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

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Message-Id: <20070601124757.fe91a002.pj@sgi.com>
Date:	Fri, 1 Jun 2007 12:47:57 -0700
From:	Paul Jackson <pj@....com>
To:	Christoph Lameter <clameter@....com>
Cc:	srinivasa@...ibm.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	vatsa@...ibm.com, dino@...ibm.com, simon.derr@...l.net,
	clameter@...ulhu.engr.sgi.com, rientjes@...gle.com
Subject: Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777
 warning

> 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@....com> 1.925.600.0401
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