linux-kernel - Re: [PATCH v3] ELF: document some de-facto PT

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Date:   Wed, 6 Dec 2023 14:58:23 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Eric Biederman <ebiederm@...ssion.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3] ELF: document some de-facto PT_* ABI quirks

*thread necromancy* Question below...

On Sat, Apr 15, 2023 at 08:37:29PM +0300, Alexey Dobriyan wrote:
> Turns out rules about PT_INTERP, PT_GNU_STACK and PT_GNU_PROPERTY
> program headers are slightly different.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
> 
> 	v3: move to Documentation/userspace-api/
> 	v2: integrate into documentation build system
> 
>  Documentation/userspace-api/ELF.rst   |   34 ++++++++++++++++++++++++++++++++++
>  Documentation/userspace-api/index.rst |    1 +
>  2 files changed, 35 insertions(+)
> 
> new file mode 100644
> --- /dev/null
> +++ b/Documentation/userspace-api/ELF.rst
> @@ -0,0 +1,34 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=================================
> +Linux-specific ELF idiosyncrasies
> +=================================
> +
> +Definitions
> +===========
> +
> +"First" program header is the one with the smallest offset in the file:
> +e_phoff.
> +
> +"Last" program header is the one with the biggest offset in the file:
> +e_phoff + (e_phnum - 1) * sizeof(Elf_Phdr).
> +
> +PT_INTERP
> +=========
> +
> +First PT_INTERP program header is used to locate the filename of ELF
> +interpreter. Other PT_INTERP headers are ignored (since Linux 2.4.11).
> +
> +PT_GNU_STACK
> +============
> +
> +Last PT_GNU_STACK program header defines userspace stack executability
> +(since Linux 2.6.6). Other PT_GNU_STACK headers are ignored.
> +
> +PT_GNU_PROPERTY
> +===============
> +
> +ELF interpreter's last PT_GNU_PROPERTY program header is used (since
> +Linux 5.8). If interpreter doesn't have one, then the last PT_GNU_PROPERTY
> +program header of an executable is used. Other PT_GNU_PROPERTY headers
> +are ignored.

Should we perhaps solve some of these in some way? What would folks
prefer the behaviors be? (I like to have things been "as expected", but
it's not very obvious here for redundant headers...)

-- 
Kees Cook