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

On 1/24/23 19:49, Ammar Faizi wrote:

• /*

• * Test that:
  

• *
  

• * - "syscall" in a FRED system doesn't clobber %rcx and %r11.
  

• * - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
  

• */
  

• ret = check_regs_result(r11, rcx, rbx);
• assert(ret != REGS_ERROR);
• /*

• * Test that we don't get a mix of REGS_SAVED and REGS_SYSRET.
  

• * It needs at least calling do_syscall() twice to assert.
  

• */
  

• if (regs_ok_state == REGS_UNDEFINED) {

• /*
  

•  * First time calling do_syscall().
  

•  */
  

• regs_ok_state = ret;
  

• } else {

• assert(regs_ok_state == ret);
  

• }

[...]

• ret = check_regs_result(ctx->uc_mcontext.gregs[REG_R11],

• 		ctx->uc_mcontext.gregs[REG_RCX],
  

• 		ctx->uc_mcontext.gregs[REG_RBX]);
  

• assert(ret != REGS_ERROR);

This instance, too, needs to be checked against regs_ok_result. It would make most sense to move that handling, and the assert() into check_regs_result() or into a separate function around it.

/* 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...

-hpa