Re: [musl] Ignoring dependencies libresolv and libcrypt

[Nikos Dragazis <ndragazis@arrikto.com>]

On 2/9/20 7:42 μ.μ., Rich Felker wrote:

> On Wed, Sep 02, 2020 at 07:14:10PM +0300, Nikos Dragazis wrote:
>> Hi everyone,
>>
>> Apologies if this has already been answered before.
>>
>> I am experimenting with musl. I see that musl produces a single DSO with
>> all symbols, as opposed to glibc which produces multiple DSOs
>> (libthread, libm, librt, etc.). I also notice that musl generates some
>> empty archives for compatibility reasons, namely the
>> lib{crypt,dl,m,pthread,resolv,rt,util,xnet}.a. These are already
>> documented in the FAQ [1].
>>
>> By looking at the code [2], I see that musl's dynamic linker ignores
>> dynamic dependencies with names lib{c,pthread,rt,m,dl,util,xnet} and
>> this makes sense based on the above.
>>
>> What doesn't make sense to me is that musl's dynamic linker does not
>> ignore dynamic dependencies with names libresolv and libcrypt. Is there
>> a reason for this?
> I believe the intent was only to reserve names that POSIX explicitly
> reserves:
>
>      If a directory specified by a -L option contains files with names
>      starting with any of the strings "libc.", "libl.", "libpthread.",
>      "libm.", "librt.", "libtrace.", "libxnet.", or "liby.", the
>      results are unspecified.
>
> https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html
>
> However it looks like the sets don't entirely match up. I'm not sure
> of the reason for the mismatch.
>
> The set of "builtin" library names should probably be broken into two
> parts: ones that will always be used (c, pthread, m, etc) and ones
> that will be used as fallbacks if no file is found (resolv, crypt,
> etc.).

It's not clear to me how the second part could be useful. Also, it would
break the scenario where one wants to use musl to run a glibc-compat
executable on a glibc-based system, because the linker would load
glibc's libresolv.so.

>
> Note that the purpose of these built-in names is twofold: (1) it's
> part of glibc ABI-compat, for running glibc binaries with their names
> in DT_NEEDED

Then definitely resolv and crypt should be part of it.

> , and this role could be moved out with the gcompat
> refactor if desired, and (2) supporting programs that (kinda
> dubiously) use dlopen with these names to access standard
> functionality.
>
> Rich