[RFC PATCH v6 2/3] selftests/x86: sysret_rip: Add more syscall tests with respect to `%rcx` and `%r11`

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

Test that:

  - REGS_SAVED: "syscall" in a FRED system doesn't clobber %rcx and
    %r11.

  - REGS_SYSRET: "syscall" in a non-FRED system sets %rcx=%rip and
    %r11=%rflags.

Test them out with trivial system calls like __NR_getppid and friends
which are extremely likely to return with SYSRET on an IDT system.

Goals of this test:

  - Ensure that the syscall behavior is consistent. It should be either
    always REGS_SAVED or always REGS_SYSRET. Not a mix of them.

  - The kernel doesn't leak its internal data when returning to
    userspace.

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>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 tools/testing/selftests/x86/sysret_rip.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 100f55981d77a29b..d45b7f0147cd25ad 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -264,8 +264,24 @@ static void test_syscall_fallthrough_to(unsigned long ip)
 	printf("[OK]\tWe survived\n");
 }
 
+/*
+ * Ensure that various system calls are consistent.
+ * We should not get a mix of REGS_SAVED and REGS_SYSRET.
+ */
+static void test_syscall_rcx_r11_consistent(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()
 {
+	int i;
+
+	for (i = 0; i < 32; i++)
+		test_syscall_rcx_r11_consistent();
+
 	/*
 	 * When the kernel returns from a slow-path syscall, it will
 	 * detect whether SYSRET is appropriate.  If it incorrectly
@@ -273,7 +289,7 @@ int main()
 	 * it'll crash on Intel CPUs.
 	 */
 	sethandler(SIGUSR1, sigusr1, 0);
-	for (int i = 47; i < 64; i++)
+	for (i = 47; i < 64; i++)
 		test_sigreturn_to(1UL<<i);
 
 	clearhandler(SIGUSR1);
@@ -284,7 +300,7 @@ int main()
 	test_syscall_fallthrough_to((1UL << 47) - 2*PAGE_SIZE);
 
 	/* These are the interesting cases. */
-	for (int i = 47; i < 64; i++) {
+	for (i = 47; i < 64; i++) {
 		test_syscall_fallthrough_to((1UL<<i) - PAGE_SIZE);
 		test_syscall_fallthrough_to(1UL<<i);
 	}
-- 
Ammar Faizi