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    SubjectRe: [git pull] drm: previous pull req + 1.

    On Sun, 21 Jun 2009, Linus Torvalds wrote:
    > Dave - no amount of userspace differences make a corrupted page table
    > acceptable.
    > This needs to be fixed. No excuses. Kernel crashes are never an issue of
    > "you used the wrong user space".

    So "corrupted page table" means that one of the reserved bits was set, and
    we get a page fault with the PF_RSVD bit on in the error code.

    Looking at the debug output, it says

    PGD 12148a067
    PUD 12148b067
    PMD 121496067
    PTE ffffc90011780237

    where the top-level entries look fine, but the PTE is total crap. It looks
    like it has filled in the page frame number with a virtual address rather
    than with an actual page

    The PTE _should_ look like this:

    - bit 63: NX
    - bits 62-52: zero (available to sw, but I don't think we use them)
    - bits 51-47: zero (reserved)
    - bits 46-12: page frame
    - bits 11-0: protection and PAT bits etc (bits 8-7 are also reserved)

    and that PTE clearly does not match.

    Strictly speaking, that "47-bit" physical address is purely theoretical. I
    think existing CPU's are limited to 40 bits or so, so there are even more
    reserved bits.

    Anyway, here's a totally UNTESTED patch that hopefully gives a warning on
    where exactly we set the invalid bits. Andy, mind trying it out? You
    should get the warnign much earlier, and it should have a much more useful

    (But this is _untested_, so maybe I screwed up and it doesn't compile or
    work. The BAD_PTE_BITS mask could also be improved upon, but that mask
    should be "good enough" - it doesn't include _all_ the bits it could,
    but it certainly has enough bits set to trigger that obviously bad case).


    arch/x86/include/asm/pgtable_64.h | 3 +++
    1 files changed, 3 insertions(+), 0 deletions(-)

    diff --git a/arch/x86/include/asm/pgtable_64.h b/arch/x86/include/asm/pgtable_64.h
    index c57a301..b95828e 100644
    --- a/arch/x86/include/asm/pgtable_64.h
    +++ b/arch/x86/include/asm/pgtable_64.h
    @@ -49,8 +49,11 @@ static inline void native_pte_clear(struct mm_struct *mm, unsigned long addr,
    *ptep = native_make_pte(0);

    +#define BAD_PTE_BITS (_PAGE_NX - (1ul << __PHYSICAL_MASK_SHIFT))
    static inline void native_set_pte(pte_t *ptep, pte_t pte)
    + WARN_ON_ONCE(pte.pte & BAD_PTE_BITS);
    *ptep = pte;

     \ /
      Last update: 2009-10-18 23:28    [W:0.033 / U:11.756 seconds]
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