Re: [PATCH] binfmt_flat: make load_flat_shared_library() work

[Sergei Poselenov <sposelenov@emcraft.com>]

Hello Arnd, all,

On Wed, 2019-05-29 at 14:05 +0200, Arnd Bergmann wrote:
> On Tue, May 28, 2019 at 12:56 PM Greg Ungerer <gerg@linux-m68k.org>
> wrote:
> > On 27/5/19 11:38 pm, Jann Horn wrote:
> > > On Sat, May 25, 2019 at 11:43 PM Andrew Morton
> > > <akpm@linux-foundation.org> wrote:
> > > > On Fri, 24 May 2019 22:18:17 +0200 Jann Horn <jannh@google.com>
> > > > wrote:
> > > > > load_flat_shared_library() is broken: It only calls
> > > > > load_flat_file() if
> > > > > prepare_binprm() returns zero, but prepare_binprm() returns
> > > > > the number of
> > > > > bytes read - so this only happens if the file is empty.
> > > > 
> > > > ouch.
> > > > 
> > > > > Instead, call into load_flat_file() if the number of bytes
> > > > > read is
> > > > > non-negative. (Even if the number of bytes is zero - in that
> > > > > case,
> > > > > load_flat_file() will see nullbytes and return a nice
> > > > > -ENOEXEC.)
> > > > > 
> > > > > In addition, remove the code related to bprm creds and stop
> > > > > using
> > > > > prepare_binprm() - this code is loading a library, not a main
> > > > > executable,
> > > > > and it only actually uses the members "buf", "file" and
> > > > > "filename" of the
> > > > > linux_binprm struct. Instead, call kernel_read() directly.
> > > > > 
> > > > > Cc: stable@vger.kernel.org
> > > > > Fixes: 287980e49ffc ("remove lots of IS_ERR_VALUE abuses")
> > > > > Signed-off-by: Jann Horn <jannh@google.com>
> > > > > ---
> > > > > I only found the bug by looking at the code, I have not
> > > > > verified its
> > > > > existence at runtime.
> > > > > Also, this patch is compile-tested only.
> > > > > It would be nice if someone who works with nommu Linux could
> > > > > have a
> > > > > look at this patch.
> > > > 
> > > > 287980e49ffc was three years ago!  Has it really been broken
> > > > for all
> > > > that time?  If so, it seems a good source of freed disk
> > > > space...
> > > 
> > > Maybe... but I didn't want to rip it out without having one of
> > > the
> > > maintainers confirm that this really isn't likely to be used
> > > anymore.
> > 
> > I have not used shared libraries on m68k non-mmu setups for
> > a very long time. At least 10 years I would think.
> 
> I think Emcraft have a significant customer base running ARM NOMMU
> Linux, I wonder whether they would have run into this (adding
> Sergei to Cc).
> My suspicion is that they use only binfmt-elf-fdpic, not binfmt-flat.
> 

We use both, acutally, but all-static. We don't support shared
libraries with bFLT or ELF FDPIC.

Kind regards,
Sergei
> The only architectures I see that enable binfmt-flat are sh, xtensa
> and h8300, but only arch/sh uses CONFIG_BINFMT_SHARED_FLAT
> for a few machine specific configurations, and I'm in turn fairly
> sure
> those machines have not run a recent kernel in many years.
> 
> The one SH nommu platform likely to have users is j2, and that is
> probably always used with musl-libc with elf-fdpic (given that
> Rich Felker maintains both the kernel port and the library).
> 
>       Arnd
>