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

----- On Jun 14, 2019, at 12:06 PM, Florian Weimer fweimer@redhat.com wrote:

* Mathieu Desnoyers:

quoted

----- On Jun 12, 2019, at 4:00 PM, Mathieu Desnoyers
mathieu.desnoyers@efficios.com wrote:

quoted

----- On Jun 10, 2019, at 4:43 PM, carlos carlos@redhat.com wrote:

quoted

On 6/6/19 7:57 AM, Florian Weimer wrote:

quoted

Let me ask the key question again: Does it matter if code observes the
rseq area first without kernel support, and then with kernel support?
If we don't expect any problems immediately, we do not need to worry
much about the constructor ordering right now.  I expect that over time,
fixing this properly will become easier.

I just wanted to chime in and say that splitting this into:

* Ownership (__rseq_handled)

* Initialization (__rseq_abi)

Makes sense to me.

I agree we need an answer to this question of ownership but not yet
initialized, to owned and initialized.

I like the idea of having __rseq_handled in ld.so.

Very good, so I'll implement this approach. Sorry for the delayed
feedback, I am traveling this week.

I had issues with cases where application or LD_PRELOAD library also
define the __rseq_handled symbol. They appear not to see the same
address as the one initialized by ld.so.

What exactly did you do?  How did you determine the addresses?  How is
__rseq_handled defined in ld.so?

The easiest way to answer these questions is through links to my github
dev branch:

https://github.com/compudj/glibc-dev/tree/glibc-rseq

specifically this commit:
https://github.com/compudj/glibc-dev/commit/c49a286497d065a7fc00aafd846e6edce14f97fc
and this attempt at using GL():
https://github.com/compudj/glibc-dev/commit/8a02acfbb6943672bfa36b4fc6f61905ee4fa180

My test programs are:

* a.c:

#include <stdio.h>
#include <linux/rseq.h>

extern __thread struct rseq __rseq_abi
__attribute__ ((tls_model ("initial-exec")));/* = {
	.cpu_id = -1,
};*/
extern int __rseq_handled;

int main()
{
	fprintf(stderr, "__rseq_handled main: %d %p\n", __rseq_handled, &__rseq_handled);
	fprintf(stderr, "__rseq_abi.cpu_id main: %d %p\n", __rseq_abi.cpu_id, &__rseq_abi);
	return 0;
}

* s.c:

#include <stdio.h>
#include <linux/rseq.h>

#if 0
__thread struct rseq __rseq_abi
__attribute__ ((tls_model ("initial-exec"))) = {
	.cpu_id = -1,
};
int __rseq_handled;

#else
extern __thread struct rseq __rseq_abi
__attribute__ ((tls_model ("initial-exec")));
extern int __rseq_handled;
#endif

void __attribute__((constructor)) myinit(void)
{
	fprintf(stderr, "__rseq_handled s.so: %d %p\n", __rseq_handled, &__rseq_handled);
	fprintf(stderr, "__rseq_abi.cpu_id s.so: %d %p\n", __rseq_abi.cpu_id, &__rseq_abi);
}

* Makefile:

LIBCPATH=/home/efficios/glibc-test/lib
KERNEL_HEADERS=/home/efficios/git/linux-percpu-dev/usr/include
CFLAGS=-I${KERNEL_HEADERS} -L${LIBCPATH} -Wl,--rpath=${LIBCPATH} -Wl,--dynamic-linker=${LIBCPATH}/ld-linux-x86-64.so.2

all:
	gcc ${CFLAGS} -o a a.c
	gcc ${CFLAGS} -shared -fPIC -o s.so s.c

Thanks,

Mathieu

Thanks,
Florian

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com