Re: SET_PERSONALITY and TASK_SIZE

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

> On Sun, 18 Jan 2009 12:18:31 +0100 Heiko Carstens <heiko.carstens@de.ibm.com> wrote:
> Hi Andrew,
> 
> while debugging I noticed the following comment in fs/binfmt_elf.c:
> 
> 			/*
> 			 * The early SET_PERSONALITY here is so that the lookup
> 			 * for the interpreter happens in the namespace of the 
> 			 * to-be-execed image.	SET_PERSONALITY can select an
> 			 * alternate root.
> 			 *
> 			 * However, SET_PERSONALITY is NOT allowed to switch
> 			 * this task into the new images's memory mapping
> 			 * policy - that is, TASK_SIZE must still evaluate to
> 			 * that which is appropriate to the execing application.
> 			 * This is because exit_mmap() needs to have TASK_SIZE
> 			 * evaluate to the size of the old image.
> 			 *
> 			 * So if (say) a 64-bit application is execing a 32-bit
> 			 * application it is the architecture's responsibility
> 			 * to defer changing the value of TASK_SIZE until the
> 			 * switch really is going to happen - do this in
> 			 * flush_thread().	- akpm
> 			 */

Cripes, that must have been a different akpm.

> At least s390 isn't doing the deferred TASK_SIZE switch. Also it seems like
> MIPS, PARISC and IA64 don't do it either. However from a quick a view I
> couldn't see that exit_mmap depends on TASK_SIZE. So is this still necessary?
> 
> And the bug I was looking for is this one: in SET_PERSONALITY we do this:
> 
> 	if (current->personality != PER_LINUX32)
> 		set_personality(PER_LINUX);
> 
> However we should use the PER_MASK if we want to check for PER_LINUX32,
> since there are more bits in the personality flags. In case any of the
> 'extra' bits is set we may incorrectly set personality to PER_LINUX even
> when we want PER_LINUX32.
> 
> Looks like more architectures should do something like:
> 
> 	if (personality(current->personality) != PER_LINUX32)
> 		...

I'm stuck in planes and hotels for the rest of the week and don't have a
git tree to dig through, sorry.  I'd prefer to hold off until next week
before diving into that one.