Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)
From: Florian Weimer
Date: Wed Jun 12 2019 - 10:48:51 EST
* 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
>> 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.
> 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.