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    SubjectRe: BUG-lockdep and freeze (was: Arrr! Linux 2.6.18)
    On Saturday 30 September 2006 23:56, Linus Torvalds wrote:
    > On Sat, 30 Sep 2006, Andi Kleen wrote:
    > >
    > > Anyways, I guess we need even more validation in the fallback code,
    > > but just terminating the kernel thread stacks should fix that particular case.
    > Why not just add the simple validation?
    > A kernel stack is one page in size. If you move to another page, you
    > terminate. It's that simple.

    No, it's not. On x86-64 it can be three or more stacks nested in
    complicated ways (process stack, interrupt stack, exception stack)
    The exception stack can happen multiple times.

    > What if the kernel stack is corrupt? Buffer overruns do that.
    > This patch seems to just paper over the _real_ problem, namely the fact
    > that the stack tracer code doesn't actually validate any of its arguments.

    It has pretty good sanity checking by first using __get_user for the stack
    data, and the regularly double checking the EIPs by looking them up
    in CFI. If it can't find them it will abort.

    > The old unwinder (well, at least for x86, and I assume x86-64 used that as
    > the beginning point) didn't have this problem at all, exactly because it
    > couldn't get on the wrong stack-page in the first place.

    In this particular case what happened is that the dwarf2 unwinder
    ended and then the fallback was in the wrong page and couldn't handle

    > The old code literally had:
    > static inline int valid_stack_ptr(struct thread_info *tinfo, void *p)
    > {
    > return p > (void *)tinfo &&
    > p < (void *)tinfo + THREAD_SIZE - 3;
    > }
    > and would refuse to touch anything that wasn't in the stack page. It was

    That was before interrupt stacks were introduced. With that it is significantly
    more complicated. On x86-64 even more because there are exception stacks.

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      Last update: 2006-10-01 00:05    [W:0.023 / U:12.264 seconds]
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