linux-kernel - Re: [PATCH] fs/binfmt_elf.c: disallow zero entry point address

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Date:   Sun, 12 Dec 2021 11:30:01 -0800
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] fs/binfmt_elf.c: disallow zero entry point address

On Sun, Dec 12, 2021 at 11:15 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, Dec 12, 2021 at 11:06 AM H.J. Lu <hjl.tools@...il.com> wrote:
> >
> > According to the ELF specification, zero entry point value means
> > there is no entry point.  Such ELF binary doesn't conform to the
> > ELF specification.
>
> Nobody cares about paper specifications.
>
> All that matters is REALITY.
>
> So let me quote my email again, since you clearly didn't actually read
> it (read that "maybe it's not supposed to work" part):
>
> > That's not my main worry - what if somebody has a code section with a
> > zero vaddr and intentionally put the entry at the beginning?
> >
> > Maybe it's not supposed to work by some paper standatd, but afaik
> > currently it _would_ work.
>
> I'm not sure this can happen currently (maybe all tools effectively
> make it so that the ELF headers etc are part of the loaded image).
>
> But no, paper specifications have absolutely no meaning if they don't
> match realty.
>
> And the reality is that I don't think we've ever checked e_entry being
> zero, which means that maybe people have used it.
>
> So convince me that the above cannot happen. I'm perfectly willing to
> be convinced, but "some random paper standard that we've never
> followed" is not the thing to quote.
>

On Linux, the start of the first PT_LOAD segment is the ELF
header and the address 0 points to the ELF magic bytes which
isn't a valid code sequence.

-- 
H.J.