tests/tcg/aarch64: Add gcsss

Validate stack switching and recursion depth.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20251008215613.300150-74-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

tests/tcg/aarch64/Makefile.target

@@ -76,7 +76,7 @@ $(SME_TESTS): CFLAGS += $(CROSS_AS_HAS_ARMV9_SME)
 endif
 
 # GCS Tests
-GCS_TESTS += gcsstr gcspushm
+GCS_TESTS += gcsstr gcspushm gcsss
 AARCH64_TESTS += $(GCS_TESTS)
 $(GCS_TESTS): gcs.h
 

tests/tcg/aarch64/gcs.h

@@ -69,3 +69,12 @@
 
 #define gcspr() \
     ({ uint64_t *r; asm volatile("mrs %0, s3_3_c2_c5_1" : "=r"(r)); r; })
+
+#define gcsss1(val) \
+    do {                                                                     \
+        asm volatile("sys #3, c7, c7, #2, %0" : : "r"(val) : "memory");      \
+    } while (0)
+
+#define gcsss2() \
+    ({ uint64_t *r;                                                          \
+       asm volatile("sysl %0, #3, c7, c7, #3" : "=r"(r) : : "memory"); r; })

tests/tcg/aarch64/gcsss.c

@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include "gcs.h"
+
+#define IN_PROGRESS(X)  ((uint64_t)(X) | 5)
+#define CAP(X)          (((uint64_t)(X) & ~0xfff) + 1)
+
+static uint64_t * __attribute__((noinline)) recurse(size_t index)
+{
+    if (index == 0) {
+        return gcspr();
+    }
+    return recurse(index - 1);
+}
+
+int main()
+{
+    void *tmp;
+    uint64_t *alt_stack, *alt_cap;
+    uint64_t *orig_pr, *orig_cap;
+    uint64_t *bottom;
+    size_t pagesize = getpagesize();
+    size_t words;
+
+    enable_gcs(0);
+    orig_pr = gcspr();
+
+    /* Allocate a guard page before and after. */
+    tmp = mmap(0, 3 * pagesize, PROT_NONE, MAP_ANON | MAP_PRIVATE, -1, 0);
+    assert(tmp != MAP_FAILED);
+
+    /* map_shadow_stack won't replace existing mappings */
+    munmap(tmp + pagesize, pagesize);
+
+    /* Allocate a new stack between the guards. */
+    alt_stack = (uint64_t *)
+        syscall(__NR_map_shadow_stack, tmp + pagesize, pagesize,
+                SHADOW_STACK_SET_TOKEN);
+    assert(alt_stack == tmp + pagesize);
+
+    words = pagesize / 8;
+    alt_cap = alt_stack + words - 1;
+
+    /* SHADOW_STACK_SET_TOKEN set the cap. */
+    assert(*alt_cap == CAP(alt_cap));
+
+    /* Swap to the alt stack, one step at a time. */
+    gcsss1(alt_cap);
+
+    assert(gcspr() == alt_cap);
+    assert(*alt_cap == IN_PROGRESS(orig_pr));
+
+    orig_cap = gcsss2();
+
+    assert(orig_cap == orig_pr - 1);
+    assert(*orig_cap == CAP(orig_cap));
+    assert(gcspr() == alt_stack + words);
+
+    /* We should be able to use the whole stack. */
+    bottom = recurse(words - 1);
+    assert(bottom == alt_stack);
+
+    /* We should be back where we started. */
+    assert(gcspr() == alt_stack + words);
+
+    /* Swap back to the original stack. */
+    gcsss1(orig_cap);
+    tmp = gcsss2();
+
+    assert(gcspr() == orig_pr);
+    assert(tmp == alt_cap);
+
+    exit(0);
+}