Re: [RFC PATCH v5 1/2] selftests/x86: sysret_rip: Handle syscall in a FRED system

Ammar Faizi
2023-01-25T10:01:40Z

On 1/25/23 4:57 PM, Ammar Faizi wrote:
> On Wed, Jan 25, 2023 at 12:39:26AM -0800, H. Peter Anvin wrote:
>>> /* Set IP and CX to match so that SYSRET can happen. */
>>> ctx->uc_mcontext.gregs[REG_RIP] = rip;
>>> ctx->uc_mcontext.gregs[REG_RCX] = rip;
>>
>> It would be interesting to have the syscall handler try both with and
>> without this (so it would end up doing both IRET and SYSCALL on legacy.)
>> Perhaps SIGUSR1 versus SIGUSR2...
>
> Just to clarify this more so I am sure I understand it correctly.
>
> Did you mean to have the same signal handler without modifiying
> 'REG_RCX' but still change 'REG_RIP'?
>
> IOW, we want to only *remove*:
>
> ctx->uc_mcontext.gregs[REG_RCX] = rip;
>
> and *keep*:
>
> ctx->uc_mcontext.gregs[REG_RIP] = rip;
>
> for the SIGUSR2 handler. Thus, inside the entry64 we will jump to the
> iret path because %rcx != %r11 upon rt_sigreturn()?

s/%rcx != %r11/%rcx != %rip/

--
Ammar Faizi