Re: SET_PERSONALITY and TASK_SIZE

On Sun, 18 Jan 2009 12:18:31 +0100 Heiko Carstens <heiko.carstens@xxxxxxxxxx> 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.
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