Re: [RFC PATCH v2 5/5] powerpc/syscalls: Allow none instead of sys_ni_syscall

Hi Arnd,

Arnd Bergmann [off-list ref] writes:

On Wed, Jan 16, 2019 at 2:27 PM Michael Ellerman [off-list ref] wrote:

quoted

sys_ni_syscall is the "not-implemented" syscall syscall, which just
returns -ENOSYS.

But unless you know that it's not obvious what it does, and even if
you do know what it means it doesn't stand out that well from other
real syscalls.

So teach the scripts to treat "none" as a synonym for
"sys_ni_syscall". This makes the table more readable.

Hmm, this actually breaks the proposed script to find bugs
in the compat handling, i.e. detecting those that have no
compat handler but only a native one.

I don't understand how? It just makes none an alias for sys_ni_syscall,
so surely the worse case is that script will need to do the reverse
transformation?

quoted

diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index c5907a2dbc86..988a7e29245f 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl

@@ -24,28 +24,28 @@
 14     common  mknod                           sys_mknod
 15     common  chmod                           sys_chmod
 16     common  lchown                          sys_lchown
-17     common  break                           sys_ni_syscall
-18     32      oldstat                         sys_stat                        sys_ni_syscall
-18     64      oldstat                         sys_ni_syscall
+17     common  break                           none
+18     32      oldstat                         sys_stat                        none
+18     64      oldstat                         none

The '64 oldstat' line can simply get dropped here, it has no value
(I failed to notice this earlier).

It does add value. It causes the syscall number to be defined in
unistd.h (now unistd_64.h).

If you remove it then that syscall number is no longer defined which
changes the uapi header and could break something.

Sure arguably it shouldn't be defined, and it's old etc. but it was
previously defined, so removing it seems risky.

For break, i.e. a syscall number without any implementation,
we use a different syntax on x86 (leaving out the sys_* entirely),
and on s390 (using '-', which is visually better than 'none' IMHO).

Except a blank compat syscall doesn't mean the syscall doesn't exist for
compat tasks, it means they get the non-compat entry point. So blank or
'-' are not explicit enough IMO because the script might have some
default logic which you can't see by looking at the table.

"none" is pretty explicit I thought. Possibly better is a literal
"ENOSYS", which stands out well and should be obvious to new comers.

We might also just remove those entirely across all architectures.
Some have already done this, and some have done it partially.
I can only see a couple of syscalls that got removed in the entire
git history (set_zone_reclaim, nfsservctl, vm86, timerfd), any other
ones are now literally pre-historic, and presumably nobody would
miss the macros when building a program that has no chance to
run on any kernel since at least 2.6.12.

I don't see the benefit, a single missing #define could be a build break
for some random pieces of software out there.

For 32-bit oldstat, I'd argue that this should actually get fixed by adding
the compat syscall logic. I think this was discussed when Firoz
first posted his patches. Something like this:

I'm not clear why we would do that? If there were programs out there
that wanted oldstat in compat mode surely we would have got a bug report
by now.

So this just wires up a syscall that no one will ever use?

cheers