[RFC PATCH v7 3/3] selftests/x86: sysret_rip: Test SYSRET with a signal handler

Ammar Faizi
2023-01-25T23:24:57Z

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

When run on a non-FRED system, test the SYSRET path. Make sure the
%rcx/%r11 clobbering behavior is consistent.

When run on a FRED system, test that %rcx/%r11 are preserved when
invoking syscall.

This is similar to what test_syscall_rcx_r11_consistent() is doing, but
with addition it's done via the SIGUSR2 signal handler.

Link: https://lore.kernel.org/lkml/8770815f-0f23-d0c5-e56a-d401827842c9@zytor.com
Suggested-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---

On Wed, 25 Jan 2023 00:39:26 -0800, "H. Peter Anvin" wrote:
> > /* Set IP and CX to match so that SYSRET can happen. */
> > ctx->uc_mcontext.gregs[REG_RIP] = rip;
> > ctx->uc_mcontext.gregs[REG_RCX] = rip;
>
> It would be interesting to have the syscall handler try both with and
> without this (so it would end up doing both IRET and SYSCALL on legacy.)
> Perhaps SIGUSR1 versus SIGUSR2...

tools/testing/selftests/x86/sysret_rip.c | 37 ++++++++++++++++++++++++
1 file changed, 37 insertions(+)

diff --git a/tools/testing/selftests/x86/sysret_rip.c b/tools/testing/selftests/x86/sysret_rip.c
index d688cb9d5ac919eb..630ee261559c14b1 100644
--- a/tools/testing/selftests/x86/sysret_rip.c
+++ b/tools/testing/selftests/x86/sysret_rip.c
@@ -269,6 +269,28 @@ 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;
@@ -286,6 +308,21 @@ int main()
for (i = 47; i < 64; i++)
test_sigreturn_to(1UL<<i);

+ /*
+ *
+ * When run on a non-FRED system, test the SYSRET path. Make
+ * sure the %rcx/%r11 clobbering behavior is consistent.
+ *
+ * When run on a FRED system, test that %rcx/%r11 are preserved
+ * when invoking syscall.
+ *
+ * This is similar to test_syscall_rcx_r11_consistent(), but via
+ * a signal handler.
+ */
+ sethandler(SIGUSR2, sigusr2, 0);
+ for (i = 0; i < 32; i++)
+ test_sysret_consistent();
+
clearhandler(SIGUSR1);

sethandler(SIGSEGV, sigsegv_for_fallthrough, 0);
--
Ammar Faizi