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    SubjectRe: [PATCH -mm 7/8] user_ns: handle file sigio
    Quoting Cedric Le Goater (clg@fr.ibm.com):
    > Serge E. Hallyn wrote:
    > > Quoting Andrew Morton (akpm@osdl.org):
    > >> On Thu, 4 Jan 2007 12:12:57 -0600
    > >> "Serge E. Hallyn" <serue@us.ibm.com> wrote:
    > >>
    > >>> A process in one user namespace could set a fowner and sigio on a file in a
    > >>> shared vfsmount, ending up killing a task in another user namespace.
    > >>>
    > >>> Prevent this by adding a user namespace pointer to the fown_struct, and
    > >>> enforcing that a process causing a signal to be sent be in the same
    > >>> user namespace as the file owner.
    > >> This patch breaks the X server (stock FC5 install) with CONFIG_USER_NS=n.
    > >> Neither the USB mouse nor the trackpad work. They work OK under GPM.
    > >>
    > >> Setting CONFIG_USER_NS=y "fixes" this. This bug was not observed in
    > >> 2.6.20-rc3-mm1 because that kernel had user-ns-always-on.patch for other
    > >> reasons. (I'll restore that patch).
    > >>
    > >> There's nothing very interesting here:
    > >
    > [ ... ]
    > >
    > > I can't see any reason for this in the code or comparative ltp runs.
    > > Cedric is testing on a fc6 laptop, hopefully he can reproduce it.
    >
    > I did reproduce it on a FC5 desktop finally.
    >
    > get_user_ns() returns NULL when CONFIG_USER_NS=n and this breaks
    > sigio_perm() which does not expect NULL values for ->user_ns.

    Argh.

    Thanks, Cedric.

    Rewriting the userns testcases right now. Clearly, in addition to
    separately testing clone and unshare, I need to add a sigioperm check,
    and have a separate set of testcases for CONFIG_USER_NS=n.

    thanks,
    -serge

    > I would fix this with the following patch.
    >
    > C.
    >
    >
    > Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>

    Acked-off-by: Serge E Hallyn <serue@us.ibm.com>

    > ---
    > include/linux/user_namespace.h | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > Index: 2.6.20-rc4-mm1/include/linux/user_namespace.h
    > ===================================================================
    > --- 2.6.20-rc4-mm1.orig/include/linux/user_namespace.h
    > +++ 2.6.20-rc4-mm1/include/linux/user_namespace.h
    > @@ -49,7 +49,7 @@
    >
    > static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
    > {
    > - return NULL;
    > + return &init_user_ns;
    > }
    >
    > static inline int unshare_user_ns(unsigned long flags,
    -
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