PATCH: Check GLIBC_IFUNC to enable/disable ifunc features

[H.J. Lu]

On Thu, Jun 30, 2016 at 9:06 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Thu, Jun 30, 2016 at 7:39 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>> 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?
>
> SInce INFUNC implementation changes over time, it won't be
> stable.
>
>> may be the env var name could reflect that it is not
>> a public api.
>
> GLIBC_IFUNC is private.
>
>>> 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.
>
> If there are security implications in IFUNC implementation,
> it is a bug in IFUNC implementation.
>
>>> 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?
>
> No.  It is in sysdeps/x86/cpu-features.c, which is shared by
> i386 and x86_64.
>
>> 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
>>
>
> I will remove strlen to be on the safe side.
>

Here is the updated patch with strlen removed.  We don't
need to check if env != NULL since it will never be NULL.


-- 
H.J.
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