linux-kernel - Re: Seg fault occurs when running statically compiled binary from kernel using call

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Message-ID: <20130710185200.GX24642@n2100.arm.linux.org.uk>
Date:	Wed, 10 Jul 2013 19:52:00 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Will Deacon <will.deacon@....com>
Cc:	Ashish Sangwan <ashishsangwan2@...il.com>,
	Namjae Jeon <namjae.jeon@...sung.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ashish Sangwan <a.sangwan@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-arm@...ts.infradead.org" <linux-arm@...ts.infradead.org>
Subject: Re: Seg fault occurs when running statically compiled binary from
	kernel using call_usermodehelper

On Wed, Jul 10, 2013 at 05:34:11PM +0100, Will Deacon wrote:
> Ok, I've finally got to the bottom of this, but I'm not sure on the best way
> to fix it.

I don't think you have!  You need to look back at the older ARM kernels
to really get to the bottom of this...

> The issue is that libc expects r0 to contain a function pointer
> to be invoked at exit (rtld_fini), to clean up after a dynamic linker. If
> this pointer is NULL, then it is ignored. We actually zero this pointer in
> our ELF_PLAT_INIT macro.
> 
> At the same time, we have this strange code called next from the ARM ELF
> loader:
> 
> 	regs->ARM_r2 = stack[2];	/* r2 (envp) */			\
> 	regs->ARM_r1 = stack[1];	/* r1 (argv) */			\
> 	regs->ARM_r0 = stack[0];	/* r0 (argc) */			\
> 
> which puts argc into r0.

You're sort of right.  It dates from the days when we had a.out binaries,
those required argc, argv and envp in r0/r1/r2 - and ARM kernels carried
this hack in  binfmt_aout.c to make it work in conjunction with the above:

static int load_aout_binary(struct linux_binprm * bprm)
{
...
        start_thread(regs, ex.a_entry, current->mm->start_stack);
#ifndef __arm__
        return 0;
#else
        return regs->ARM_r0;
#endif
}

ELF, on the other hand, never had that hack - ELF has always been zero
in r0, and it's always retrieved the argc/argv/envp off the stack.

As the above hack got dropped from the kernel (I don't think it ever made
it into mainline), I think we should be safe getting rid of this
initialization of regs->ARM_r0 to r2, leaving them all as zeros.

We should probably also remove the selection of HAVE_AOUT from
arch/arm/Kconfig too as this definitely won't work with any recent
kernel (certainly not without binfmt_aout.c hacked in the above way.)
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