Re: [Xen-devel] Crash with paravirt-ops 2.6.31.6 kernel

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Ian Campbell <Ian.Campbell@xxxxxxxxxx> 23.11.09 17:44 >>>
>On Mon, 2009-11-23 at 16:31 +0000, Jan Beulich wrote:
>> >> It does not happen on XenSource 2.6.18 kernel
>> >
>> >I assume that this kernel (perhaps coincidentally) manages to use
>> >FLAT_USER_CS32 for compat mode processes.
>> >
>> >> , or the Debian 2.6.26 kernel.
>> >
>> >This was a forward ported 2.6.18-style kernel so I guess the same reason
>> >as 2.6.18.
>> 
>> If your analysis was right, 2.6.18 as well as our forward ported kernels
>> should also be affected (both ia32_sysenter_target and ia32_cstar_target
>> store __USER32_CS to the frame, and return via HYPERVISOR_iret), yet
>> supposedly they don't have the problem (though I can't say why that
>> would be). So perhaps there's some other yet un-described aspect to
>> this, or I'm being confused by something...
>
>I didn't try any of these kernels myself so I don't really know what
>happens.

Okay, I think I spotted the relevant difference: 2.6.18 and forward ports
set VGCF_in_syscall only when returning from 64-bit system calls (through
ret_from_sys_call) - 32-bit syscalls (regardless of the entry path taken)
return through int_ret_from_sys_call. 32-bit guest kernels shouldn't be
affected by this, as compat mode returns from the hypervisor
(compat_restore_all_guest) always use iret.

Jan


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