[PATCH v8 09/20] aarch64: Add glibc.cpu.aarch64_gcs tunable

[Adhemerval Zanella Netto]

On 14/01/25 13:03, Yury Khrustalev wrote:
> From: Szabolcs Nagy <szabolcs.nagy@arm.com>
> 
> This tunable is for controlling the GCS status. It is used to work
> out the argument to the PR_SET_SHADOW_STACK_STATUS prctl syscall.
> 
> By default it is 0, so GCS is disabled, value 1 will enable GCS.
> 
> The status is stored into GL(dl_aarch64_gcs) early and only applied
> later, since enabling GCS is tricky: it must happen on a top level
> stack frame. Using GL instead of GLRO because it may need updates
> depending on loaded libraries that happen after readonly protection
> is applied, however library marking based GCS setting is not yet
> implemented.
> 
> Describe new tunable in the manual.

LGTM with a nit below.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  manual/tunables.texi                          |  9 +++++
>  sysdeps/aarch64/dl-tunables.list              |  5 +++
>  .../unix/sysv/linux/aarch64/cpu-features.c    |  8 ++++
>  .../unix/sysv/linux/aarch64/dl-procruntime.c  | 37 +++++++++++++++++++
>  4 files changed, 59 insertions(+)
>  create mode 100644 sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c
> 
> diff --git a/manual/tunables.texi b/manual/tunables.texi
> index 1ac58b9ae9..118f490010 100644
> --- a/manual/tunables.texi
> +++ b/manual/tunables.texi
> @@ -669,6 +669,15 @@ This tunable is specific to x86-64 and effective only when the lazy
>  binding is disabled.
>  @end deftp
>  
> +@deftp Tunable glibc.cpu.aarch64_gcs
> +This tunable enables or disables Guarded Control Stack for a process.
> +Value @code{0} will leave GCS disabled and value @code{1} will enable
> +GCS via a @code{prctl} system call. Default is @code{0}, so GCS is
> +disabled by default.
> +
> +This tunable is specific to AArch64.
> +@end deftp
> +
>  @node Memory Related Tunables
>  @section Memory Related Tunables
>  @cindex memory related tunables
> diff --git a/sysdeps/aarch64/dl-tunables.list b/sysdeps/aarch64/dl-tunables.list
> index 5c1a172559..4b28341b72 100644
> --- a/sysdeps/aarch64/dl-tunables.list
> +++ b/sysdeps/aarch64/dl-tunables.list
> @@ -21,5 +21,10 @@ glibc {
>      name {
>        type: STRING
>      }
> +    aarch64_gcs {
> +      type: UINT_64
> +      minval: 0

Maybe add a 'maxval: 1' here to limit the possible tunable values. 

> +      default: 0
> +    }
>    }
>  }
> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> index 7ac228303f..1ecf6cd176 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> @@ -176,4 +176,12 @@ init_cpu_features (struct cpu_features *cpu_features)
>  
>    /* Check if MOPS is supported.  */
>    cpu_features->mops = GLRO (dl_hwcap2) & HWCAP2_MOPS;
> +
> +#ifndef HWCAP_GCS
> +#define HWCAP_GCS (1UL << 32)
> +#endif
> +
> +  if (GLRO (dl_hwcap) & HWCAP_GCS)
> +    /* GCS status may be updated later by binary compatibility checks.  */
> +    GL (dl_aarch64_gcs) = TUNABLE_GET (glibc, cpu, aarch64_gcs, uint64_t, 0);
>  }
> diff --git a/sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c b/sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c
> new file mode 100644
> index 0000000000..044544aa78
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/aarch64/dl-procruntime.c
> @@ -0,0 +1,37 @@
> +/* Data for processor runtime information.  AArch64 version.
> +   Copyright (C) 2024-2025 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   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/>.  */
> +
> +#ifndef PROCINFO_CLASS
> +# define PROCINFO_CLASS
> +#endif
> +
> +#if !IS_IN (ldconfig)
> +# if !defined PROCINFO_DECL && defined SHARED
> +  ._dl_aarch64_gcs
> +# else
> +PROCINFO_CLASS unsigned long _dl_aarch64_gcs
> +# endif
> +# ifndef PROCINFO_DECL
> += 0
> +# endif
> +# if !defined SHARED || defined PROCINFO_DECL
> +;
> +# else
> +,
> +# endif
> +#endif