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

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Message-ID: <20121015162732.GG2616@ZenIV.linux.org.uk>
Date:	Mon, 15 Oct 2012 17:27:32 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Daniel Mack <zonque@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [revert request for commit 9fff2fa] Re: [git pull] signals pile 3

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...
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