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Date: Sun, 10 Feb 2019 01:53:02 +0300
From: Alexey Izbyshev <>
Subject: Re: dlsym(handle) may search in unrelated libraries

(Replying to

> On Thu, Feb 07, 2019 at 07:36:38PM +0100, Markus Wichmann wrote:
> > On Thu, Feb 07, 2019 at 11:54:47AM -0500, Rich Felker wrote:
> > > I'm not sure if it makes sense to build both of these lists at the
> > > same time. We currently try to avoid allocating dependency lists for
> > > libraries loaded at startup in order not to allocate and setup data
> > > that might never be used, defering until if/when dlopen is called on
> > > them. I've wanted to do the ctor order walk without allocating a
> > > dependency list, but I don't know a good way to do so. Note that the
> > > code that runs ctors cannot allocate because, at the time it runs,
> > > dlopen has already "committed" the load and can't back it out. It has
> > > to have all resources it needs for the walk precommitted.
> >
> > Depth first means stack. Means recursion or explicit stack. Explicit is
> > going to be hard, considering there is no known limit to dependency
> > nesting depth. We could argue that the user better make their stack size
> > ulimit large enough for the main thread, but I hazard a guess that
> > nobody expects dlopen() to use overly much stack in other threads.
> >
> > Alright, what's the algorithm here?
> >
> > init(lib):
> >     if (!lib.inited):
> >         foreach d in lib.deps init(d)
> >         start_init(lib)
> >         lib.inited = 1
> >
> > That about right? Because that means we need a stack as high as the
> > nesting depth of dependencies. We can get a pessimistic estimation from
> > the number of deps loaded, but that may be way too high (see below).
> Yes, but you can also avoid recursion just by looping to the deepest
> dependency with !inited, then going back to the root. For a one-time
> operation at dlopen-time or program-start time, the quadratic search
> for each !inited seems unlikely to be a problem:
> > > Since the beginning of a list of breadth-first dependencies is just
> > > the direct dependencies, if we recorded the number of direct
> > > dependencies for each dso, the breadth-first lists could be used to
> > > perform a depth-first walk to run ctors; this can be made
> > > non-recursive, at the cost of quadratic time but with trivially-tiny
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > constant factor, by simply restarting at the root of the tree after
>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > each node and finding the deepest not-yet-constructed dso. This
>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We'd need breadth-first lists for all dsos to implement DFS as you 
suggest because a single breadth-first list for the root is not enough 
to reconstruct all edges of the dependency graph even if we know the 
number of direct dependencies for each dso (we need to process each edge 
for DFS to work correctly). Or am I misunderstanding your suggestion?

If we're willing to add some fields to struct dso to implement the 
correct ctor order, I'd suggest to use a non-recursive DFS with an 
explicit stack instead. We'd need just two fields in each stack slot 
(dso and the number of processed DT_NEEDED entries), so the memory 
overhead is low compared to sizeof(struct dso). The stack can be 
preallocated before "committing" dlopen(). As Markus said, the upper 
bound is the number of dsos loaded with this dlopen() call because all 
other dsos are already constructed, so we don't need to visit them. This 
stack can be freed immediately after ctors are run. We already have 
dso->constructed to mark visited dsos, and we already use another list 
for finalization (dso->fini_next), and that list would be built 
naturally by DFS.


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