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

From: Florian Weimer
Date: Wed Jun 12 2019 - 10:27:44 EST

* Mathieu Desnoyers:

>> It's the registration from 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

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))

_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.