linux-kernel - Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an ELF file

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Message-ID: <20190618133223.GD2790@e103592.cambridge.arm.com>
Date:   Tue, 18 Jun 2019 14:32:25 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an
 ELF file

On Tue, Jun 18, 2019 at 02:55:12PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 18, 2019 at 02:47:00PM +0200, Florian Weimer wrote:
> > * Peter Zijlstra:
> > 
> > > I'm not sure I read Thomas' comment like that. In my reading keeping the
> > > PT_NOTE fallback is exactly one of those 'fly workarounds'. By not
> > > supporting PT_NOTE only the 'fine' people already shit^Hpping this out
> > > of tree are affected, and we don't have to care about them at all.
> > 
> > Just to be clear here: There was an ABI document that required PT_NOTE
> > parsing.
> 
> URGH.
> 
> > The Linux kernel does *not* define the x86-64 ABI, it only
> > implements it.  The authoritative source should be the ABI document.
> >
> > In this particularly case, so far anyone implementing this ABI extension
> > tried to provide value by changing it, sometimes successfully.  Which
> > makes me wonder why we even bother to mainatain ABI documentation.  The
> > kernel is just very late to the party.
> 
> How can the kernel be late to the party if all of this is spinning
> wheels without kernel support?

PT_GNU_PROPERTY is mentioned and allocated a p_type value in hjl's
spec [1], but otherwise seems underspecified.

In particular, it's not clear whether a PT_GNU_PROPERTY phdr _must_ be
emitted for NT_GNU_PROPERTY_TYPE_0.  While it seems a no-brainer to emit
it, RHEL's linker already doesn't IIUC, and there are binaries in the
wild.

Maybe this phdr type is a late addition -- I haven't attempted to dig
through the history.


For arm64 we don't have this out-of-tree legacy to support, so we can
avoid exhausitvely searching for the note: no PT_GNU_PROPERTY ->
no note.

So, can we do the same for x86, forcing RHEL to carry some code out of
tree to support their legacy binaries?  Or do we accept that there is
already a de facto ABI and try to be compatible with it?


>From my side, I want to avoid duplication between x86 and arm64, and
keep unneeded complexity out of the ELF loader where possible.

Cheers
---Dave


[1] https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI