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

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Message-ID: <d54fe81be77b9edd8578a6d208c72cd7c0b8c1dd.camel@intel.com>
Date:   Tue, 18 Jun 2019 07:58:37 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Martin <Dave.Martin@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, 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, 2019-06-18 at 14:32 +0100, Dave Martin wrote:
> 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.

Hi Florian,

The kernel looks at only ld-linux.  Other applications are loaded by ld-linux. 
So the issues are limited to three versions of ld-linux's.  Can we somehow
update those??

Thanks,
Yu-cheng