Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)

[Florian Weimer]

* Mathieu Desnoyers:

> ----- On Jun 12, 2019, at 4:22 PM, Florian Weimer fweimer@xxxxxxxxxx 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 ?

In my experience, __libc_multiple_libcs is not reliable.  I have not yet
figured out why.

Thanks,
Florian