[RFC PATCH v3 2/2] selftests/x86: sysret_rip: Add more syscall tests with respect to `%rcx` and `%r11`

Ammar Faizi
2023-01-24T10:10:10Z

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

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.

Link: https://lore.kernel.org/lkml/25b96960-a07e-a952-5c23-786b55054126@zytor.com
Co-developed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Acked-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
tools/testing/selftests/x86/sysret_rip.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index b0d271c19ddd7834..bf90fac95a264e2d 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -267,8 +267,17 @@ static void test_syscall_fallthrough_to(unsigned long ip)
printf("[OK]\tWe survived\n");
}

+static void test_syscall_rcx_r11(void)
+{
+ do_syscall(__NR_getpid, 0, 0, 0, 0, 0, 0);
+ do_syscall(__NR_gettid, 0, 0, 0, 0, 0, 0);
+ do_syscall(__NR_getppid, 0, 0, 0, 0, 0, 0);
+}
+
int main()
{
+ test_syscall_rcx_r11();
+
/*
* When the kernel returns from a slow-path syscall, it will
* detect whether SYSRET is appropriate. If it incorrectly
--
Ammar Faizi