Re: [PATCH v2 07/12] elf: Move vDSO setup to rtld (BZ#24967)

[Florian Weimer <fweimer at redhat dot com>]

* Adhemerval Zanella:

>> This IFUNC resolver is still not valid because _rtld_global_ro has a
>> relocation dependency.
>
> Afaik with current ld guaranties, .rel{a}.dyn will be sort prior ifunc
> so _rtld_global_ro should be reallocated prior the ifunc itself.

But that's intra-DSO.  It does not necessarily affect cross-DSO
relocation ordering.  In theory, the DT_NEEDED ordering should make this
work, except for the static dlopen case, where you get a
default-initialized _rtld_global_ro from the inactive (inner) loader.
The latter is why some architectures use _dl_var_init for essential
non-loader data, and why getauxval is currently broken for something
that is dlopen'ed from a statically linked program (bug 20802).

>> What we should do instead is to patch the vDSO function pointers (and
>> pretty much all shared variables, including the page size) into
>> libc.so.6 right after loading it (before relocation).  I had hoped to
>> post a patch for this, but the prerequisite
>> 
>>   <https://sourceware.org/ml/libc-alpha/2019-11/msg00977.html>
>> 
>> has not been reviewed.  It provides _dl_lookup_direct, which we could
>> use to get access to this global variables structure very efficiently.
>
> We can go on this way (which is a rather complex solution with might add
> even more pitfalls). However another much simple possibility is to make 
> gettimeoday and time use normal function that calls the vDSO directly.  
> On x86_64 haswell (i7-4790K) I see just 7% increase of latency to by 
> using INLINE_VSYSCALL with my patch, which I think it quite acceptable. 

It's not more complex overall because we already need _dl_var_init on
some architectures.  This new mechanism would replace that and do away
with undo-RELRO/_dl_var_init/reapply-RELRO sequence.

Thanks,
Florian