Re: [RFC PATCH v1 1/2] selftests/x86: sysret_rip: Handle syscall in a FRED system

H. Peter Anvin
2023-01-24T01:43:12Z

On 1/23/23 16:26, Ammar Faizi wrote:
> +
> +static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
> + unsigned long arg3, unsigned long arg4,
> + unsigned long arg5, unsigned long arg6)
> +{
> + register unsigned long r11 asm("%r11");
> + register unsigned long r10 asm("%r10");
> + register unsigned long r8 asm("%r8");
> + register unsigned long r9 asm("%r9");
> + unsigned long rcx, rbx;
> +
> + r11 = r11_sentinel;
> + rcx = rcx_sentinel;
> + r10 = arg4;
> + r8 = arg5;
> + r9 = arg6;
> +
> + asm volatile (
> + "movq -8(%%rsp), %%r12\n\t" /* Don't clobber redzone. */
> + "pushq %[rflags_sentinel]\n\t"
> + "popf\n\t"
> + "movq %%r12, -8(%%rsp)\n\t"
> + "leaq 1f(%%rip), %[rbx]\n\t"
> + "syscall\n"
> + "1:"
> +
> + : "+a" (nr_syscall),
> + "+r" (r11),
> + "+c" (rcx),
> + [rbx] "=b" (rbx)
> +
> + : [rflags_sentinel] "g" (rflags_sentinel),
> + "D" (arg1), /* %rdi */
> + "S" (arg2), /* %rsi */
> + "d" (arg3), /* %rdx */
> + "r" (r10),
> + "r" (r8),
> + "r" (r9)
> +
> + : "r12", "memory"
> + );
> +
> + /*
> + * 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.
> + *
> + */
> + assert(check_regs_result(r11, rcx, rbx) != REGS_ERROR);
> + return nr_syscall;
> +}
> +

So as per Andrew's comment, add:

register void * rsp asm("%rsp");

...

"+r" (rsp) /* clobber the redzone */

... as the right way to avoid redzone problems.

-hpa