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    SubjectRe: + remove-the-likelypid-check-in-copy_process.patch added to -mm tree
    On 03/17, Eric W. Biederman wrote:
    >
    > Oleg Nesterov <oleg@tv-sign.ru> writes:
    >
    > > On 03/17, Oleg Nesterov wrote:
    > >>
    > >> > Well the initial kernel process does not have a struct pid so when
    > >> > it's children start doing:
    > >> > attach_pid(p, PIDTYPE_PGID, task_group(p));
    > >> > attach_pid(p, PIDTYPE_SID, task_session(p));
    > >> > We will get an oops.
    > >>
    > >> So far this is the only reason to have init_struct_pid. Because the
    > >> boot CPU (swapper) forks, right?
    > >
    > > Damn. I am afraid I was not clear again :) Not init_struct_pid, but
    > >
    > > + .pids = { \
    > > + [PIDTYPE_PID] = INIT_PID_LINK(PIDTYPE_PID), \
    > > + [PIDTYPE_PGID] = INIT_PID_LINK(PIDTYPE_PGID), \
    > > + [PIDTYPE_SID] = INIT_PID_LINK(PIDTYPE_SID), \
    > > + }, \
    > >
    > > for INIT_TASK().
    > >
    > >> > So a dummy unhashed struct pid was added for the idle threads.
    > >> > Allowing several special cases in the code to be removed.
    > >> >
    > >> > With that chance the previous special case to force the idle thread
    > >> > init session 1 pgrp 1 no longer works because attach_pid no longer
    > >> > looks at the pid value but instead at the struct pid pointers.
    > >> >
    > >> > So we had to add the __set_special_pids() to continue to keep init
    > >> > in session 1 pgrp 1. Since /sbin/init calls setsid() that our setting
    > >> > the sid and the pgrp may not be strictly necessary. Still is better
    > >> > to not take any chances.
    > >>
    > >> Yes, yes, I see. But my (very unclear, sorry) question was: shouldn't we
    > >> change INIT_SIGNALS then? /sbin/init inherits ->pgrp == ->_session == 1,
    > >> in that case __set_special_pids(1,1) does nothing.
    > >
    > > ... and thus /sbin/init remains attached to the .pids above, no?
    >
    > The problem is that we dynamically allocate the struct pid for
    > pid_t == 1 when we fork init.
    >
    > Which means we don't have access to it at compile time so we can
    > no longer make INIT_SIGNALS set ->gprp == ->session == 1.

    Yes! I meant we should change INIT_SIGNALS(), currently it does

    #define INIT_SIGNALS(sig) {
    ...
    .pgrp = 1,
    { .__session = 1},

    and this confuses (I think) set_special_pids(1,1) above. Because
    __set_special_pids() still deals with pid_t, not "struct pid".

    Unless I missed something, we should kill these 2 initializations
    above.

    Oleg.

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