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Date: Thu, 15 Mar 2018 12:17:05 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: Bracken Dawson <abdawson@...il.com>, musl@...ts.openwall.com
Subject: Re: Program with constructor function segfaults frequently
 with musl

* Szabolcs Nagy <nsz@...t70.net> [2018-03-15 12:01:44 +0100]:
> * 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

ah this does not explain the type signature, the right link is
http://www.sco.com/developers/gabi/latest/ch4.sheader.html#init_array

> (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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