Messages in this thread | | | Date | Thu, 26 Jul 2007 09:29:50 -0500 | From | "Eric Van Hensbergen" <> | Subject | Re: net/9p/mux.c: use-after-free |
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On 7/25/07, Andrew Morton <akpm@linux-foundation.org> wrote: > On Wed, 25 Jul 2007 13:43:16 -0500 "Eric Van Hensbergen" <ericvh@gmail.com> wrote: > > > mtmp = ERR_PTR(PTR_ERR(m->tagpool)); > > odd. What does ERR_PTR(PTR_ERR(...)) do? >
I kind of assumed it was a necessry evil to get the casting right. A quick grep shows it in 42 other places within the kernel. Unpacking the macros it looks like:
(void *)(long)(struct p9_idpool *)
So all that you would really need is (void *) or ERR_PTR -- but that might look confusing in the code. Of course, broadening the context a bit:
m->tagpool = p9_idpool_create(); if (!m->tagpool) { mtmp = ERR_PTR(PTR_ERR(m->tagpool)); kfree(m); return mtmp; }
m->tagpool must be zero to enter the code at all, so we are returning a NULL pointer, not really an error -- which is probably wrong (I don't think it will properly trigger IS_ERR_VALUE) -- so we should probably be returning -ENOMEM.
Of course, we really should be seeing an ERR_PTR returned from p9_idpool_create, not 0 -- checking that code, it either returns -ENOMEM or the correct value, never 0, so the check is wrong as well. It should be:
m->tagpool = p9_idpool_create(); if (IS_ERR(m->tagpool)) { mtmp = ERR_PTR(-ENOMEM); kfree(m); return mtmp; }
We could have done: ERR_PTR(m->tagpool); or kept the long: ERR_PTR(PTR_ERR(m->tagpool)); but I think returning an explicit error code keeps the code more clear.
So, which is the correct approach?
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