Messages in this thread | | | Date | Mon, 21 Sep 2009 23:41:21 +0900 | From | Tejun Heo <> | Subject | Re: [PATCH] fix error handling in load_module() |
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Hello, Andrew.
Andrew Morton wrote: > My reverse engineering of the secret, undocumented percpu_modfree() > indicates that its mad inventor intended that percpu_modfree(NULL) be a > valid thing to do. > > It calls free_percpu(), all implementations of which appear to secretly > support free_percpu(NULL).
Eh... unfortunately, the original percpu_modfree() implementation didn't seem to support it.
> So why did your machine crash? > > This: > > void free_percpu(void *ptr) > { > void *addr = __pcpu_ptr_to_addr(ptr); > struct pcpu_chunk *chunk; > unsigned long flags; > int off; > > if (!ptr) > return; > > is dangerous. The implementation of __pcpu_ptr_to_addr() can be > overridden by asm/percpu.h and there's no reason why the compiler won't > choose to pass a NULL into __pcpu_ptr_to_addr(). > > But there doesn't appear to be any overriding of __pcpu_ptr_to_addr() > in 2.6.31 and the default __pcpu_ptr_to_addr() looks like it will > handle a NULL pointer OK. > > So again, why did your machine crash? > > From: Andrew Morton <akpm@linux-foundation.org> > > __pcpu_ptr_to_addr() can be overridden by the architecture and might not > behave well if passed a NULL pointer. So avoid calling it until we have > verified that its arg is not NULL. > > Cc: Rusty Russell <rusty@rustcorp.com.au> > Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org> > --- > > mm/percpu.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff -puN mm/percpu.c~percpu-avoid-calling-__pcpu_ptr_to_addrnull mm/percpu.c > --- a/mm/percpu.c~percpu-avoid-calling-__pcpu_ptr_to_addrnull > +++ a/mm/percpu.c > @@ -957,7 +957,7 @@ static void pcpu_reclaim(struct work_str > */ > void free_percpu(void *ptr) > { > - void *addr = __pcpu_ptr_to_addr(ptr); > + void *addr; > struct pcpu_chunk *chunk; > unsigned long flags; > int off; > @@ -965,6 +965,8 @@ void free_percpu(void *ptr) > if (!ptr) > return; > > + addr = __pcpu_ptr_to_addr(ptr); > +
__pcpu_ptr_to_addr() and reverse should be simple arithmetic transformations. The sole reason why it's defined as overridable was mostly because I didn't know whether all archs could be unified to use the same macro (and different variants were used early during development) but yeap no harm in being careful.
Thanks.
-- tejun
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