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    SubjectQ: Do si_time and si_utime need to be 64bit for y2038?

    Arnd,

    I am looking at the siginfo si_utime and si_stime fields of type clock_t
    on 32bit architectures except for x32 these are 32bit fields. For y2038
    do we want to extend these fields to 64bit like x32 does? Or is it not
    a problem for these fields to be 32bit?

    I care right now because I am trying to figure out how
    copy_siginfo_to_user32 and copy_siginfo_to_user need to evolve.

    If we are going to extend existing architectures with 64bit variations
    of si_utime and si_stime copy_siginfo_to_user and copy_siginfo_to_user32
    needs an additional parameter describing which variant they should be
    copying.

    It looks like posix does not define si_stime and and si_utime so we only
    have to be backwards compatible with ourselves for whatever that is
    worth.

    I am wondering if perhaps the general solution might be to just add
    two extra fields si_stime64 and si_utime64 and always fill those in.

    Arnd do you have any ideas?


    Andy Lutomirski <luto@amacapital.net> writes:

    >> On Apr 10, 2018, at 6:26 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
    >>
    >>
    >> Andy,
    >>
    >> I am looking at copy_siginfo_to_user32 and find it very unfortunate
    >> that x86 with _sigchld_x32 needs to be the odd man out. I am looking
    >> at ways to simplify the special case.
    >>
    >> The core of the special case comes from:
    >> exit_to_usermode_loop
    >> do_signal
    >> handle_signal
    >> setup_rt_frame
    >>
    >>
    >> In setup_rt_frame the code looks at ksig to see which kind of signal
    >> frame should be written for the signal.
    >>
    >> This leads to the one case in the kernel where copy_siginfo_to_user32
    >> does not use is_ia32_syscall() or is_x32_syscall() to see which kind of
    >> signal frame it needs to create.
    >>
    >> Andy, since you have been all over the entry point code in recent years
    >> do you know if we allow tasks that can do both ia32 and x86_64 system
    >> calls? That seems to be what we the testing of ksig to see which kind
    >> of signal frame to setup is all about.
    >
    > We do :(
    >
    >> If we don't allow mixed abi's on x86_64 then can I see if I have a ia32
    >> task in setup_rt_frame by just calling is_ia32_syscall()?
    >>
    >> If we do allow mixed abi's do you know if it would be safe to
    >> temporarily play with orig_ax or current_thread_info()->status?
    >
    > Maybe, but it’s a real minefield. I think the right fix is to use
    > sa_flags's SA_X32_ABI bit instead for the sigchld bit. In general,
    > the is_..._syscall() helpers can't be expected to return anything
    > valid in any context other than a syscall, and handle_signal() is not
    > a syscall.
    >
    >>
    >> My goal is to write two wrappers: copy_siginfo_to_user32_ia32, and
    >> copy_siginfo_to_user32_x32 around the ordinary copy_siginfo_to_user32.
    >> With only a runtime test to see which ABI we need to implement.
    >>
    >> Aka change:
    >>> case SIL_CHLD:
    >>> to->si_pid = from->si_pid;
    >>> to->si_uid = from->si_uid;
    >>> to->si_status = from->si_status;
    >>> #ifdef CONFIG_X86_X32_ABI
    >>> if (x32_ABI) {
    >>> to->_sifields._sigchld_x32._utime = from->si_utime;
    >>> to->_sifields._sigchld_x32._stime = from->si_stime;
    >>> } else
    >>> #endif
    >>> {
    >>> to->si_utime = from->si_utime;
    >>> to->si_stime = from->si_stime;
    >>> }
    >>> break;
    >> to something like:
    >>> case SIL_CHLD:
    >>> to->si_pid = from->si_pid;
    >>> to->si_uid = from->si_uid;
    >>> to->si_status = from->si_status;
    >>> #ifdef CONFIG_X86_X32_ABI
    >>> if (!is_ia32_syscall()) {
    >>> to->_sifields._sigchld_x32._utime = from->si_utime;
    >>> to->_sifields._sigchld_x32._stime = from->si_stime;
    >>> } else
    >>> #endif
    >>> {
    >>> to->si_utime = from->si_utime;
    >>> to->si_stime = from->si_stime;
    >>> }
    >>> break;
    >>
    >
    > Makes sense, but can you get to sa_flags in there instead?

    Almost. copy_siginfo_to_user32 is called in 3 places: setup_rt_frame32
    (or whatever the arch names the function for setting up the 32bit signal
    frame), ptrace, and compat_binfmt_elf.

    So except for ptrace and compat_binfmt_elf sa_flags are available so
    that is a possibility. And for those we can fake something up if
    needed.

    Stepping back it really looks like the question is really do
    we want/need 64bit time in siginfo for 32bit architectures to
    make the code y2038 safe?

    If so passing an extra parameter to copy_siginfo_to_user32 and
    copy_siginfo_to_user is a no-brainer. If not we are at x86 and
    in particular x32 is weird. So I am asking Arnd above if he
    has any idea which way things should evolve.

    > FWIW, I have a branch here:
    >
    > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=execve
    >
    > that contains a *massive* cleanup of some other x86 signal stuff. I
    > need to dust it off and test it better.

    It looks interesting, and except for the last patch "Drop the separate
    compat signal delivery code" looks orthogonal to what I am doing.

    What I have seen other architectures do in that last case are instead of
    #ifdefs to #define functions to their compat counterparts on 64bit.
    Something like:
    #define copy_siginfo_to_user copy_siginfo_to_user32

    Eric

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