[PATCH] binfmt_flat: Remove shared library support

[Kees Cook]

On Wed, Apr 20, 2022 at 12:59:37PM -0400, Rich Felker wrote:
> On Wed, Apr 20, 2022 at 09:17:22AM -0700, Palmer Dabbelt wrote:
> > On Wed, 20 Apr 2022 07:58:03 PDT (-0700), ebiederm at xmission.com wrote:
> > >
> > >In a recent discussion[1] it was reported that the binfmt_flat library
> > >support was only ever used on m68k and even on m68k has not been used
> > >in a very long time.
> > >
> > >The structure of binfmt_flat is different from all of the other binfmt
> > >implementations becasue of this shared library support and it made
> > >life and code review more effort when I refactored the code in fs/exec.c.
> > >
> > >Since in practice the code is dead remove the binfmt_flat shared libarary
> > >support and make maintenance of the code easier.
> > >
> > >[1] https://lkml.kernel.org/r/81788b56-5b15-7308-38c7-c7f2502c4e15@linux-m68k.org
> > >Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> > >---
> > >
> > >Can the binfmt_flat folks please verify that the shared library support
> > >really isn't used?
> > 
> > I don't actually know follow the RISC-V flat support, last I heard it was still
> > sort of just in limbo (some toolchain/userspace bugs th at needed to be sorted
> > out).  Damien would know better, though, he's already on the thread.  I'll
> > leave it up to him to ack this one, if you were even looking for anything from
> > the RISC-V folks at all (we don't have this in any defconfigs).
> 
> For what it's worth, bimfmt_flat (with or without shared library
> support) should be simple to implement as a binfmt_misc handler if
> anyone needs the old shared library support (or if kernel wanted to
> drop it entirely, which I would be in favor of). That's how I handled
> old aout binaries I wanted to run after aout was removed: trivial
> binfmt_misc loader.

Yeah, I was trying to understand why systems were using binfmt_flat and
not binfmt_elf, given the mention of elf2flat -- is there really such a
large kernel memory footprint savings to be had from removing
binfmt_elf?

But regardless, yes, it seems like if you're doing anything remotely
needing shared libraries with binfmt_flat, such a system could just use
ELF instead.

-- 
Kees Cook