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    SubjectRe: [RFC][PATCH][v2] Define/use siginfo_from_ancestor_ns()
    Oleg Nesterov [oleg@redhat.com] wrote:
    | On 11/15, Sukadev Bhattiprolu wrote:
    | >
    | > Subject: [PATCH] Define/use siginfo_from_ancestor_ns()
    |
    | Imho, the main problem with this patch is that it tries to do many
    | different things at once, and each part is suboptimal/incomplete.
    |
    | This needs several patches. Not only because this is easier to review,
    | but also because each part needs the good changelog.

    I agree I sent this as an RFC to show the overall changes.
    I do plan to include the following two patches, which should address
    the issue of ->nsproxy being NULL.
    https://lists.linux-foundation.org/pipermail/containers/2008-November/014187.html
    https://lists.linux-foundation.org/pipermail/containers/2008-November/014188.html

    |
    | Also. I don't think we should try do solve the "whole" problem right
    | now. For example, if we add/use siginfo_from_ancestor_ns(), we should
    | also change sys_sigaction(SIG_IGN). As I said, imho we should start
    | with:
    |
    | - cinit can't be killed from its namespace
    |
    | - the parent ns can always SIGKILL cinit
    |
    | This solves most of problems, and this is very simple.

    Yes, I agree and am trying to solve only those two :-) I moved out
    changes to __do_notify() and others to separate patches, but maybe
    we can simplify this patch further.

    |
    | As for .si_pid mangling, this again needs a separate patch.

    I thought we were going to use SIG_FROM_USER to decide if the siginfo
    does in fact have a ->si_pid (so we don't need the switch statement
    we had in an earlier patch).

    |
    | Sukadev, I don't have a time today, I'll return tomorrow with more
    | comments on this...

    No problem. Thanks for the comments so far.

    |
    | > +static int sig_ignored(struct task_struct *t, int sig, int same_ns)
    | > {
    | > void __user *handler;
    | >
    | > @@ -68,6 +68,14 @@ static int sig_ignored(struct task_struct *t, int sig)
    | > handler = sig_handler(t, sig);
    | > if (!sig_handler_ignored(handler, sig))
    | > return 0;
    | > + /*
    | > + * ignores SIGSTOP/SIGKILL signals to init from same namespace.
    | > + *
    | > + * TODO: Ignore unblocked SIG_DFL signals also here or drop them
    | > + * in get_signal_to_deliver() ?
    | > + */
    | > + if (is_container_init(t) && same_ns && sig_kernel_only(sig))
    | > + return 1;
    |
    | No, no. is_container_init() is slow and unneeded, same_ns is bogus,
    | the usage of sig_kernel_only() is suboptimal. The comment is not
    | right too...

    Maybe in a separate patch, but same_ns is needed to ensure container-init
    does not ignore signals from ancestor namespace - no ?

    I was undecided between the above sig_kernel_only() check and
    'handler == SIG_DFL' (hence the TODO).

    |
    | As I already said, this problem is not namespace-specific, we need
    | some changes for the global init too.

    Right I used is_container_init() since it includes global init().
    Again, maybe it could have been separate patches for just global_init
    first.

    But I see from your patch that we could use SIGNAL_UNKILLABLE instead
    of is_container_init(). That is more efficient.

    |
    | Actually, I already did the patch, I'll send it soon.

    Ok. I will review that.

    |
    | > static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
    | > int group)
    | > {
    | > struct sigpending *pending;
    | > struct sigqueue *q;
    | > + int from_ancestor_ns;
    | > +
    | > + from_ancestor_ns = 0;
    | > + if (siginfo_from_user(info)) {
    | > + /* if t can't see us we are from parent ns */
    | > + if (task_pid_nr_ns(current, task_active_pid_ns(t)) == 0)
    | ^^^^^^^^^^^^^^^^^^
    |
    | ->nsproxy may be NULL, but we can use task_pid(t)->numbers[-1].ns

    Eric's patch of generalizing task_active_pid_ns() should fix this. It
    was reviewed several times, so I did not send it, but yes, I should
    have mentioned it.

    |
    | > @@ -864,6 +902,9 @@ static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
    | > * and sent by user using something other than kill().
    | > */
    | > return -EAGAIN;
    | > +
    | > + if (from_ancestor_ns)
    | > + return -ENOMEM;
    |
    | This change alone needs a fat comment in changelog. But I don't think
    | we need it now. Until we change the dequeue path to check "from_ancestor_ns".

    Ok.

    |
    | > +static inline int siginfo_from_ancestor_ns(siginfo_t *info)
    | > +{
    | > + return SI_FROMUSER(info) && (info->si_pid == 0);
    | > +}
    |
    | Yes, this is problem... I doubt we can rely on !si_pid here.
    | More on this later.
    |
    | > @@ -2296,10 +2347,25 @@ sys_rt_sigqueueinfo(pid_t pid, int sig, siginfo_t __user *uinfo)
    | > Nor can they impersonate a kill(), which adds source info. */
    | > if (info.si_code >= 0)
    | > return -EPERM;
    | > - info.si_signo = sig;
    | > + info.si_signo = sig | SIG_FROM_USER;
    | >
    | > /* POSIX.1b doesn't mention process groups. */
    | > - return kill_proc_info(sig, &info, pid);
    | > + rcu_read_lock();
    | > + spid = find_vpid(pid);
    | > + /*
    | > + * A container-init (cinit) ignores/drops fatal signals unless sender
    | > + * is in an ancestor namespace. Cinit uses 'si_pid == 0' to check if
    | > + * sender is an ancestor. See siginfo_from_ancestor_ns().
    | > + *
    | > + * If signalling a descendant cinit, set si_pid to 0 so it does not
    | > + * get ignored/dropped.
    | > + */
    | > + if (!pid_nr_ns(spid, task_active_pid_ns(current)))
    | > + info.si_pid = 0;
    | > + error = kill_pid_info(sig, &info, spid);
    |
    | Can't understand. We set SIG_FROM_USER, If signalling a descendant task
    | (not only cinit), send_signal() will clear .si_pid anyway?

    Good point. We had gone back and forth on this and I thought one of the
    emails mentioned this check. Maybe I misread that.

    But yes, its not needed since send_signal() does it.



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