Re: [PATCH 2/3] fs/binfmts: Better handling of binfmt loops

[Zach Levis]

Quoting Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:

> On Tue, 30 Jul 2013 16:16:51 -0700 Zach Levis <zml@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Quoting Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> >
> > > On Thu, 25 Jul 2013 08:40:44 -0700 Zach Levis  
> > <zml@xxxxxxxxxxxxxxxxxx> wrote:
> > >
> > >> With these changes, when a binfmt loop is encountered,
> > >> the ELOOP will propogate back to the 0 depth. At this point the
> > >> argv and argc values will be reset to what they were originally and an
> > >> attempt is made to continue with the following binfmt handlers.
> > >
> > > hm, why?  What problem does this fix?  What value does the change offer
> > > to our users?
> >
> > This is used when the binfmt_misc,script,etc options are configured in
> > a way that would previously prevent executables from launching that
> > could be executed with a different binfmt but don't because of a loop
> > in a prior binfmt.
> >
> > Example: a qemu is configured to run 64-bit ELFs on an otherwise
> > 32-bit system. The system's owner switches to running with 64-bit
> > executables, but forgets to disable the binfmt_misc option that
> > redirects 64bit ELFs to qemu. Since the qemu executable is a 64-bit
> > ELF now, binfmt_misc keeps on matching it with the qemu rule,
> > preventing the execution of any 64-bit binary.
>
> So the admin can unforget to make that change and no longer has a problem.
>
> > With this patch, an error is printed and search_binary_handler()
> > continues on to the next handler, allowing the original executable to
> > run normally so the user can (hopefully) fix their misconfiguration
> > more easily.
>
> Is all this really worth changing the kernel for?  It sounds
> a bit marginal.
I'd say it is worth changing because this is a self-contained fix that  
doesn't have much in the way of side effects for normal execution but  
can help recover a system. Apart from storing a pointer to the  
provided arguments, the only changes this requires are in the code for  
error handling.


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