Re: [Xen-devel] [BUGFIX][PATCH 1/4] dbg_rw_guest_mem: need to call put_gfn in error path.

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

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;
 }


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel