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Date: Sat, 4 Mar 2017 11:58:18 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: Reviving planned ldso changes
* Rich Felker <dalias@...c.org> [2017-03-02 20:30:26 -0500]:
> Here's a v4 of the patch that saves the "init parent" we descended
> from so that it can return where it left off. There are a couple
> gratuitous hunks left over adding setting of "needed_by" where it made
> sense to be set, but it's not actually used anymore. They could be
> dropped if desired but are probably nice to keep for the sake of
> consistency of data, even thoough it's data we don't use.
>
> I believe this can be extended to allow concurrent dlopen by amending
> the case in the tree-walk where a dependency isn't constructed yet but
> already has an "init parent" to check whether it's
> pending-construction in the calling thread (recursive dlopen from a
> ctor) or another thread; in the former case (as now) treat it as
> already-constructed; in the latter, wait on a condvar that gets
> signaled at the end of each construction, then continue the loop
> without advancing p. There are probably some subtleties I'm missing,
> though.
...
> static void do_init_fini(struct dso *p)
> {
> size_t dyn[DYN_CNT];
> - int need_locking = libc.threads_minus_1;
> - /* Allow recursive calls that arise when a library calls
> - * dlopen from one of its constructors, but block any
> - * other threads until all ctors have finished. */
> - if (need_locking) pthread_mutex_lock(&init_fini_lock);
> - for (; p; p=p->prev) {
> - if (p->constructed) continue;
> + pthread_mutex_lock(&init_fini_lock);
> + /* Construct in dependency order without any recursive state. */
> + while (p && !p->constructed) {
> + /* The following loop descends into the first dependency
> + * that is neither alredy constructed nor pending
> + * construction due to circular deps, stopping only
> + * when it reaches a dso with no remaining dependencies
> + * to descend into. */
> + while (p->deps && p->deps[p->next_dep]) {
> + if (!p->deps[p->next_dep]->constructed &&
> + !p->deps[p->next_dep]->init_parent) {
> + p->deps[p->next_dep]->init_parent = p;
> + p = p->deps[p->next_dep++];
i think the root may be visited twice because it
has no init_parent, which may be problematic with
the concurrent dlopen (and can cause unexpected
ctor order: the root node is not constructed last
if there is a cycle through it)
i think only checking init_parent of a dep is
enough and the root node can have a dummy parent
that is guaranteed to be not a dependency (ldso?)
and constructed so it stops the loop.
> + } else {
> + p->next_dep++;
> + }
> + }
> p->constructed = 1;
> decode_vec(p->dynv, dyn, DYN_CNT);
> if (dyn[0] & ((1<<DT_FINI) | (1<<DT_FINI_ARRAY))) {
> @@ -1233,17 +1248,19 @@ static void do_init_fini(struct dso *p)
> size_t *fn = laddr(p, dyn[DT_INIT_ARRAY]);
> while (n--) ((void (*)(void))*fn++)();
> }
> - if (!need_locking && libc.threads_minus_1) {
> - need_locking = 1;
> - pthread_mutex_lock(&init_fini_lock);
> - }
> - }
> - if (need_locking) pthread_mutex_unlock(&init_fini_lock);
> + /* Revisit "parent" dso which caused the just-constructed
> + * dso to be pulled in as a dependency. On the next loop
> + * iteration we will either descend to construct a sibling
> + * of the just-constructed dso, or finish constructing the
> + * parent if no unfinished deps remain. */
> + p = p->init_parent;
> + }
> + pthread_mutex_unlock(&init_fini_lock);
> }
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