Re: [PATCH v3] AArch64: Add ifunc masking tunable

On Mon, Jul 06, 2026 at 12:49:16PM +0000, Wilco Dijkstra wrote:
> v3: keep the tst-cpu-tunable-static-pie test
> 
> Remove the glibc.cpu.name tunable since it's unused and out of date.
> Add support for glibc.cpu.hwcaps to adjust ifunc selection for debugging
> and benchmarking (undocumented since this is for developers only).

The 'glibc.cpu.hwcaps' tunable is documented in the Glibc manual with
this note:

  This tunable is specific to i386, x86-64, s390x, powerpc and
  loongarch.

Now in this patch we are adding support for aarch64 but in a way that
does not follow what is documented for other targets. This might result
in confusion. Maybe we should add a comment to the manual that this
tunable is partially supported on aarch64 but only allows to turn off
some hwcaps and only those that don't affect security?

> Only allow disabling of features that are (a) used by ifuncs, (b) safe
> to disable to a more generic ifunc without any security impact.
>
> ...
> 
> diff --git a/sysdeps/aarch64/dl-tunables.list b/sysdeps/aarch64/dl-tunables.list
> index a2ccba0b293a2d728ec6986f7ddd5b9793c1de17..c876f3fe0d4891be425f0c022e4adcaa11f86af4 100644
> --- a/sysdeps/aarch64/dl-tunables.list
> +++ b/sysdeps/aarch64/dl-tunables.list
> @@ -18,7 +18,7 @@
>  
>  glibc {
>    cpu {
> -    name {
> +    hwcaps {
>        type: STRING
>      }

OK

>      aarch64_bti {
> diff --git a/sysdeps/unix/sysv/linux/aarch64/Makefile b/sysdeps/unix/sysv/linux/aarch64/Makefile
> index d1fcb48aa250dffd8b7e5b41f5b290512dc797e2..baefac15b5985414a5044766048cab5e84e4636f 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/Makefile
> +++ b/sysdeps/unix/sysv/linux/aarch64/Makefile
> @@ -17,7 +17,7 @@ tests += \
>  tests-static += \
>    tst-cpu-tunable-static-pie \
>    # tests-static
> -tst-cpu-tunable-static-pie-TUNABLES = glibc.cpu.name=generic
> +tst-cpu-tunable-static-pie-TUNABLES = glibc.cpu.hwcaps=-midr,-sve,-mops
>  endif
>  endif

OK

>  
> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> index 9a87332ac49c0272930200ece5276eb5a4a2fae4..6a13638368673870b9a9e35bbb43108612561ae9 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> @@ -33,56 +33,39 @@
>     to see when pointer have been correctly tagged.  */
>  #define MTE_ALLOWED_TAGS (0xfffe << PR_MTE_TAG_SHIFT)
>  
> -static const char cpu_list_name[] = {
> -  "kunpeng920\0"
> -  "kunpeng950\0"
> -  "a64fx\0"
> -  "generic\0",
> -};
> -
> -static const uint64_t cpu_list_midr[] = {
> -  0x481FD010,
> -  0x480FD060,
> -  0x460F0010,
> -  0x0,
> -};

OK

> -
> -static uint64_t
> -get_midr_from_mcpu (const struct tunable_str_t *mcpu)
> +static void
> +TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *val)
>  {
> -  const char *name = cpu_list_name;
> -  size_t offset = 0;
> -  for (int i = 0; i < array_length (cpu_list_midr); i++)
> +  struct cpu_features *cpu_features = &GLRO(dl_aarch64_cpu_features);
> +  struct tunable_str_comma_state_t cs;
> +  tunable_str_comma_init (&cs, val);
> +
> +  struct tunable_str_comma_t n;
> +  while (tunable_str_comma_next (&cs, &n))
>      {
> -      size_t len = strlen (name);
> -      if (tunable_strcmp (mcpu, cpu_list_name + offset, len))
> -	return cpu_list_midr[i];
> -      offset += len;
> +      /* Support disabling of features to select more generic ifuncs.  */
> +      if (!n.disable)
> +	continue;
> +      if (tunable_str_comma_strcmp_cte (&n, "midr"))

Should this (and similar values below) be '-midr' with the '-' used to
make sure we only support disabling?

> +	cpu_features->midr_el1 = 0;
> +      else if (tunable_str_comma_strcmp_cte (&n, "zva"))
> +	cpu_features->zva_size = 0;
> +      else if (tunable_str_comma_strcmp_cte (&n, "sve"))
> +	cpu_features->sve = false;
> +      else if (tunable_str_comma_strcmp_cte (&n, "sve2"))
> +	cpu_features->sve2 = false;
> +      else if (tunable_str_comma_strcmp_cte (&n, "mops"))
> +	cpu_features->mops = false;
>      }
> -  return UINT64_MAX;
>  }
>  
> ...
> 
> @@ -108,4 +91,8 @@ init_cpu_features (struct cpu_features *cpu_features)
>    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);
> +
> +  /* Allow override by glibc.cpu.hwcaps tunable after setting features.  */
> +  TUNABLE_GET (glibc, cpu, hwcaps, tunable_val_t *,
> +	       TUNABLE_CALLBACK (set_hwcaps));
>  }

OK

Thanks,
Yury