Allocate a small guard region around the shadow call stack.
This lets us do two things:
1) Make setjmp and longjmp compatible with shadow call stack.
To avoid leaking the shadow call stack address into memory, only the
lower log2(SCS_SIZE) bits of x18 are stored to jmp_buf. This requires
allocating an additional guard page so that we're guaranteed to be
able to allocate a sufficiently aligned SCS.
2) SCS overflow detection. Overflows now result in a SIGSEGV instead
of corrupting the allocation that comes after it.
Change-Id: I04d6634f96162bf625684672a87fba8b402b7fd1
Test: bionic-unit-tests
diff --git a/libc/private/bionic_constants.h b/libc/private/bionic_constants.h
index 9ae1c8d..e64c826 100644
--- a/libc/private/bionic_constants.h
+++ b/libc/private/bionic_constants.h
@@ -19,4 +19,14 @@
#define NS_PER_S 1000000000
+// Size of the shadow call stack. This must be a power of 2.
+#define SCS_SIZE (8 * 1024)
+
+// The shadow call stack is allocated at an aligned address within a guard region of this size. The
+// guard region must be large enough that we can allocate an SCS_SIZE-aligned SCS while ensuring
+// that there is at least one guard page after the SCS so that a stack overflow results in a SIGSEGV
+// instead of corrupting the allocation that comes after it.
+// TODO(b/118642754): Use a larger guard region.
+#define SCS_GUARD_REGION_SIZE (SCS_SIZE * 2)
+
#endif // _BIONIC_CONSTANTS_H_