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    SubjectRe: [PATCH] /proc/kcore: fix seeking
    On Fri, Jan 14, 2011 at 05:44:42PM +0800, Américo Wang wrote:
    > On Tue, Jan 11, 2011 at 05:23:23PM +0100, Frederic Weisbecker wrote:
    > >On Wed, Jan 12, 2011 at 12:04:37AM +0800, Américo Wang wrote:
    > >> On Mon, Jan 10, 2011 at 09:42:29AM -0500, Dave Anderson wrote:
    > >> >From: Dave Anderson <anderson@redhat.com>
    > >> >
    > >> >Commit 34aacb2920667d405a8df15968b7f71ba46c8f18
    > >> >("procfs: Use generic_file_llseek in /proc/kcore")
    > >> >broke seeking on /proc/kcore. This changes it back
    > >> >to use default_llseek in order to restore the original
    > >> >behavior.
    > >> >
    > >> >The problem with generic_file_llseek is that it only
    > >> >allows seeks up to inode->i_sb->s_maxbytes, which is
    > >> >2GB-1 on procfs, where the memory file offset values in
    > >> >the /proc/kcore PT_LOAD segments may exceed or start
    > >> >beyond that offset value.
    > >> >
    > >>
    > >> Is the race solved? Using default_llseek() still races
    > >> with read_kcore() on fpos, AFAIK.
    > >
    > >Hmm, how does it race there?
    > >
    > >read_kcore() manipulates fpos, which can't be changed behind
    > >us inside the read callback as it's a snapshot. Also read_kcore()
    > >can change the value of fpos, which is writed back to file->fpos
    > >from sys_read().
    > >
    > >So the last resulting race here the natural one between
    > >seeking and reading, which is up to the user to take care
    > >of.
    >
    > Hmm, I just read the changelog of commit
    > 34aacb2920667d405a8df15968b7f71ba46c8f18, which claims to fix
    > the race. So anything changed in vfs layer after that?


    Ah it didn't fix any race, it just got rid of the bkl, OTOH
    I said in my changelog:

    "/proc/kcore has no llseek and then falls down to use default_llseek.
    This is racy against read_kcore() that directly manipulates fpos
    but it doesn't hold the bkl there so using it in llseek doesn't
    protect anything."

    So I think this just testifies my crude misunderstanding of the code when I wrote
    that changelog. I didn't realize fpos is a copy of the file offset that is writed back
    later. Hence my changelog was buggy.
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