Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

[Brian Gerst]

On Tue, Mar 24, 2015 at 4:17 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 03/24/2015 05:55 PM, Brian Gerst wrote:
>>>> Might be nice to place a more generic description there, which
>>>> registers are expected to be saved by user-space calling in here, etc.
>>>
>>> __kernel_vsyscall entry point has the same ABI in any 32-bit vDSO,
>>> the good old int 0x80 calling convention:
>>>
>>> syscall# in eax,
>>> params in ebx/ecx/edx/esi/edi/ebp,
>>> all registers are preserved by the syscall.
>>>
>>> (I think we don't guarantee that all flags are preserved:
>>> I have a testcase where DF gets cleared).
>>
>> DF should always be clear on any function call per the C ABI.  But,
>> eflags should be preserved, at least the non-privileged bits.  I'd
>> like to see that testcase.
>
> The testcase is a simplistic example of how to find and use
> 32-bit vDSO to perform system calls.
>
> It also sets flags.DF before syscall, and checks whether registers
> are preserved, including flags.DF.
>
> On 32-bit kernel (on Intel CPU, where vDSO uses SYSENTER), I see this:
>
> $ ./test32_syscall_vdso
> Result:1
>
> whereas on 64-bit it is
>
> ./test32_syscall_vdso
> Result:0
>
> "Result:1" means that DF was cleared.
>
> See attached source.
>

It looks like 32-bit native does not restore eflags before sysexit,
while 64-bit compat does.

--
Brian Gerst
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