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

Ammar Faizi
2023-01-25T09:57:34Z

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()?

--
Ammar Faizi