Re: [PATCH v2 0/5] Fix ELF / FDPIC ELF core dumping, and use mmap_sem properly in there

Greg Ungerer
2020-05-01T07:14:43Z

On 1/5/20 2:54 am, Linus Torvalds wrote:
> On Thu, Apr 30, 2020 at 7:10 AM Greg Ungerer <gerg@linux-m68k.org> wrote:
>>
>>> in load_flat_file() - which is also used to loading _libraries_. Where
>>> it makes no sense at all.
>>
>> I haven't looked at the shared lib support in there for a long time,
>> but I thought that "id" is only 0 for the actual final program.
>> Libraries have a slot or id number associated with them.
>
> Yes, that was my assumption, but looking at the code, it really isn't
> obvious that that is the case at all.
>
> 'id' gets calculated from fields that very much look like they could
> be zero (eg by taking the top bits from another random field).
>
>>> Most of that file goes back to pre-git days. And most of the commits
>>> since are not so much about binfmt_flat, as they are about cleanups or
>>> changes elsewhere where binfmt_flat was just a victim.
>>
>> I'll have a look at this.
>
> Thanks.
>
>> Quick hack test shows moving setup_new_exec(bprm) to be just before
>> install_exec_creds(bprm) works fine for the static binaries case.
>> Doing the flush_old_exec(bprm) there too crashed out - I'll need to
>> dig into that to see why.
>
> Just moving setup_new_exec() would at least allow us to then join the
> two together, and just say "setup_new_exec() does the credential
> installation too".
>
> So to some degree, that's the important one.
>
> But that flush_old_exec() does look odd in load_flat_file(). It's not
> like anything but executing a binary should flush the old exec.
> Certainly not loading a library, however odd that flat library code
> is.
>
> My _guess_ is that the reason for this is that "load_flat_file()" also
> does a lot of verification of the file and does that whole "return
> -ENOEXEC if the file format isn't right". So we don't want to flush
> the old exec before that is done, but we obviously also don't want to
> flush the old exec after we've actually loaded the new one into
> memory..

Yeah, that is what it looks like. Looking at the history, the introduction
of setup_new_exec() [in commit 221af7f87b974] was probably just
added where the the existing flush_old_exec() was.

> So the location of flush_old_exec() makes that kind of sense, but it
> would have made it better if that flat file support had a clear
> separation of "check the file" from "load the file".
>
> Oh well. As mentioned, the whole "at least put setup_new_exec() and
> install_exec_creds() together" is the bigger thing.
>
> But if it's true that nobody really uses the odd flat library support
> any more and there are no testers, maybe we should consider ripping it
> out...

I am for that. If nobody pipes up and complains I'll look at taking it out.

Regards
Greg