Re: [musl] Shared library loading

[Alexander Scherbatiy <alexander.scherbatiy@bell-sw.com>]

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 liba.so: No such file or
>> directory (needed by /root/load-lib-sample/bin/b/libb.so)
>> 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.

   Thanks,
   Alexander.

>
> This means, in your case, the DSO handle for liba does not get a short
> name, so when libb is loaded, it is not detected that liba has already
> been loaded (only the shortname is used for that purpose). And since
> your files are in different directories, liba is also not found to be
> loaded again. To work around this, you can set LD_LIBRARY_PATH to the
> directory containing liba. In that case, when loading libb, the
> dependency will be found with the environment search, and since liba was
> already loaded, the loop in lines 1085ff. will find the handle and even
> assign a shortname. The shortname is only used for checking of
> duplicates.
>
> Ciao,
> Markus