When shadow stack is enabled, selftests/x86/sigreturn_64 triggers a fault
when doing sigreturn to 32-bit context but the task's shadow stack pointer
is above 32-bit address range.  Fix it by:

- Allocate a small shadow stack below 32-bit address,
- Switch to the new shadow stack,
- Run tests,
- Switch back to the original 64-bit shadow stack.

Signed-off-by: Yu-cheng Yu <redacted>
---
 tools/testing/selftests/x86/sigreturn.c | 28 +++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/tools/testing/selftests/x86/sigreturn.c b/tools/testing/selftests/x86/sigreturn.c
index 57c4f67f16ef..5bcd74d416ff 100644
--- a/tools/testing/selftests/x86/sigreturn.c
+++ b/tools/testing/selftests/x86/sigreturn.c

@@ -45,6 +45,14 @@
 #include <stdbool.h>
 #include <sys/ptrace.h>
 #include <sys/user.h>
+#include <x86intrin.h>
+#include <asm/prctl.h>
+#include <sys/prctl.h>
+
+#ifdef __x86_64__
+int arch_prctl(int code, unsigned long *addr);
+#define ARCH_CET_ALLOC_SHSTK 0x3004
+#endif
 
 /* Pull in AR_xyz defines. */
 typedef unsigned int u32;

@@ -766,6 +774,20 @@ int main()
 	int total_nerrs = 0;
 	unsigned short my_cs, my_ss;
 
+#ifdef __x86_64__
+	/* Alloc a shadow stack within 32-bit address range */
+	unsigned long arg, ssp_64, ssp_32;
+	ssp_64 = _get_ssp();
+
+	if (ssp_64 != 0) {
+		arg = 0x1001;
+		arch_prctl(ARCH_CET_ALLOC_SHSTK, &arg);
+		ssp_32 = arg + 0x1000 - 8;
+		asm volatile("RSTORSSP (%0)\n":: "r" (ssp_32));
+		asm volatile("SAVEPREVSSP");
+	}
+#endif