Re: [revert request for commit 9fff2fa] Re: [git pull] signals pile 3

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Oct 15, 2012 at 05:07:10PM +0100, Catalin Marinas wrote:
> On Sun, Oct 14, 2012 at 08:56:11PM +0100, Al Viro wrote:
> > On Sun, Oct 14, 2012 at 08:24:03PM +0100, Al Viro wrote:
> > 
> > > Russell, could you recall what those had been about?  I'm not sure if that
> > > had been oopsable that far back (again, oops scenario is userland stack
> > > page getting swapped out before we get to start_thread(), leading to
> > > direct read from an absent page in start_thread() by plain ldr, without
> > > anything in exception table about that insn), but it looks very odd
> > > regardless of that problem.
> > 
> > BTW, arm64 has copied that logics, so it also seems to be unsafe and very
> > odd - there we definitely have only ELF to cope with.  arm64 folks Cc'd...
> 
> Good point. We don't need this on arm64 and probably neither on arm (at
> least since EABI).
> 
> Setting x0 may cause other issues as well. The dynamic loader simply
> ignores the startup registers but for static binaries the _start code in
> glibc expects r0 to contain a function pointer to be registered with
> atexit() in __libc_start_main() or NULL. Since we pass argc in there,
> for static binaries the rtld_fini argument to __libc_start_main() is
> neither NULL nor something meaningful.

The value left there by start_thread() will not reach the userland anyway...