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Date:   Mon, 27 May 2019 10:37:46 -0400 (EDT)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Jann Horn <jannh@google.com>
cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k@vger.kernel.org, Russell King <linux@armlinux.org.uk>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] binfmt_flat: make load_flat_shared_library() work
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On Mon, 27 May 2019, 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.

Last time I played with this, I couldn't figure out the toolchain to 
produce shared libs. Only static executables worked fine. If I recall, 
existing binfmt_flat distros don't use shared libs either.

For shared lib support on no-MMU target, binfmt_elf_fdpic is a much 
saner choice.


Nicolas