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Date: Fri, 22 May 2020 14:25:56 -0400
From: Rich Felker <>
To: Alexander Scherbatiy <>
Cc:, Markus Wichmann <>
Subject: Re: Shared library loading

On Fri, May 22, 2020 at 09:07:20PM +0300, Alexander Scherbatiy wrote:
> On 21.05.2020 18:21, Markus Wichmann wrote:
> >On Thu, May 21, 2020 at 02:27:24PM +0300, Alexander Scherbatiy wrote:
> >>Hello,
> >>
> >>I use Alpine Linux 3.11.6 with musl libc (x86_64) version 1.1.24.
> >>
> >>I have two shared libraries "a" and "b" each of which is placed in its own
> >>directory and lib "b" depends on "a".
> >>First, I use dlopen to load lib "a" with "RTLD_NOW | RTLD_GLOBAL" flags.
> >>Second, I load lib "b" with  flag "RTLD_LAZY".
> >>
> >>The "b" library loading works on my Ubuntu 19.10 and fails on Alpine 3.11.6
> >>with message:
> >>   dlopen failed: Error loading shared library No such file or
> >>directory (needed by /root/load-lib-sample/bin/b/
> >>Should it work on Alpine with musl libc as well?
> >[...]
> >
> >I shall leave the "Should it work" to others who know more about this
> >than me. But I will tackle the "Why doesn't it work". It appears however
> >the behavior is intentional (whether the consequences are as well
> >remains to be seen). Namely in line 1140f. of dynlink.c (revision
> >72658c65):
> >
> >
> >|/* Add a shortname only if name arg was not an explicit pathname. */
> >|if (pathname != name) p->shortname = strrchr(p->name, '/')+1;
>   It would be interesting to know which task this check is supposed
> to solve.

The concept here is that non-pathname library names should be loaded
from the library path and not replaced by something (typically a
"module" or "plug-in") in a different, explicitly-loaded location that
happens to have a colliding base filename.

For example suppose your application loads modules from $libdir/myapp/
and has a module named "". Unbeknownst to you, there's also a
"" in the system paths, and some library you potentially load
indirectly (maybe the GPU driver for some video hardware you've never
heard of) depends on "".

If dlopen("$libdir/myapp/") had put "" in the
namespace such that it would satisfy future load requests for the name
"", the subsequent load would break due to getting the wrong
(unrelated) library.


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