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    SubjectRe: Word-at-a-time dcache name accesses (was Re: .. anybody know of any filesystems that depend on the exact VFS 'namehash' implementation?)
    On 03/02/2012 04:57 PM, Linus Torvalds wrote:
    > On Fri, Mar 2, 2012 at 4:38 PM, H. Peter Anvin <hpa@zytor.com> wrote:
    >>
    >> My biggest concern is what happens when this happens to be at the end of
    >> mapped kernel memory and we overrun the page?
    >
    > Yes. It's very unlikely, and it never happens with the dentry data
    > itself (the name is always aligned for those).
    >
    > But it *can* happen if:
    >
    > - the page contains the filename we copied from user space
    >
    > - the page is the last page mapped
    >
    > - the filename is PATH_MAX in size (or very close)
    >
    > - the last component is sufficiently unaligned
    >
    > but I was thinking we'd just make sure not to free the last page, and
    > just solve it that way.
    >
    > I was playing around with other ideas (take the page fault and fix it
    > up), but those are all really complicated when the notion of "don't
    > use the last page" is so much simpler.
    >

    Note that does mean we need a guard page after each and every
    discontiguous RAM range, not just the last one. Raising that issue
    since we have had serious bugs in that area in the past.

    -hpa




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