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

Ammar Faizi
2023-01-24T06:47:15Z

On Tue, Jan 24, 2023 at 01:41:30PM +0700, Ammar Faizi wrote:
> On Mon, Jan 23, 2023 at 10:16:01PM -0800, H. Peter Anvin wrote:
> > 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;
> }
>

And oh, on top of that. Add a comment, just to make it clear...

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 3da827713831acbc..3d783f5baee1b66a 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -131,6 +131,10 @@ static long do_syscall(long nr_syscall, unsigned long arg1, unsigned long arg2,
ret = check_regs_result(r11, rcx, rbx);
assert(ret != REGS_ERROR);

+ /*
+ * Check that we don't get a mix of REGS_SAVED and REGS_SYSRET.
+ * Need at least 2 times 'syscall' invoked from this function.
+ */
if (regs_ok_state == REGS_INIT_VAL)
regs_ok_state = ret;
else

--
Ammar Faizi