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

* Mathieu Desnoyers:

----- On Jun 12, 2019, at 4:22 PM, Florian Weimer fweimer@redhat.com wrote:

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* Mathieu Desnoyers:

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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