Re: globals in (out of) crt0

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Sun, Nov 27, 2016 at 03:20:26AM +0000, David Holland wrote:
> While discussing the Emacs problems with Joerg (see PR 51654 for the
> gory details) the conclusion we came to is that the best real fix (as
> opposed to the workaround of disabling ASLR) is to shift the
> __ps_strings global from crt0 to libc; then it won't be referenced
> directly by programs, won't be in the linked program's BSS, and won't
> be accidentally saved when Emacs dumps. Instead, libc will provide a
> private entry point to assign the value of __ps_strings. (This also
> turns out to simplify the piece of libc initialization that is
> crashing Emacs.)

The function calls don't really help as libc has to support old crt0.o
anyway. Ignoring compat, there is no need for three different calls,
just provide a way to pass down __ps_strings. The other two variables
are derived from it anyway. That was the reason why ___start no longer
takes as many arguments.

>  - I put declarations for the hooks in unistd.h. I'd rather use a
> private header so they aren't exposed, but there isn't currently one
> shared by libc and crt0. Should I set one up?

There is the precedence case with rtld.h.

>  - I put the hooks in the .text.startup section like _libc_init but
> I'm not sure if that's actually appropriate -- what is and isn't
> supposed to go in .text.startup?

It's basically a subsection for functions used only once during initial
startup, so that the linker can try to group them together to avoid
wasting TLB space etc.

Joerg