Re: [PATCH][BZ #19329] Fix race between tls allocation at thread creation and dlopen

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 13/01/16 02:34, Carlos O'Donell wrote:
> On 01/11/2016 11:32 AM, Szabolcs Nagy wrote:
> > 2016-01-11  Szabolcs Nagy  <szabolcs.nagy@arm.com>
> >
> >      [BZ #19329]
> >      * elf/dl-open.c (dl_open_worker): Write GL(dl_tls_generation)
> >      atomically.
> >      * elf/dl-tls.c (_dl_allocate_tls_init): Read GL(dl_tls_generation),
> >      GL(dl_tls_max_dtv_idx), slotinfo entries and listp->next atomically.
> >      (_dl_add_to_slotinfo): Write the slotinfo entries and listp->next
> >      atomically.
>
> You are headed in the right direction. I like where this patch is going,
> but don't have enough time to review this in detail yet.

will there be time before 2.23?

> At first glance your patch lacks sufficient concurrency documentation to
> be acceptable. You need to document which acquires the releases
> synchronizes-with. Please grep for "CONCURRENCY NOTES" to see the level
> of detail we need to maintain these kinds of changes.

i wanted to avoid documenting all the mess in the dynamic linker,
but i can improve the comments.

i see the following bugs:

1) pthread_create is not synced with dlopen
2) pthread_create is not synced with dlclose
3) tls access is not synced with dlopen
4) tls access is not synced with dlclose
5) tls access can oom crash
6) tls access is not as-safe
7) dlopen holds a global lock during ctors

i can fix 1) by adding some atomics (this is the current patch).

for 2) that's not enough, because dlclose has to wait with the
free(link_map) while pthread_create is initializing the tls.

i guess 3) can be fixed similarly to 1) but i don't have a
test case for that.

the rest is harder to fix.

is it ok to only fix 1) for 2.23?
or should i try to fix 3) as well (races on the same globals)?