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

H. Peter Anvin
2023-01-24T09:10:24Z

On January 23, 2023 10:41:20 PM PST, Ammar Faizi <ammarfaizi2@gnuweeb.org> wrote:
>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;
> }
>

Works for me, although I would call it REGS_UNDEFINED myself.