linux-kernel - Re: [KJ] audit return code handling for kernel

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Date:	Sat, 29 Jul 2006 09:14:19 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	kernel-janitors@...l.org, linux-kernel@...r.kernel.org,
	schwidefsky@...ibm.com
Subject: Re: [KJ] audit return code handling for kernel_thread [2/11]

On Sat, Jul 29, 2006 at 10:37:04AM +0100, Russell King wrote:
> On Fri, Jul 28, 2006 at 04:07:13PM -0400, nhorman@...driver.com 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
Neil

> -- 
> Russell King
>  Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
>  maintainer of:  2.6 Serial core
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