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Date:   Wed, 12 Jun 2019 10:36:03 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
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        Peter Zijlstra <peterz@infradead.org>,
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        Will Deacon <will.deacon@arm.com>,
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Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup
 and thread creation (v10)
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Thread-Topic: glibc: Perform rseq(2) registration at C startup and thread creation (v10)
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----- On Jun 12, 2019, at 4:22 PM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>>> It's the registration from libc.so which needs some care.  In
>>> particular, we must not override an existing registration.
>>
>> OK, so it could check if __rseq_abi.cpu_id is -1, and only
>> perform registration if it is the case. Or do you have another
>> approach in mind ?
> 
> No, __rseq_abi will not be shared with the outer libc, so the inner libc
> will always see -1 there, even if the outer libc has performed
> registration.
> 
> libio/vtables.c has some example what you can do:
> 
>  /* In case this libc copy is in a non-default namespace, we always
>     need to accept foreign vtables because there is always a
>     possibility that FILE * objects are passed across the linking
>     boundary.  */
>  {
>    Dl_info di;
>    struct link_map *l;
>    if (!rtld_active ()
>        || (_dl_addr (_IO_vtable_check, &di, &l, NULL) != 0
>            && l->l_ns != LM_ID_BASE))
>      return;
>  }
> 
> _IO_vtable_check would have to be replaced with your own function; the
> actual function doesn't really matter.
> 
> The rtld_active check covers the static dlopen case, where
> rtld_active () is false in the inner libc.

Then out of curiosity, would it also work if I check for

if (!__libc_multiple_libcs)

in LIBC_START_MAIN ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com