[RFC PATCH v8 3/3] selftests/x86: sysret_rip: Test SYSRET with a signal handler

[Ammar Faizi]

The current test_sigreturn_to() goes to the slow-path syscall with
IRET due to non-canonical addresses. It uses the SIGUSR1 signal to
perform the test.

Add a similar test that goes to the SYSRET path instead of IRET using
the SIGUSR2 signal. There are two cases:

  A) 'syscall' in a FRED system preserves %rcx and %r11.

  B) 'syscall' in a non-FRED system sets %rcx=%rip and %r11=%rflags.

The __raise(SIGUSR2) call verifies the 'syscall' behavior consistency
when dealing with a signal handler. It must always be (A) or always be
(B). Not a mix of them.

Cc: Xin Li <xin3.li@xxxxxxxxx>
Link: https://lore.kernel.org/lkml/8770815f-0f23-d0c5-e56a-d401827842c9@xxxxxxxxx
Suggested-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
---
 tools/testing/selftests/x86/sysret_rip.c | 30 ++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index 1531593b50d02150..746801675fe77e9c 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -274,6 +274,28 @@ static void test_syscall_rcx_r11_consistent(void)
 	do_syscall(__NR_getppid, 0, 0, 0, 0, 0, 0);
 }
 
+static unsigned long usr2_rcx;
+static unsigned long usr2_r11;
+
+static void sigusr2(int sig, siginfo_t *info, void *ctx_void)
+{
+	ucontext_t *ctx = (ucontext_t*)ctx_void;
+
+	usr2_r11 = ctx->uc_mcontext.gregs[REG_R11];
+	usr2_rcx = ctx->uc_mcontext.gregs[REG_RCX];
+
+	check_regs_result(ctx->uc_mcontext.gregs[REG_R11],
+			  ctx->uc_mcontext.gregs[REG_RCX],
+			  ctx->uc_mcontext.gregs[REG_RBX]);
+}
+
+static void test_sysret_consistent(void)
+{
+	printf("[RUN]\ttest_sysret_consistent\n");
+	__raise(SIGUSR2);
+	printf("[OK]\tRCX = %#lx;  R11 = %#lx\n", usr2_rcx, usr2_r11);
+}
+
 int main()
 {
 	int i;
@@ -291,6 +313,14 @@ int main()
 	for (i = 47; i < 64; i++)
 		test_sigreturn_to(1UL<<i);
 
+	/*
+	 * test_sigreturn_to() above will test the IRET path. Now test
+	 * the SYSRET path.
+	 */
+	sethandler(SIGUSR2, sigusr2, 0);
+	for (i = 0; i < 32; i++)
+		test_sysret_consistent();
+
 	clearhandler(SIGUSR1);
 
 	sethandler(SIGSEGV, sigsegv_for_fallthrough, 0);
-- 
Ammar Faizi