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 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.

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.

~Andrew

> +            if ( gfn != INVALID_GFN )
> +                put_gfn(dp, gfn);
>              break;
> +        }
>  
>          va = map_domain_page(mfn);
>          va = va + (addr & (PAGE_SIZE-1));


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