Re: PATCH: Check GLIBC_IFUNC to enable/disable ifunc features

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 30/06/16 03:56, H.J. Lu wrote:
> The environment
> variable, GLIBC_IFUNC=xxx=0:yyy=1:zzz=0...., can be used to enable
> CPU/ARCH feature yyy, disable CPU/ARCH feature yyy and zzz, where the
> feature name is case-sensitive and has to match the ones in
> cpu-features.h.  It can be used by glibc developers to override the
> IFUNC selection to improve performance for a particular workload or
> tune for a new processor.
> 

since it is for glibc devs only is it expected to be
stable across glibc versions?

may be the env var name could reflect that it is not
a public api.

> Since all CPU/ARCH features are hardware optimizations without security
> implication, except for Prefer_MAP_32BIT_EXEC, which can only be disabled,
> we check GLIBC_IFUNC for programs, including set*id ones.
> 

i think it can have security implications, but probably not significant.

> NOTE: the IFUNC selection may change over time.  Please check all
> multiarch implementations when experimenting.
> 

> +init_cpu_features (struct cpu_features *cpu_features, char **env)
...
> +  while (*env != NULL)
> +    {
> +      const char *p, *end;
> +      size_t len = sizeof ("GLIBC_IFUNC=");
> +
> +      end = *env;
> +      for (p = end; *p != '\0'; p++)
> +	if (--len == 0 && equal (end, "GLIBC_IFUNC=", 12))
> +	  {
> +	    len = strlen (p);

is this x86_64 only?

how can strlen work before ifunc is done?
(i think strlen is ifunc resolved on i386)

i know ld.so is careful about this, but i think
with static linking ifunc resolved functions
should not be called before apply_irel is done