len == -1) && (errno == EINTR) )
(gdb) p len
$4 = -1
(gdb) p errno
$5 = 14
(gdb) 

I try it on source dom0:
gdb --args xl dump-core 1 vmcore 
GNU gdb (GDB) Red Hat Enterprise Linux (7.2-56.el6)
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /usr/sbin/xl...done.
(gdb) b main
Breakpoint 1 at 0x406ad8: file xl.c, line 298.
(gdb) r
Starting program: /usr/sbin/xl dump-core 1 vmcore
[Thread debugging using libthread_db enabled]

Breakpoint 1, main (argc=4, argv=0x7fffffffe438) at xl.c:298
298	    void *config_data = 0;
Missing separate debuginfos, use: debuginfo-install bzip2-libs-1.0.5-7.el6_0.x86_64 glibc-2.12-1.80.el6.x86_64 libuuid-2.17.2-12.7.el6.x86_64 yajl-1.0.7-3.el6.x86_64 zlib-1.2.3-27.el6.x86_64
(gdb) b xc_core.c:482
Breakpoint 2 at 0x7ffff794459d: file xc_core.c, line 482.
(gdb) c
Continuing.

Breakpoint 2, xc_domain_dumpcore_via_callback (xch=0x6262d0, domid=1, args=0x7fffffffe1a0, dump_rtn=0x7ffff79440c0 <local_file_dump>) at xc_core.c:482
482	    live_shinfo = xc_map_foreign_range(xch, domid, PAGE_SIZE,
(gdb) p live_shinfo 
$1 = (shared_info_any_t *) 0x0
(gdb) n
484	    if ( !live_shinfo && !info.hvm )
(gdb) p live_shinfo 
$2 = (shared_info_any_t *) 0x7ffff7ffb000
(gdb) p *live_shinfo 
Cannot access memory at address 0x7ffff7ffb000            <======== We also cannot access live_shinfo
(gdb) b 763
Breakpoint 4 at 0x7ffff7945588: file xc_core.c, line 763.
(gdb) c
Continuing.

Breakpoint 4, xc_domain_dumpcore_via_callback (xch=0x6262d0, domid=1, args=0x7fffffffe1a0, dump_rtn=0x7ffff79440c0 <local_file_dump>) at xc_core.c:763
763	        sts = dump_rtn(xch, args, (char*)live_shinfo, PAGE_SIZE);
(gdb) s
local_file_dump (xch=0x6262d0, args=0x7fffffffe1a0, buffer=0x7ffff7ffb000 <Address 0x7ffff7ffb000 out of bounds>, length=4096) at xc_core.c:931
931	    if ( write_exact(da->fd, buffer, length) == -1 )
(gdb) p buffer
$3 = 0x7ffff7ffb000 <Address 0x7ffff7ffb000 out of bounds>
(gdb) p *buffer
Cannot access memory at address 0x7ffff7ffb000
(gdb) n
937	    if ( length >= (DUMP_INCREMENT * PAGE_SIZE) )            <===== But we can write live_shinfo to corefile. Why???