From: Florian Weimer <fweimer@redhat.com>
To: Rich Felker <dalias@libc.org>
Cc: GNU C Library <libc-alpha@sourceware.org>, GCC <gcc@gcc.gnu.org>,
Binutils <binutils@sourceware.org>
Subject: Re: Run (some?) ELF constructors after applying RELRO protection
Date: Mon, 11 Jun 2018 19:50:32 +0200 [thread overview]
Message-ID: <98e7d3dd-16b2-83aa-4025-768f913eaf41@redhat.com> (raw)
In-Reply-To: <20180611145013.GG1392@brightrain.aerifal.cx>
On 06/11/2018 04:50 PM, Rich Felker wrote:
> On Tue, Feb 27, 2018 at 11:01:23AM +0100, Florian Weimer wrote:
>> I think it would be a nice addition to the toolchain if it were
>> possible to programatically initialize data in the RELRO section.
>> We do this in glibc, but I don't think this is currently supported
>> for general use.
>>
>> One important application is to allocate a memory region with mmap,
>> on which protection flags can be changed as needed. This way, the
>> application can have a read-only path to its own configuration data,
>> for example.
>>
>> Do you think this would be worthwhile to implement? Any suggestions
>> how we should do it, without needing binutils/GCC/glibc updates?
>
> This weakens protection of the actual relro section (because there's a
> window where it's writable but application code is running; in the
> case of thread creation from ctors, or dlopen in a multithreaded
> program, this is a nontrivial window) and has no benefit, except
> saving a page of memory, over the application just calling mprotect
> itself.
My latest proposal suggests to make this opt-in:
<https://sourceware.org/ml/gnu-gabi/2018-q2/msg00000.html>
> If the application already has to annotate that the data is
> going to be read-only after ctors, it can just page-align/page-pad the
> data itself and call mprotect with minimal additional effort, and no
> complex interaction between application code and relro (which is about
> RELocations not arbitrary data protection).
Is this really supported? We currently do not provide programmatic
access to the largest supported page size of a target architecture, I
think. The link editor knows of it, of course, but beyond that, it's a
bit of a mystery. It's not just about cross-compilation. Even if you
check the run-time page size, it might not give you the right answer.
Thanks,
Florian
next prev parent reply other threads:[~2018-06-11 17:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-27 10:01 Run (some?) ELF constructors after applying RELRO protection Florian Weimer
2018-06-11 14:50 ` Rich Felker
2018-06-11 17:50 ` Florian Weimer [this message]
2018-06-11 18:59 ` Rich Felker
2018-07-05 19:49 ` Florian Weimer
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