From: Szabolcs Nagy <szabolcs.nagy@arm.com>
Use the dynamic linker start code to enable GCS in the dynamic linked
case after _dl_start returns and before _dl_start_user which marks
the point after which user code may run.
Like in the static linked case this ensures that GCS is enabled on a
top level stack frame.
---
sysdeps/aarch64/Makefile | 4 +++-
sysdeps/aarch64/dl-start.S | 23 +++++++++++++++++++++--
sysdeps/aarch64/rtld-global-offsets.sym | 5 +++++
3 files changed, 29 insertions(+), 3 deletions(-)
@@ -35,7 +35,9 @@ endif
ifeq ($(subdir),elf)
sysdep-rtld-routines += dl-start
sysdep-dl-routines += tlsdesc dl-tlsdesc
-gen-as-const-headers += dl-link.sym
+gen-as-const-headers += \
+ dl-link.sym \
+ rtld-global-offsets.sym
tests-internal += tst-ifunc-arg-1 tst-ifunc-arg-2
@@ -18,6 +18,7 @@
<https://www.gnu.org/licenses/>. */
#include <sysdep.h>
+#include <rtld-global-offsets.h>
ENTRY (_start)
/* Create an initial frame with 0 LR and FP */
@@ -25,11 +26,30 @@ ENTRY (_start)
mov x29, #0
mov x30, #0
+ /* Load and relocate all library dependencies. */
mov x0, sp
PTR_ARG (0)
bl _dl_start
/* Returns user entry point in x0. */
mov PTR_REG (21), PTR_REG (0)
+
+ /* Use GL(dl_aarch64_gcs) to set the shadow stack status. */
+ adrp x16, _rtld_local
+ add PTR_REG (16), PTR_REG (16), :lo12:_rtld_local
+ ldr x1, [x16, GL_DL_AARCH64_GCS_OFFSET]
+ cbz x1, L(skip_gcs_enable)
+
+ /* Enable GCS before user code runs. Note that IFUNC resolvers and
+ LD_AUDIT hooks may run before, but should not create threads. */
+#define PR_SET_SHADOW_STACK_STATUS 75
+ mov x0, PR_SET_SHADOW_STACK_STATUS
+ mov x2, 0
+ mov x3, 0
+ mov x4, 0
+ mov x8, #SYS_ify(prctl)
+ svc 0x0
+L(skip_gcs_enable):
+
.globl _dl_start_user
.type _dl_start_user, %function
_dl_start_user:
@@ -40,8 +60,7 @@ _dl_start_user:
/* Compute envp. */
add PTR_REG (3), PTR_REG (2), PTR_REG (1), lsl PTR_LOG_SIZE
add PTR_REG (3), PTR_REG (3), PTR_SIZE
- adrp x16, _rtld_local
- add PTR_REG (16), PTR_REG (16), :lo12:_rtld_local
+ /* Run the init functions of the loaded modules. */
ldr PTR_REG (0), [x16]
bl _dl_init
/* Load the finalizer function. */
@@ -3,8 +3,13 @@
#include <ldsodefs.h>
#define GLRO_offsetof(name) offsetof (struct rtld_global_ro, _##name)
+#define GL_offsetof(name) offsetof (struct rtld_global, _##name)
-- Offsets of _rtld_global_ro in libc.so
GLRO_DL_HWCAP_OFFSET GLRO_offsetof (dl_hwcap)
GLRO_DL_HWCAP2_OFFSET GLRO_offsetof (dl_hwcap2)
+
+-- Offsets of _rtld_global in libc.so
+
+GL_DL_AARCH64_GCS_OFFSET GL_offsetof (dl_aarch64_gcs)