linux-kernel - Re: [PATCH] binfmt_elf: fix return value in case of interpreter load failure

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Message-Id: <20130415145324.d63be917d438b3f4ec37f845@linux-foundation.org>
Date:	Mon, 15 Apr 2013 14:53:24 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Matthieu CASTET <matthieu.castet@...rot.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] binfmt_elf: fix return value in case of interpreter
 load failure

On Fri, 12 Apr 2013 16:49:50 +0200 Matthieu CASTET <matthieu.castet@...rot.com> wrote:

> Hi Andrew,
> 
> thanks for your quick review.
> 
> Andrew Morton a __crit :
> > On Thu, 11 Apr 2013 15:53:09 +0200 Matthieu CASTET <matthieu.castet@...rot.com> wrote:
> > 
> >> The current code return the address instead of using PTR_ERR.
> > 
> > I don't understand what you mean here - please describe this error in
> > much more detail.  Help people to identify the section of code which
> > is being discussed.
> 
> I was speaking of
> 
> 
>  elf_entry = load_elf_interp(&loc->interp_elf_ex,
>                         interpreter,
>                         &interp_map_addr,
>                         load_bias);
> [...]
>         if (BAD_ADDR(elf_entry)) {
>             force_sig(SIGSEGV, current);
>             retval = IS_ERR((void *)elf_entry) ?
>                     (int)elf_entry : -EINVAL;
>             goto out_free_dentry;
>         }
> 
> and was expecting we should use PTR_ERR when IS_ERR is true to match what is
> done in [1].
> 
> But didn't saw that PTR_ERR((void *)elf_entry) and (int)elf_entry are equivalent.
> 
> > 
> >> Also the check is done after adding e_entry. This can cause weird behaviour
> >> because -errno + loc->interp_elf_ex.e_entry can produce a valid address.
> > 
> > Which check?
> 
> I am really confused here. Reading again the code this can't happen because if
> load_elf_interp return -errno
> 
> 
> We don't enter this condition
> >         if (!IS_ERR((void *)elf_entry)) {
> >             /*
> >              * load_elf_interp() returns relocation
> >              * adjustment
> >              */
> >             interp_load_addr = elf_entry;
> >             elf_entry += loc->interp_elf_ex.e_entry;
> >         }
> we still have -errno here
> >         if (BAD_ADDR(elf_entry)) {
> >             force_sig(SIGSEGV, current);
> >             retval = IS_ERR((void *)elf_entry) ?
> >                     (int)elf_entry : -EINVAL;
> >             goto out_free_dentry;
> >         }
> 
> 
> Sorry for my mistake.
> 
> The only valid remaining part of my patch is to return SIGKILL when
> load_elf_interp fail (IS_ERR((void *)elf_entry) is true) (for example load
> address of linker is bad) instead of SIGSEGV. This will follow what is done when
> loading binary.
> 
> But is it even worth doing?

SIGSEGV can be caught so that would be a user-visible change.  I just
don't know what the implications of such a change would be :(

(hopefully cc's Oleg)
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