[PATCH v7 18/23] aarch64: Handle GCS marking

[Adhemerval Zanella Netto]

On 03/01/25 12:41, Yury Khrustalev wrote:
> From: Szabolcs Nagy <szabolcs.nagy@arm.com>
> 
>  - Handle GCS marking
>  - Use l_searchlist.r_list for gcs (allows using the
>    same function for static exe)
> ---
>  sysdeps/aarch64/Makefile  |  4 ++-
>  sysdeps/aarch64/dl-gcs.c  | 71 +++++++++++++++++++++++++++++++++++++++
>  sysdeps/aarch64/dl-prop.h | 15 ++++++---
>  sysdeps/aarch64/linkmap.h |  1 +
>  4 files changed, 85 insertions(+), 6 deletions(-)
>  create mode 100644 sysdeps/aarch64/dl-gcs.c
> 
> diff --git a/sysdeps/aarch64/Makefile b/sysdeps/aarch64/Makefile
> index ca8b96f550..74479604f2 100644
> --- a/sysdeps/aarch64/Makefile
> +++ b/sysdeps/aarch64/Makefile
> @@ -9,7 +9,9 @@ LDFLAGS-rtld += -Wl,-z,force-bti,--fatal-warnings
>  endif
>  
>  ifeq ($(subdir),elf)
> -sysdep-dl-routines += dl-bti
> +sysdep-dl-routines += \
> +  dl-bti \
> +  dl-gcs
>  
>  tests += tst-audit26 \
>  	 tst-audit27
> diff --git a/sysdeps/aarch64/dl-gcs.c b/sysdeps/aarch64/dl-gcs.c
> new file mode 100644
> index 0000000000..dc62908d13
> --- /dev/null
> +++ b/sysdeps/aarch64/dl-gcs.c
> @@ -0,0 +1,71 @@
> +/* AArch64 GCS functions.
> +   Copyright (C) 2024-2025 Free Software Foundation, Inc.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <unistd.h>
> +#include <errno.h>
> +#include <libintl.h>
> +#include <ldsodefs.h>
> +
> +static void
> +fail (struct link_map *l, const char *program)
> +{
> +  if (program && program[0])
> +    _dl_fatal_printf ("%s: %s: %s\n", program, l->l_name, N_("not GCS compatible"));
> +  else if (program)
> +    _dl_fatal_printf ("%s\n", N_("not GCS compatible"));
> +  else
> +    _dl_signal_error (0, l->l_name, "dlopen", N_("not GCS compatible"));
> +}

I don't think N_ would make a difference here, it is not support on loader
and adding on the translation database does not add much without revamping
how to translation during program loading.

> +
> +static void
> +unsupported (void)
> +{
> +  _dl_fatal_printf ("%s\n", N_("unsupported GCS policy"));
> +}
> +
> +static void
> +check_gcs (struct link_map *l, const char *program)
> +{
> +  bool for_dlopen = program == NULL;
> +  if (!l->l_mach.gcs)
> +    {
> +      if (GLRO(dl_aarch64_gcs_policy) == 2 || for_dlopen)
> +	fail (l, program);
> +      if (GLRO(dl_aarch64_gcs_policy) == 1)
> +	GL(dl_aarch64_gcs) = 0;
> +      else
> +	unsupported ();
> +    }
> +}
> +

This requires to update the glibc.cpu.aarch64_gcs_policy entry at 
manual/tunables.texi with the description of what each value represents.

> +/* Apply GCS policy for L and its dependencies.
> +   PROGRAM is NULL when this check is invoked for dl_open.  */
> +
> +void
> +_dl_gcs_check (struct link_map *l, const char *program)
> +{
> +  /* GCS is disabled.  */
> +  if (GL(dl_aarch64_gcs) == 0)
> +    return;
> +  /* GCS marking is ignored.  */
> +  if (GLRO(dl_aarch64_gcs_policy) == 0)
> +    return;
> +
> +  check_gcs (l, program);
> +  for (unsigned int i = 0; i < l->l_searchlist.r_nlist; i++)
> +    check_gcs (l->l_searchlist.r_list[i], program);
> +}
> diff --git a/sysdeps/aarch64/dl-prop.h b/sysdeps/aarch64/dl-prop.h
> index 361fc593da..abca2be7fa 100644
> --- a/sysdeps/aarch64/dl-prop.h
> +++ b/sysdeps/aarch64/dl-prop.h
> @@ -24,16 +24,21 @@ extern void _dl_bti_protect (struct link_map *, int) attribute_hidden;
>  extern void _dl_bti_check (struct link_map *, const char *)
>      attribute_hidden;
>  
> +extern void _dl_gcs_check (struct link_map *, const char *)
> +    attribute_hidden;
> +
>  static inline void __attribute__ ((always_inline))
>  _rtld_main_check (struct link_map *m, const char *program)
>  {
>    _dl_bti_check (m, program);
> +  _dl_gcs_check (m, program);
>  }
>  
>  static inline void __attribute__ ((always_inline))
>  _dl_open_check (struct link_map *m)
>  {
>    _dl_bti_check (m, NULL);
> +  _dl_gcs_check (m, NULL);
>  }
>  
>  static inline void __attribute__ ((always_inline))
> @@ -45,10 +50,6 @@ static inline int
>  _dl_process_gnu_property (struct link_map *l, int fd, uint32_t type,
>  			  uint32_t datasz, void *data)
>  {
> -  if (!GLRO(dl_aarch64_cpu_features).bti)
> -    /* Skip note processing.  */
> -    return 0;
> -
>    if (type == GNU_PROPERTY_AARCH64_FEATURE_1_AND)
>      {
>        /* Stop if the property note is ill-formed.  */
> @@ -57,7 +58,11 @@ _dl_process_gnu_property (struct link_map *l, int fd, uint32_t type,
>  
>        unsigned int feature_1 = *(unsigned int *) data;
>        if (feature_1 & GNU_PROPERTY_AARCH64_FEATURE_1_BTI)
> -	_dl_bti_protect (l, fd);
> +	if (GLRO(dl_aarch64_cpu_features).bti)
> +	  _dl_bti_protect (l, fd);
> +
> +      if (feature_1 & GNU_PROPERTY_AARCH64_FEATURE_1_GCS)
> +	l->l_mach.gcs = 1;
>  
>        /* Stop if we processed the property note.  */
>        return 0;
> diff --git a/sysdeps/aarch64/linkmap.h b/sysdeps/aarch64/linkmap.h
> index df6d3c66e8..e56c890aea 100644
> --- a/sysdeps/aarch64/linkmap.h
> +++ b/sysdeps/aarch64/linkmap.h
> @@ -23,4 +23,5 @@ struct link_map_machine
>    ElfW(Addr) plt;	  /* Address of .plt */
>    void *tlsdesc_table;	  /* Address of TLS descriptor hash table.  */
>    bool bti_fail;	  /* Failed to enable Branch Target Identification.  */
> +  bool gcs;		  /* Guarded Control Stack marking.  */
>  };