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

[Zach Levis]

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.

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.

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