Messages in this thread | | | Date | Sat, 04 Apr 2009 23:46:15 -0400 | From | Masami Hiramatsu <> | Subject | Re: [BUG][-tip] kprobes on module functions hits kernel BUG in text_poke on x86-32 |
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Masami Hiramatsu wrote: > Masami Hiramatsu wrote: >> Mathieu Desnoyers wrote: >>> * Masami Hiramatsu (mhiramat@redhat.com) wrote: >>>> Hi, >>>> >>>> I found text_poke() problem on x86-32 with the latest-tip tree. >>>> When I put a kprobe on a module function, text_poke() hit a BUG. >>>> >>>> This bug can be reproduced on x86-32, but not on x86-64. >>>> And inserting kprobes on a kernel-core function is OK. >>>> >>>> Thank you, >>>> >>> Hi Masami, >>> >>> OK, so text_poke safety net saves the day :) >>> >>> Basically, what we have here is the BUG_ON I have put : >>> >>> for (i = 0; i < len; i++) >>> BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]); >>> >>> Which checks that the modification is really preceivable in the kernel >>> code, triggers this bug. Only for modules you say. >>> >>> It might not be this, but.. let's try something simple (this could be >>> completely unrelated, but won't take long to test): can you try to add a >>> vmalloc_sync_all() at the beginning of text_poke ? This would make sure >>> that lazily-populated TLB entries, which include module code and data on >>> x86, will be populated. I wonder if we hit this problem because >>> vmalloc_to_page would be returning a mapping to a yet unpopulated TLB >>> entry, if it is ever possible. >> Hmm, from the result of my test, vmalloc_sync_all() didn't change anything... >> >>> If that's not this, then I guess we have some problem with setting a >>> fixmap to a page returned by vmalloc on x86 32.
Hmm, ok. AFAICS, fixmap is only for lowmem, and pkmap is only for highmem.
So, I think we have some options;
A) Separate text_poke into __text_poke and __text_poke_highmem. And use pkmap_atomic in __text_poke_highmem. This way doesn't require any additional change except adding KM_TEXT_POKE0/1 in km_type.
B) Add set_fixmap_page and use it in text_poke. This will require changes in paravirt_ops and pgtable.c. We need to ensure there is no side effects.
C) Change pkmap_atomic_prot to map lowmem only if the page's pgprot is different from user specified pgprot. And use it instead of fixmap. This also requires KM_TEXT_POKE0/1, however we can remove FIX_TEXT_POKE0/1.
etc...
I think A) is for short-term solution. I guess it will be acceptable for next release. But for long-term, C) might be better.
Thank you,
-- Masami Hiramatsu
Software Engineer Hitachi Computer Products (America) Inc. Software Solutions Division
e-mail: mhiramat@redhat.com
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