Re: Build on linux 2.6 and run on linux 2.4?

[Rich Felker <dalias@aerifal.cx>]

On Thu, Mar 20, 2014 at 07:14:12PM -0400, Rich Felker wrote:
> On Thu, Mar 20, 2014 at 04:58:49PM -0400, John Mudd wrote:
> > Probably a dumb question. Is part of the value of musl that I can build an
> > app using musl on linux 2.6 and then run the binary on linux 2.4?
> 
> The version of Linux you build on has no bearing on the binary that
> comes out, so that's not a problem.
> 
> However, Linux 2.4 is not officially supported since it lacks a lot of
> functionality needed to provide a modern POSIX conforming environment.
> The most notable is that it can't do threads. If you're ok with that,
> the other problems might be small enough that you don't mind. I
> remember some people in Freenode #musl trying out 2.4 recently and
> finding that a few of the busybox applets didn't work right, though,
> due to missing statfs64 syscall.
> 
> This page has details on which kernel versions added which syscalls:
> 
> http://man7.org/linux/man-pages/man2/syscalls.2.html

I looked through this list, and statfs64/fstatfs64 seem to be the only
two syscalls where the 32-bit off_t version was all that's available
in 2.4 and the 64-bit version was added in 2.6. So if we could provide
a fallback for them, this might make 2.4 a fairly viable target for
single-threaded programs built with musl. (Despite 2.4 being ancient
and generally a bad idea to use, there are some devices where
replacing the kernel is hard or where the patches to support the
device have not been ported to 2.6, so this isn't just a useless
exercise.)

The big question that remains of course is how ugly the fallbacks
would be, and whether we'd need a lot more arch-specific definitions
_purely_ for 2.4 fallback.

Rich