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Date: Tue, 24 May 2022 09:27:23 +0200
From: Joakim Sindholt <opensource@...sha.com>
To: musl@...ts.openwall.com
Subject: Re: Linking musl with -shared -static - is this supported?
On Tue, 24 May 2022 07:54:43 +0200, Kuba Kazimierczak <kuba@...imierczak.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@...ry=0, req=req@...ry=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
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