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Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)
- From: Florian Weimer <fweimer at redhat dot com>
- To: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- Cc: carlos <carlos at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>, Szabolcs Nagy <szabolcs dot nagy at arm dot com>, libc-alpha <libc-alpha at sourceware dot org>, Thomas Gleixner <tglx at linutronix dot de>, Ben Maurer <bmaurer at fb dot com>, Peter Zijlstra <peterz at infradead dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Boqun Feng <boqun dot feng at gmail dot com>, Will Deacon <will dot deacon at arm dot com>, Dave Watson <davejwatson at fb dot com>, Paul Turner <pjt at google dot com>, Rich Felker <dalias at libc dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, linux-api <linux-api at vger dot kernel dot org>
- Date: Wed, 12 Jun 2019 16:43:58 +0200
- Subject: Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)
- References: <20190503184219.19266-1-mathieu.desnoyers@efficios.com> <87muj2k4ov.fsf@oldenburg2.str.redhat.com> <1528929896.22217.1559326257155.JavaMail.zimbra@efficios.com> <87o93d4lqb.fsf@oldenburg2.str.redhat.com> <117220011.27079.1559663870037.JavaMail.zimbra@efficios.com> <87wohzorj0.fsf@oldenburg2.str.redhat.com> <1329439108.43041.1560348962006.JavaMail.zimbra@efficios.com> <87blz27uio.fsf@oldenburg2.str.redhat.com> <512441263.43096.1560350163180.JavaMail.zimbra@efficios.com>
* Mathieu Desnoyers:
> ----- 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 ?
In my experience, __libc_multiple_libcs is not reliable. I have not yet
figured out why.
Thanks,
Florian