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SubjectRe: Q: sys_personality() && misc oddities
Date
I don't have any special insight either, just poking around as you are.

I suspect that most of the logic and code in the kernel for this just
predates 64-bit stuff, and so was written with long==int and then maybe
tweaked slightly later.

I don't think we're ever going to need or want a 64-bit personality word.
(There are still 10 bits unused in the middle for more flags.)

Though the high bit might be set on 32-bit, there still should not really
be a danger of misinterpreting a value as an error code--as long as we
haven't used up all 10 of those middle bits. The test userland (glibc)
uses is not "long < 0" but "u_long > -4095UL". So as long as at least
one bit in 0xff00 remains clear, it won't match.

For 64-bit you want to avoid sign-extension of the old value just so it
looks valid (even though it won't look like an error code). I think the
most sensible thing is to change the task_struct field to 'unsigned int'.
Then:

u_long old = current->personality;

will do what:

u_long old = (unsigned int) current->personality;

would do now, i.e. zero-extend rather than sign-extend.

I think the 0xffffffff check is intended specifically for personality(-1)
to be a no-op that returns the old value, and nothing more. So I'd just
make that check be "((int) personality != -1)" instead.

OTOH, this:

set_personality(personality);
if (current->personality != personality)
return -EINVAL;

will then both do the job in set_personality() and return -EINVAL, when
some high bits are set. So, perhaps you are right about checking high
bits. Then I'd make it:

if ((int) personality != -1) {
if (unlikely((unsigned int) personality != personality))
return -EINVAL;
set_personality(personality);
if (current->personality != personality)
return -EINVAL;
}

> __set_personality(). What is the point to check
> ep == current_thread_info()->exec_domain ? This buys nothing afaics.
> IOW, it could be simplified:

Agreed.

> Now let's look at the caller, sys_personality()
>
> set_personality(personality);
> if (current->personality != personality)
> return -EINVAL;
>
> but __set_personality() always sets current->personality = personality,
> what is the point to check equality?

Good point. I suspect that at some point in the past set_personality()
would sometimes refuse to make the change.

> IOW, when we should return -EINVAL? Perhaps, lookup_exec_domain() should
> return NULL instead of default_exec_domain when the search in exec_domains
> fails? And probably we shouldn't change task->personality/exec_domain in
> this case? It is really strange that sys_personality() can return -EINVAL
> but change ->personality.

Agreed.

> But this can probably break exec. alpha does set_personality(PER_OSF4)
> but I do not see the corresponding register_exec_domain(). On the other
> hand, I do not understand why it puts PER_OSF4 into PER_MASK. PER_OSF4
> is only used by sys_osf_readv/sys_osf_writev.

No idea about that.


Thanks,
Roland


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