Re: [musl] Linking musl with -shared -static - is this supported?

[Joakim Sindholt <opensource@zhasha.com>]

On Tue, 24 May 2022 07:54:43 +0200, Kuba Kazimierczak <kuba@kazimierczak.name> wrote:
> Hi, I'm on Alpine Linux, musl version 1.2.2. I'm attempting to create a 
> statically-linked shared object using "-shared -static" and use it in a 
> driver program as follows:

Musl does support building a statically linked shared object but it does
not (and cannot) support loading said object into another process with
its own libc, regardless of which libc it is.

> /tmp # cat lib.c
> #include <stdlib.h>
> 
> unsigned int add(unsigned int a, unsigned int b)
> {
>      free(malloc(1));
>      return (a+b);
> }
> /tmp # cat main.c
> #include <stdio.h>
> 
> extern unsigned int add(unsigned int a, unsigned int b);
> 
> int main(void)
> {
>      printf("%d\n", add(1,2));
>      return 0;
> }
> /tmp # clang-14 -g -c -fPIC lib.c
> /tmp # clang-14 -shared -static -o liblib.so lib.o
> /tmp # clang-14 -g -L. main.c -o main -llib
> /tmp # LD_LIBRARY_PATH=$PWD ./main
> Segmentation fault
> 
> The segfault happens in musl's malloc implementation internals:
> 
> #0  0x00007f9c882adc7a in get_random_secret () at 
> src/malloc/mallocng/glue.h:45
> #1  __malloc_alloc_meta () at src/malloc/mallocng/malloc.c:50
> #2  0x00007f9c882ae11b in alloc_group (req=1, sc=0) at 
> src/malloc/mallocng/malloc.c:179
> #3  alloc_slot (sc=sc@entry=0, req=req@entry=1) at 
> src/malloc/mallocng/malloc.c:291
> #4  0x00007f9c882ae646 in __libc_malloc_impl (n=1) at 
> src/malloc/mallocng/malloc.c:369
> #5  0x00007f9c882ad208 in add (a=1, b=2) at lib.c:5
> #6  0x000055909082f22e in main () at main.c:7
> 
> I see get_random_secret is trying to use struct __libc.argv, which is 
> probably not initialized in this case.
> 
> Is this use case supported at all?

You are entirely correct. The DSO you've built does not get to run its
instance of the dynamic linker which leaves all the libc-global data
uninitialized. Unfortunately just running the init won't give you a
functional environment.

Someone else asked roughly the same question half a year ago:
https://www.openwall.com/lists/musl/2022/01/01/1

Joakim