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Re: [Xen-devel] [BUGFIX][PATCH 1/4] dbg_rw_guest_mem: need to call put_gfn in error path.
On 01/06/14 08:13, Andrew Cooper wrote:
On 06/01/14 12:43, Don Slutz wrote:
On 01/05/14 18:09, Andrew Cooper wrote:
On 04/01/2014 17:52, Don Slutz wrote:
Using a 1G hvm domU (in grub) and gdbsx:
(gdb) set arch i8086
warning: A handler for the OS ABI "GNU/Linux" is not built into this
configuration
of GDB. Attempting to continue with the default i8086 settings.
The target architecture is assumed to be i8086
(gdb) target remote localhost:9999
Remote debugging using localhost:9999
Remote debugging from host 127.0.0.1
0x0000d475 in ?? ()
(gdb) x/1xh 0x6ae9168b
Will reproduce this bug.
With a debug=y build you will get:
Assertion '!preempt_count()' failed at preempt.c:37
For a debug=n build you will get a dom0 VCPU hung (at some point) in:
[ffff82c4c0126eec] _write_lock+0x3c/0x50
ffff82c4c01e43a0 __get_gfn_type_access+0x150/0x230
ffff82c4c0158885 dbg_rw_mem+0x115/0x360
ffff82c4c0158fc8 arch_do_domctl+0x4b8/0x22f0
ffff82c4c01709ed get_page+0x2d/0x100
ffff82c4c01031aa do_domctl+0x2ba/0x11e0
ffff82c4c0179662 do_mmuext_op+0x8d2/0x1b20
ffff82c4c0183598 __update_vcpu_system_time+0x288/0x340
ffff82c4c015c719 continue_nonidle_domain+0x9/0x30
ffff82c4c012938b add_entry+0x4b/0xb0
ffff82c4c02223f9 syscall_enter+0xa9/0xae
And gdb output:
(gdb) x/1xh 0x6ae9168b
0x6ae9168b: 0x3024
(gdb) x/1xh 0x6ae9168b
0x6ae9168b: Ignoring packet error, continuing...
Reply contains invalid hex digit 116
The 1st one worked because the p2m.lock is recursive and the PCPU
had not yet changed.
crash reports (for example):
crash> mm_rwlock_t 0xffff83083f913010
struct mm_rwlock_t {
lock = {
raw = {
lock = 2147483647
},
debug = {<No data fields>}
},
unlock_level = 0,
recurse_count = 1,
locker = 1,
locker_function = 0xffff82c4c022c640 <__func__.13514>
"__get_gfn_type_access"
}
Signed-off-by: Don Slutz <dslutz@xxxxxxxxxxx>
---
xen/arch/x86/debug.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/xen/arch/x86/debug.c b/xen/arch/x86/debug.c
index 3e21ca8..eceb805 100644
--- a/xen/arch/x86/debug.c
+++ b/xen/arch/x86/debug.c
@@ -161,8 +161,11 @@ dbg_rw_guest_mem(dbgva_t addr, dbgbyte_t *buf,
int len, struct domain *dp,
mfn = (has_hvm_container_domain(dp)
? dbg_hvm_va2mfn(addr, dp, toaddr, &gfn)
: dbg_pv_va2mfn(addr, dp, pgd3));
- if ( mfn == INVALID_MFN )
+ if ( mfn == INVALID_MFN ) {
Xen coding style would have this brace on the next line.
Will fix.
Content-wise, I think it would be better to fix up the error path in
dbg_hvm_va2mfn() so it doesn't exit with INVALID_MFN having taken a
reference on the gfn.
That would only fix "p2m_is_readonly(gfntype) and writing memory" case
(see cover letter). To fix "the requested vaddr does not exist" case,
I would need to add a check for INVALID_MFN in dbg_hvm_va2mfn() also.
As It currently stands it is a smaller fix.
-Don Slutz
Size really doesn't matter.
What does matter is that the caller of dbg_hvm_va2mfn() should not have
to cleanup a reference taken when it returns an error.
Anyway, "the requested vaddr does not exist" is covered by
paging_gva_to_gfn(), which will exit before taking the ref on the gfn.
This is a false statement. A valid GFN is returned in this case. The lock is
taken by get_gfn().
The following (completely untested) patch ought to do:
diff --git a/xen/arch/x86/debug.c b/xen/arch/x86/debug.c
index 3e21ca8..3372adb 100644
--- a/xen/arch/x86/debug.c
+++ b/xen/arch/x86/debug.c
@@ -63,10 +63,17 @@ dbg_hvm_va2mfn(dbgva_t vaddr, struct domain *dp, int
toaddr,
if ( p2m_is_readonly(gfntype) && toaddr )
{
DBGP2("kdb:p2m_is_readonly: gfntype:%x\n", gfntype);
- return INVALID_MFN;
+ mfn = INVALID_MFN;
}
DBGP2("X: vaddr:%lx domid:%d mfn:%lx\n", vaddr, dp->domain_id, mfn);
+
+ if ( mfn == INVALID_MFN )
+ {
+ put_gfn(dp, *gfn);
+ *gfn = INVALID_GFN;
+ }
+
return mfn;
}
However this patch (which I have now tested) works fine. I will post it soon
as part of v2 of this patch set.
-Don Slutz
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