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

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Message-ID: <20190610165730.GM28398@e103592.cambridge.arm.com>
Date:   Mon, 10 Jun 2019 17:57:31 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Florian Weimer <fweimer@...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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Mon, Jun 10, 2019 at 09:29:04AM -0700, Yu-cheng Yu wrote:
> On Fri, 2019-06-07 at 19:01 +0100, Dave Martin wrote:
> > On Thu, Jun 06, 2019 at 01:06:41PM -0700, Yu-cheng Yu wrote:
> > > An ELF file's .note.gnu.property indicates features the executable file
> > > can support.  For example, the property GNU_PROPERTY_X86_FEATURE_1_AND
> > > indicates the file supports GNU_PROPERTY_X86_FEATURE_1_IBT and/or
> > > GNU_PROPERTY_X86_FEATURE_1_SHSTK.
> > > 
> > > With this patch, if an arch needs to setup features from ELF properties,
> > > it needs CONFIG_ARCH_USE_GNU_PROPERTY to be set, and a specific
> > > arch_setup_property().
> > > 
> > > For example, for X86_64:
> > > 
> > > int arch_setup_property(void *ehdr, void *phdr, struct file *f, bool inter)
> > > {
> > > 	int r;
> > > 	uint32_t property;
> > > 
> > > 	r = get_gnu_property(ehdr, phdr, f, GNU_PROPERTY_X86_FEATURE_1_AND,
> > > 			     &property);
> > > 	...
> > > }
> > 
> > Although this code works for the simple case, I have some concerns about
> > some aspects of the implementation here.  There appear to be some bounds
> > checking / buffer overrun issues, and the code seems quite complex.
> > 
> > Maybe this patch tries too hard to be compatible with toolchains that do
> > silly things such as embedding huge notes in an executable, or mixing
> > NT_GNU_PROPERTY_TYPE_0 in a single PT_NOTE with a load of junk not
> > relevant to the loader.  I wonder whether Linux can dictate what
> > interpretation(s) of the ELF specs it is prepared to support, rather than
> > trying to support absolutely anything.
> 
> To me, looking at PT_GNU_PROPERTY and not trying to support anything is a
> logical choice.  And it breaks only a limited set of toolchains.
> 
> I will simplify the parser and leave this patch as-is for anyone who wants to
> back-port.  Are there any objections or concerns?

No objection from me ;)  But I'm biased.

Hopefully this change should allow substantial simplification.  For one
thing, PT_GNU_PROPERTY tells its file offset and size directly in its
phdrs entry.  That should save us a lot of effort on the kernel side.

Cheers
---Dave