Re: [PATCH 10/19] LoongArch: Add signal handling support

[Arnd Bergmann <arnd@arndb.de>]

On Fri, Jul 9, 2021 at 4:49 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> Arnd Bergmann <arnd@arndb.de> writes:
> > On Fri, Jul 9, 2021 at 11:24 AM Huacai Chen <chenhuacai@gmail.com> wrote:
> >> On Thu, Jul 8, 2021 at 9:30 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > Most such system calls currently go through set_user_sigmask or
> > set_compat_user_sigmask, which only differ on big-endian.
> > I would actually like to see these merged together and have a single
> > helper checking for in_compat_syscall() to decide whether to do
> > the word-swap for 32-bit bit-endian tasks or not, but that's a separate
> > discussion (and I suspect that Eric won't like that version, based on
> > other discussions we've had).
>
> Reading through get_compat_sigset is the best argument I have ever seen
> for getting rid of big endian architectures.  My gut reaction is we
> should just sweep all of the big endian craziness into a corner and let
> it disappear as the big endian architectures are retired.

A nice thought, but not going to happen any time soon as long as IBM
makes money from s390.

> Perhaps we generalize the non-compat version of the system calls and
> only have a compat version of the system call for the big endian
> architectures.
>
> I really hope loongarch and any new architectures added to the tree all
> are little endian.

It is. Most of the architectures merged over the last years only support
little-endian kernels, even those that can theoretically do both in hardware
(c-sky, riscv). arm64 and arc support big-endian in theory, but this is
rarely used.

Most of the server and workstation class hardware from the last
century is big-endian though. OpenRISC was the last architecture
we support that is big-endian only, but this was designed 21 years
ago now.

> > What I think you need for loongarch though is to change
> > set_user_sigmask(), get_compat_sigset() and similar functions to
> > behave differently depending on the user space execution context,
> > converting the 64-bit masks for loongarch/x86/arm64 tasks into
> > 128-bit in-kernel masks, while copying the 128-bit mips masks
> > as-is. This also requires changing the sigset_t and _NSIG
> > definitions so you get a 64-bit mask in user space, but a 128-bit
> > mask in kernel space.
> >
> > There are multiple ways of achieving this, either by generalizing
> > the common code, or by providing an architecture specific
> > implementation to replace it for loongarch only. I think you need to
> > try out which of those is the most maintainable.
>
> I believe all of the modern versions of the system calls that
> take a sigset_t in the kernel also take a sigsetsize.  So the most
> straight forward thing to do is to carefully define what happens
> to sigsets that are too big or too small when set.
>
> Something like defining that if a sigset is larger than the kernel's
> sigset size all of the additional bits must be zero, and if the sigset
> is smaller than the kernel's sigset size all of the missing bits
> will be set to zero in the kernel's sigset_t.  There may be cases
> I am missing bug for SIG_SETMASK, SIG_BLOCK, and SIG_UNBLOCK those
> look like the correct definitions.

Right, that would work as well. It is a change in behavior though,
since currently kernels just reject any non-default sigsetsize, and there
is a chance that this causes problems when some project relies on
being able to pass an arbitrary sigsetsize value, and then someone
tries running this on an older kernel.

> Another option would be to simply have whatever translates the system
> calls in userspace to perform the work of verifying the extra bits in
> the bitmap are unused before calling system calls that take a sigset_t
> and just ignoring the extra bits.

This is why I asked about how the current loongarch code does it.
qemu must already be doing something like this to run mips code on
non-mips architectures or vice-versa. FEX is another project doing
this, but they also want to add support for foreign syscalls (x86 on
arm64 mainly).

Based on previous discussions, I can see us coming up with a more
general way to handle ioctl() commands using some lookup table
to decide which commands need what kind of translation (32-bit
compat, foreign architecture, usercopy, ...). This needs some more
planning and discussion, but if we end up doing it, it would be
plausible to have a more general way for dealing with more than two
ABIs in a syscall.

       Arnd