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

H. Peter Anvin
2023-01-25T10:20:26Z

On January 25, 2023 1:57:15 AM PST, Ammar Faizi <ammarfaizi2@gnuweeb.org> 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()?
>

I guess it would depend on what they "normally" are. My #1 impulse would be to leave them both unchanged.