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

On Mon, Jan 23, 2023 at 05:40:23PM -0800, H. Peter Anvin wrote:

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.

Fixed in v2.