Re: [PATCH] binfmt_flat: Remove shared library support

[Rob Landley <rob@landley.net>]

On 4/21/22 07:43, Rich Felker wrote:
> On Thu, Apr 21, 2022 at 08:52:59AM +0200, Geert Uytterhoeven wrote:
>> On Thu, Apr 21, 2022 at 1:53 AM Greg Ungerer <gerg@linux-m68k.org> wrote:
>> > On 21/4/22 00:58, Eric W. Biederman 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@xmission.com>
>> > > ---
>> > >
>> > > Can the binfmt_flat folks please verify that the shared library support
>> > > really isn't used?
>> >
>> > I can definitely confirm I don't use it on m68k. And I don't know of
>> > anyone that has used it in many years.
>> >
>> >
>> > > Was binfmt_flat being enabled on arm and sh the mistake it looks like?
>> 
>> I think the question was intended to be
>> 
>>     Was *binfmt_flat_shared_flat* being enabled on arm and sh the
>>     mistake it looks like?
> 
> Early in my work on j2, I tried to research the history of shared flat
> support on sh, and it turned out the mainline tooling never even
> supported it, and the out-of-line tooling I eventually found was using
> all sorts of wrong conditionals for how it did the linking and elf2flt
> conversion, e.g. mere presence of any PIC-like relocation in any file
> made it assume the whole program was PIC-compatible. There's no way
> that stuf was ever used in any meaningful way. It just didn't work.
> 
> Quickly dropped that and got plain ELF (no shared text/xip, but no
> worse than the existing flat support) working, and soon after, FDPIC.
> 
> The whole binfmt_flat ecosystem is a mess with no good reason to
> exist.

FYI when I had to come up to speed on this in 2014 I did a writeup on my own
research:

https://landley.net/notes-2014.html#07-12-2014

The lack of a canonical "upstream" elf2flt repository was probably the biggest
problem at the time.

(There's a reason I grabbed fdpic hard and tried to make that work everywhere.)

> Rich

Rob