Re: [PATCH] Allow recursion in binfmt_script and binfmt_misc

[Andrew Morton <akpm@linux-foundation.org>]

On Sat,  6 Sep 2008 18:09:55 +0300
"Kirill A. Shutemov" <kirill@shutemov.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.

> @@ -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"?

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?