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

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Message-ID: <20130730161651.Horde.PkwqiJir309R_EljyDbxlqA@imap.linux.ibm.com>
Date:	Tue, 30 Jul 2013 16:16:51 -0700
From:	Zach Levis <zml@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, Zach Levis <zach@...hsthings.com>
Subject: Re: [PATCH 2/3] fs/binfmts: Better handling of binfmt loops

Quoting Andrew Morton <akpm@...ux-foundation.org>:

> On Thu, 25 Jul 2013 08:40:44 -0700 Zach Levis <zml@...ux.vnet.ibm.com> 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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