Re: [KJ] audit return code handling for kernel_thread [2/11]

From: Neil Horman
Date: Sat Jul 29 2006 - 09:16:04 EST

On Sat, Jul 29, 2006 at 10:37:04AM +0100, Russell King wrote:
> On Fri, Jul 28, 2006 at 04:07:13PM -0400, nhorman@xxxxxxxxxxxxx wrote:
> > Audit/Cleanup of kernel_thread calls, specifically checking of return codes.
> > Problems seemed to fall into 3 main categories:
> >
> > 1) callers of kernel_thread were inconsistent about meaning of a zero return
> > code. Some callers considered a zero return code to mean success, others took
> > it to mean failure. a zero return code, while not actually possible in the
> > current implementation, should be considered a success (pid 0 is/should be
> > valid). fixed all callers to treat zero return as success
> >
> > 2) caller of kernel_thread saved return code of kernel_thread for later use
> > without ever checking its value. Callers who did this tended to assume a
> > non-zero return was success, and would often wait for a completion queue to be
> > woken up, implying that an error (negative return code) from kernel_thread could
> > lead to deadlock. Repaired by checking return code at call time, and setting
> > saved return code to zero in the event of an error.
> This is inconsistent with your assertion that pid 0 "is/should be valid"
> above. If you want '0' to mean "not valid" then it's not a valid return
> value from kernel_thread() (and arguably that's true, since pid 0 is
> permanently allocated to the idle thread.)
No its, not, but I can see how my comments might be ambiguous. I want zero to be
a valid return code, since we never actually return zero, but we certainly could
if we wanted to. Note that kernel_thread returns an int (not an unsigned int),
and as such assuming that a non-zero return code implies success ignores the
fact that kernel_thread can return a negative value, which indicates failure.
This is what I found, and what my patch fixes.

> I don't particularly care whether you decide to that returning pid 0 from
> kernel_thread is valid or not, just that your two points above are at least
> consistent with each other.
My comments in (2) should be made more clear by changing "assume a non-zero
return was success" to "assume a negative return was success". This is what my
patch fixes.

Thanks & Regards

> --
> Russell King
> Linux kernel 2.6 ARM Linux -
> maintainer of: 2.6 Serial core
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