Re: [musl] building statically linked DSOs with musl

[Sebastien Bourdeauducq <sb@m-labs.hk>]

On 1/1/22 20:56, Joakim Sindholt wrote:
> Musl will set up its own internal global libc structure
> with a bunch of values during the initial dynamic loading phase; among
> the members is libc.auxv, which is what __vdsosym will look through when
> trying to find the VDSO. Since you never ran musl's dynamic linker (and
> even if your host binary was musl-based, not the one that would have
> initialized the libc.auxv baked in to your statically linked DSO) it
> won't have set up this and a whole host of other things.

Thanks for the hint.

libc.auxv seems to be set up by __dls2b, which itself is called by 
__dls2 via find_sym(&ldso, "__dls2b", 0).
How does this code work when a program is statically linked against musl?

On 1/3/22 05:41, Markus Wichmann wrote:
 > There can be different heaps, but not different brk heaps. Basically,
 > brk() manages a global variable generated by the kernel. You can work
 > around that problem by installing a seccomp filter that makes brk()
 > always fail, forcing the allocators to fall back to mmap().

mimalloc exclusively uses mmap() already, so that will be OK and will 
not require a workaround. It shouldn't matter if the other allocator 
uses brk().

 > The thread pointer does not match up,

Seems it does (okay, probably by accident):
https://git.musl-libc.org/cgit/musl/tree/arch/x86_64/pthread_arch.h?id=cfdfd5ea3ce14c6abf7fb22a531f3d99518b5a1b#n4
https://github.com/bminor/glibc/blob/b92a49359f33a461db080a33940d73f47c756126/sysdeps/x86/nptl/thread_pointer.h#L30

 > and so no functions accessing the thread pointers will work. That
 > includes functions that read the current thread pointer, so you cannot
 > know in advance which those are. All the implementation-defined
 > structures mismatch, so none of the locking functions work, and neither
 > do the semaphore functions.

AFAIK I would not need to share locks or semaphores across the plugin. I 
need to see if the rest of this thread business has other consequences.

Sébastien