Re: Program with constructor function segfaults frequently with musl

[Bracken Dawson <abdawson@gmail.com>]

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Sadly my use case is to set a given mnt namespace before go becomes
multi-threaded, which happens before the go main() function, so I do depend
on reading argv in the constructor, I mean I could use a file or something
else, but would rather not.

I guess this is just something I can get away with today in glibc that musl
will never support.

Thanks for looking though.

:wq

On 15 March 2018 at 11:17, Szabolcs Nagy <nsz@port70.net> wrote:

> * Szabolcs Nagy <nsz@port70.net> [2018-03-15 12:01:44 +0100]:
> > * Bracken Dawson <abdawson@gmail.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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