Re: [RFC PATCH v2 2/2] selftests/x86: sysret_rip: Add more syscall tests with respect to `%rcx` and `%r11`

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

On Mon, Jan 23, 2023 at 10:16:01PM -0800, H. Peter Anvin wrote:
> On 1/23/23 18:27, 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.
> > 
> > Test them out with a trivial system call like __NR_getppid and friends
> > which are extremely likely to return with SYSRET on an IDT system; check
> > that it returns a nonnegative value and then save the result.
> > 
> 
> "Nonnegative" here should be "valid"; it is an implementation detail that
> the error value is -1.

Copy-paste error, will fix that!

> However, you are not checking that you don't get a mix of REGS_SAVED and
> REGS_SYSRET, which is a major part of the point.

Good point!

What do you think of adding this on top of patch #1?

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 75c72d34dbc5840c..3da827713831acbc 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -47,11 +47,14 @@ static const unsigned long rcx_sentinel = 0x5ca1ab1e0b57ac1e;
 static const unsigned long rflags_sentinel = 0x200a93;
 
 enum regs_ok {
-	REGS_ERROR  = -1,	/* Invalid register contents */
-	REGS_SAVED  =  0,	/* Registers properly preserved */
-	REGS_SYSRET =  1	/* Registers match syscall/sysret */
+	REGS_INIT_VAL	= -2,	/* For init value checker, never returned */
+	REGS_ERROR 	= -1,	/* Invalid register contents */
+	REGS_SAVED 	=  0,	/* Registers properly preserved */
+	REGS_SYSRET	=  1	/* Registers match syscall/sysret */
 };
 
+static enum regs_ok regs_ok_state = REGS_INIT_VAL;
+
 /*
  * Returns:
  *  0 = %rcx and %r11 preserved.
@@ -86,6 +89,7 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
 	register unsigned long r9 asm("%r9");
 	register void *rsp asm("%rsp");
 	unsigned long rcx, rbx;
+	enum regs_ok ret;
 
 	r11 = r11_sentinel;
 	rcx = rcx_sentinel;
@@ -124,7 +128,14 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
 	 * - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
 	 *
 	 */
-	assert(check_regs_result(r11, rcx, rbx) != REGS_ERROR);
+	ret = check_regs_result(r11, rcx, rbx);
+	assert(ret != REGS_ERROR);
+
+	if (regs_ok_state == REGS_INIT_VAL)
+		regs_ok_state = ret;
+	else
+		assert(ret == regs_ok_state);
+
 	return nr_syscall;
 }
 
-- 
Ammar Faizi