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Date: Thu, 31 Mar 2016 21:53:01 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: vfork on ARM
On Fri, Apr 01, 2016 at 11:42:47AM +1100, Patrick Oppenlander wrote:
> I'm looking at what would be involved in using musl on a nommu arm system.
>
> As far as I know SYS_vfork is available on ARM, but musl is
> currently falling back to fork.
>
> Are there any plans to support vfork on ARM and other architectures?
It's trivial to add vfork, but the usefulness is limited without other
changes:
1. To my knowledge, all nommu ARM systems are thumb[2]-only, so
supporting them as targets requires adapting all the asm files to
support building as thumb. This is a task in progress and, as long as
we only care about thumb2 (available on armv7-m, i.e. Corext-M3 and
up, I think) it's almost done.
2. For pre-v7, there's no way to do atomics without kernel help, and
no established kernel API for this as far as I know. For v7-m this is
probably not a problem.
3. Running on nommu without shareable program text is not much fun;
execve is really slow (memcpy of full program) and you need lots of
memory. Some people at ST have implemented an FDPIC abi for ARM which
solves this problem, but it's not upstream in the toolchain or kernel,
and the relocation types it needs are not officially assigned. Getting
it officially stabilized, supported, and forward-ported to modern tool
versions is going to be a lot of work. Here are some slides on it:
http://www.slideshare.net/linaroorg/sfo15406-arm-fdpic-toolset-kernel-libraries-for-cortexm-cortexr-mmuless-cores
Without FDPIC, it's possible to build a toolchain that produces
static-PIE executables that will work on nommu (with my recently
committed kernel patch for running non-FDPIC PIE ELF files on nommu,
and some additional work still needed to hook it up to work on ARM)
but these cannot use a shared mapping of the program.
If you or anyone else is up for helping with these tasks that would be
great.
Rich
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