[PATCH] RFC: Provide a function to reset IFUNC PLTs

[Adhemerval Zanella Netto]

On 07/03/23 05:40, Florian Weimer via Libc-alpha wrote:
> * Jan Kratochvil via Libc-alpha:
> 
>> Some projects snapshot+restore process images to migrate them to
>> a different machine. The target machine may have different (particularly
>> lower) set of CPU features. Restored process does crash in glibc IFUNC
>> functions which have been already set in PLTs due to the former more
>> rich CPU features on the snapshotting machine.
>>
>> Providing a glibc function which can be called during the restore.
>>
>> I understand the code may need more adjustments before its upstreaming
>> but is this an acceptable approach?
> 
> Do you have a high-level overview how this is supposed to work?  Is CRIU
> expected to call the new _dl_reset_ifunc symbol after restore?
> 
> Use of <sys/platform/x86.h> is somewhat rare.  Not even GCC uses it
> AFAIK.  It has its own cached CPU data used for target clones and
> similar features.  Re-running those IFUNC resolvers will just give the
> same results.

I am not sure if the ifunc reset will be really safe without adding CRIU
migrate sync points, to avoid suspend execution in a context that the
ifunc variants are already being executed or its address is being in a
function point (for instance in PLT code).

Besides, I also not sure if adding way to remove RELRO protection won't
add more security issues (we can disable for sesuid binaries, but even
though it is not a good security practice).

> 
> Distributions have started to deploy alternate builds in
> glibc-hwcaps/x86-64-v3 directories:
> 
>   openSUSE Tumbleweed gains optional x86-64-v3 optimization
>   <https://news.opensuse.org/2023/03/02/tw-gains-optional-optimizations/>
> 
> This might be a common situation fairly soon.  This is not IFUNC-based
> at all, so any IFUNC-based approach is not going to help with that.
> 
> Practically speaking, I think cluster heterogeneity needs to be hidden
> at the cluster level.


For glibc standpoint I think heterogeneity should be handled by masking
off the higher cpu features sets so the programs always run with the 
lowest ifunc variants set.