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Date: Thu, 15 Mar 2018 12:01:44 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: Bracken Dawson <abdawson@...il.com>
Cc: musl@...ts.openwall.com
Subject: Re: Program with constructor function segfaults frequently
 with musl

* Bracken Dawson <abdawson@...il.com> [2018-03-15 10:38:31 +0000]:
> I have been having trouble getting a cgo program to run with musl, it has
> been segfaulting frequently and with 'No stack' when run under gdb.
> 
> I have managed to reproduce such a failure in pure c with a very small
> example:
> 
> ```
> #include <stdio.h>
> #include <stdlib.h>
> #include <getopt.h>
> 
> __attribute__((constructor)) void enter_namespace(int argc, char *argv[]) {

the arguments passed to ctors are not part of the elf abi
http://www.sco.com/developers/gabi/latest/ch5.dynamic.html#init_fini
(and it cannot really work for dynamically loaded libraries anyway:
the application can arbitrarily clobber argv by that time)

glibc passes these arguments as an extension (the semantics
for dlopened libraries is unclear), which happens to work
since the calling convention of functions with no arguments
allows this on all supported targets.

(note that there are security hardenning solutions that check
the call site function signature against the callee and abort on
mismatch and such extension would not work with that)

is this cgo that tries to capture argv in a ctor or some other
c library? (in either case you should first try to solve it
portably without depending on the glibc extension)

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