Re: [PATCH v2] signal: add procfd_signal() syscall

[Christian Brauner <christian@brauner.io>]

On Thu, Nov 29, 2018 at 11:13:57PM -0600, Eric W. Biederman wrote:
> Arnd Bergmann <arnd@arndb.de> writes:
> 
> > On Thu, Nov 29, 2018 at 9:14 PM Andy Lutomirski <luto@amacapital.net> wrote:
> >> > On Nov 29, 2018, at 11:55 AM, Christian Brauner <christian@brauner.io> wrote:
> >> >> On Thu, Nov 29, 2018 at 11:22:58AM -0800, Andy Lutomirski wrote:
> >> >>> On Thu, Nov 29, 2018 at 11:17 AM Christian Brauner <christian@brauner.io> wrote:
> >> >>>> On November 30, 2018 5:54:18 AM GMT+13:00, Andy Lutomirski <luto@amacapital.net> wrote:
> >> >>
> >> >> The #1 fix would add a copy_siginfo_from_user64() or similar.
> >> >
> >> > Thanks very much! That all helped a bunch already! I'll try to go the
> >> > copy_siginfo_from_user64() way first and see if I can make this work. If
> >> > we do this I would however only want to use it for the new syscall first
> >> > and not change all other signal syscalls over to it too. I'd rather keep
> >> > this patchset focussed and small and do such conversions caused by the
> >> > new approach later. Does that sound reasonable?
> >>
> >> Absolutely. I don’t think we can change old syscalls — the ABI is set in stone.
> >> But for new syscalls, I think the always-64-bit behavior makes sense.
> >
> > It looks like we already have a 'struct signalfd_siginfo' that is defined in a
> > sane architecture-independent way, so I'd suggest we use that.
> 
> Unfortunately it isn't maintained very well.  What you can
> express with signalfd_siginfo is a subset what you can express with
> siginfo.  Many of the linux extensions to siginfo for exception
> information add pointers and have integers right after those pointers.
> Not all of those linux specific extentions for exceptions are handled
> by signalfd_siginfo (it needs new fields).
> 
> As originally defined siginfo had the sigval_t union at the end so it
> was perfectly fine on both 32bit and 64bit as it only had a single
> pointer in the structure and the other fields were 32bits in size.
> 
> Although I do feel the pain as x86_64 has to deal with 3 versions
> of siginfo.  A 64bit one.  A 32bit one for ia32.  A 32bit one for x32
> with a 64bit si_clock_t.
> 
> > We may then also want to make sure that any system call that takes a
> > siginfo has a replacement that takes a signalfd_siginfo, and that this
> > replacement can be used to implement the old version purely in
> > user space.
> 
> If you are not implementing CRIU and reinserting exceptions to yourself.
> At most user space wants the ability to implement sigqueue:
> 
> AKA:
> sigqueue(pid_t pid, int sig, const union sigval value);
> 
> Well sigqueue with it's own si_codes so the following would cover all
> the use cases I know of:
> int sendnewsig(pid_t pid, int sig, int si_code, const union sigval value);
> 
> The si_code could even be set to SI_USER to request that the normal
> trusted SI_USER values be filled in.  si_code values of < 0 if not
> recognized could reasonably safely be treated as the _rt member of
> the siginfo union.  Or perhaps better we error out in such a case.
> 
> If we want to be flexible and not have N kinds of system calls that
> is the direction I would go.  That is simple, and you don't need any of
> the rest.
> 
> 
> Alternatively we abandon the mistake of sigqueueinfo and not allow
> a signal number in the arguments that differs from the si_signo in the
> siginfo and allow passing the entire thing unchanged from sender to
> receiver.  That is maximum flexibility.
> 
> signalfd_siginfo just sucks in practice.  It is larger that siginfo 104
> bytes versus 48.  We must deliver it to userspace as a siginfo so it
> must be translated.  Because of the translation signalfd_siginfo adds
> no flexiblity in practice, because it can not just be passed through.
> Finallay signalfd_siginfo does not have encodings for all of the
> siginfo union members, so it fails to be fully general.
> 
> Personally if I was to define signalfd_siginfo today I would make it:
> struct siginfo_subset {
> 	__u32 sis_signo;
> 	__s32 sis_errno;
> 	__s32 sis_code;
>         __u32 sis_pad;
> 	__u32 sis_pid;
> 	__u32 sis_uid;
> 	__u64 sis_data (A pointer or integer data field);
> };
> 
> That is just 32bytes, and is all that is needed for everything
> except for synchronous exceptions.  Oh and that happens to be a proper
> subset of a any sane siginfo layout, on both 32bit and 64bit.
> 
> This is one of those rare times where POSIX is sane and what linux
> has implemented is not.

Thanks for the in-depth explanation. So your point is that we are better
off if we stick with siginfo_t instead of struct signalfd_siginfo in
procfd_signal()?