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Message-ID: <98176a37-92c6-0da5-c14a-f61b4f8b13ec@m-labs.hk>
Date: Mon, 3 Jan 2022 09:37:00 +0800
From: Sebastien Bourdeauducq <sb@...abs.hk>
To: musl@...ts.openwall.com
Subject: Re: building statically linked DSOs with musl
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
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