[PATCH v5 13/23] aarch64: Enable GCS in dynamic linked exe

[Carlos O'Donell]

On 12/6/24 8:29 AM, Yury Khrustalev wrote:
> 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.

Internal only changes.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>  sysdeps/aarch64/Makefile                |  4 +++-
>  sysdeps/aarch64/dl-start.S              | 23 +++++++++++++++++++++--
>  sysdeps/aarch64/rtld-global-offsets.sym |  5 +++++
>  3 files changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/sysdeps/aarch64/Makefile b/sysdeps/aarch64/Makefile
> index 141d7d9cc2..ca8b96f550 100644
> --- a/sysdeps/aarch64/Makefile
> +++ b/sysdeps/aarch64/Makefile
> @@ -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
>  
> diff --git a/sysdeps/aarch64/dl-start.S b/sysdeps/aarch64/dl-start.S
> index d645484e79..bca5c408af 100644
> --- a/sysdeps/aarch64/dl-start.S
> +++ b/sysdeps/aarch64/dl-start.S
> @@ -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

OK. Inline the syscall, and use a constant value.

> +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.  */
> diff --git a/sysdeps/aarch64/rtld-global-offsets.sym b/sysdeps/aarch64/rtld-global-offsets.sym
> index 23cdaf7d9e..6c0690bb95 100644
> --- a/sysdeps/aarch64/rtld-global-offsets.sym
> +++ b/sysdeps/aarch64/rtld-global-offsets.sym
> @@ -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)