Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777 warning

Paul Jackson
2007-06-01T19:48:14Z

> The allocated array has 0 bytes. Any dereference is an error.

Well, then, that doesn't work either.

We -agree- we can't have this failing. You suggested (if I understood
correctly, which I doubt) that we set the pointer to NULL, and noted
that derefencing a NULL pointer would fail. I agree such derefences
would fail, and tried to point out that this would be bad. We can't
have pointer dereferences failing in the kernel ... duh.

Your reply seems like a non sequiter to me, pointing out that having
a pointer to an array of 0 bytes fails as well. Ok - that's bad too.

So we cannot have a NULL pointer, used unchecked, and we cannot have
a non-NULL pointer to a zero byte array, used unchecked.

As I said, and as I thought Srinivasa coded, with these patch lines:

@@ -1772,6 +1779,9 @@ static ssize_t cpuset_tasks_read(struct
{
struct ctr_struct *ctr = file->private_data;

+ if (!ctr->buf) /* No tasks in this cpuset */
+ return 0;
+
return simple_read_from_buffer(buf, nbytes, ppos, ctr->buf, ctr->bufsz);
}

we actually have to check the NULL-ness (or emptiness) of this thing,
down where it would be used, to avoid using NULL or empty things.

Summary - Srinivasa's patch still looks ok to me. This followup
discussion between you and I is just confusing me - sorry.

--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <pj@sgi.com> 1.925.600.0401