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Date: Fri, 29 Mar 2013 12:54:12 -0700
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Subject: Re: ipc,sem: sysv semaphore scalability
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Dave Jones <davej@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@surriel.com>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        hhuang@redhat.com, "Low, Jason" <jason.low2@hp.com>,
        Michel Lespinasse <walken@google.com>,
        Larry Woodman <lwoodman@redhat.com>,
        "Vinod, Chegu" <chegu_vinod@hp.com>,
        Stanislav Kinsbursky <skinsbursky@parallels.com>
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On Fri, Mar 29, 2013 at 12:33 PM, Peter Hurley <peter@hurleysoftware.com> wrote:
> On Fri, 2013-03-29 at 11:43 -0700, Linus Torvalds wrote:
>> I think I foud at least one bug in the MSG_COPY stuff: it leaks the
>> "copy" allocation if
>>
>>     mode == SEARCH_LESSEQUAL
>>
>> but maybe I'm misreading it.
>
> Yes, you're misreading it. copy_msg() returns the 'copy' address when
> copying is successful. So this patch double-frees 'copy'.

No it doesn't.

The case where "copy_msg()" *is* called and is successful, msg gets
set to "copy", my patch sets "copy" to NULL, so no, it doesn't
double-free.

That said, it looks like if MSG_COPY is set, we cannot trigger the
"mode == SEARCH_LESSEQUAL" case, because it forces msgtyp to 0, which
in turn forces mode = SEARCH_ANY.

So the actual leak case seems to not be possible, although that's
*really* hard to see from looking at the code. The code is just an
unreadable mess.

              Linus