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SubjectRe: [PATCH] oops in sd_shutdown
At +0200 04:49 AM 8/12/2003, Andries Brouwer wrote:
>On Mon, Aug 11, 2003 at 06:13:50PM -0700, Jeff Woods wrote:
>>Looking only at the above code snippet, I'd suggest something more like:
>>+ if (!sdp ||
>This is not meaningful.

How is it not meaningful? The next action in the expression is to
dereference the pointer and if it has a NULL value then I expect the
dereference to fail. [But I am a complete newbie with respect to Linux
kernel and driver code so perhaps my understanding is in error. If so,
please enlighten me.]

>A general kind of convention is that a pointer will be NULL either by
>mistake, when it is uninitialized, or on purpose, when no object is
>present or no action (other than the default) needs to be performed.

Of course. But in this case, the next action the code will attempt is to
dereference that pointer which I expect would fail if it's NULL. If you're
telling me the pointer cannot be NULL, then fine [which I tried to indicate
was a possibility in my first email in this thread], but if the pointer
might *ever* be NULL (and dereferencing a NULL pointer in this context is
as bad as it usually is) then there is no point in proceeding and returning
from the function immediately seems like a reasonable thing to do in case
of a shutdown function. (I can see possible value in additionally
reporting an error or warning somehow if feasible, but that's not germane
to whether checking the pointers for NULL is a prudent action.

>But that general idea is broken by container_of(), which just subtracts a
>constant. So, one should check before subtracting that the pointer is
>non-NULL. Checking afterwards is meaningless.

As I tried to indicate in the opening statement I have not looked at any
other code than what you included in the patch diff beginning this thread
so I don't see any reference to anything that indicates some function
called container_of() [which sounds like it might be some C++ routine...
and I was under the impression this code is C rather than C++]. The diff
includes the beginning of the function including initialization of both the
sdp and sdkp pointers. One bug the patch fixes is the implicit dereference
of sdkp in the original version of the "if" statement I suggest be
modified. My version of the patch (1) reduces the number of lines changed,
(2) results in fewer lines, (3) improves the transparency of the code, and
(4) additionally checks for a (perhaps unlikely or even improbable)
potential unanticipated runtime error, all of which makes me believe it is
an improvement.

Jeff Woods <>

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