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

[H. Peter Anvin]

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