linux-kernel - Re: [PATCH] Allow recursion in binfmt_script and binfmt

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Date:	Tue, 9 Sep 2008 09:45:55 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, xemul@...nvz.org,
	viro@...iv.linux.org.uk
Subject: Re: [PATCH] Allow recursion in binfmt_script and binfmt_misc

On Mon, Sep 08, 2008 at 03:39:21PM -0700, Andrew Morton wrote:
> On Sat,  6 Sep 2008 18:09:55 +0300
> "Kirill A. Shutemov" <kirill@...temov.name> wrote:
> 
> > binfmt_script and binfmt_misc disallow recursion to avoid stack overflow
> > using sh_bang and misc_bang. It causes problem in some cases:
> > 
> > $ echo '#!/bin/ls' > /tmp/t0
> > $ echo '#!/tmp/t0' > /tmp/t1
> > $ echo '#!/tmp/t1' > /tmp/t2
> > $ chmod +x /tmp/t*
> > $ /tmp/t2
> > zsh: exec format error: /tmp/t2
> > 
> > Similar problem with binfmt_misc.
> > 
> > This patch introduces field 'recursion_depth' into struct linux_binprm
> > to track recursion level in binfmt_misc and binfmt_script. If recursion
> > level more then BINPRM_MAX_RECURSION it generates -ENOEXEC.
> > 
> >
> > ...
> >
> > --- a/include/linux/binfmts.h
> > +++ b/include/linux/binfmts.h
> > @@ -34,8 +34,7 @@ struct linux_binprm{
> >  #endif
> >  	struct mm_struct *mm;
> >  	unsigned long p; /* current top of mem */
> > -	unsigned int sh_bang:1,
> > -		     misc_bang:1;
> > +	unsigned char recursion_depth;
> >  #ifdef __alpha__
> >  	unsigned int taso:1;
> >  #endif
> 
> That's a strange position in which to add the new field.  It will prevent
> the compiler from using the same word for sh_bang, misc_bang and taso.
> 
> I fixed that up while fixing linux-next rejects.

Thanks.

> > @@ -61,6 +60,7 @@ struct linux_binprm{
> >  #define BINPRM_FLAGS_EXECFD_BIT 1
> >  #define BINPRM_FLAGS_EXECFD (1 << BINPRM_FLAGS_EXECFD_BIT)
> >  
> > +#define BINPRM_MAX_RECURSION 4
> 
> Why "4"?

It's enough for my goals. :)

We can increase it later if any user will need it.

> Why make linux_binprm.recursion_depth a u8?  There would be 
> practically (or actually) zero cost to making it 32-bit.
> Admittedly a depth >256 would be a bit odd, but did we gain 
> anything from this restriction?

No.

Should I repost patch with unsigned int recursion_depth?

-- 
Regards,  Kirill A. Shutemov
 + Belarus, Minsk
 + ALT Linux Team, http://www.altlinux.com/

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