Re: [PATCH v5 04/10] ARM: syscall: always store thread_info->abi_syscall

["Arnd Bergmann" <arnd@arndb.de>]

On Fri, Aug 4, 2023, at 01:17, Kees Cook wrote:
> On Mon, Jul 26, 2021 at 04:11:35PM +0200, Arnd Bergmann wrote:
>> From: Arnd Bergmann <arnd@arndb.de>
>> 
>> The system call number is used in a a couple of places, in particular
>> ptrace, seccomp and /proc/<pid>/syscall.
>
> *thread necromancy*
>
> Hi!
>
> So, it seems like the seccomp selftests broke in a few places due to
> this change (back in v5.15). I really thought kernelci.org was running
> the seccomp tests, but it seems like the coverage is spotty.
>
> Specifically, the syscall_restart selftest fails, as well as syscall_errno
> and syscall_faked (both via seccomp and PTRACE), starting with this patch.

Thanks for tracking this down!

>> The last one apparently never worked reliably on ARM for tasks that are
>> not currently getting traced.
>> 
>> Storing the syscall number in the normal entry path makes it work,
>> as well as allowing us to see if the current system call is for OABI
>> compat mode, which is the next thing I want to hook into.
>> 
>> Since the thread_info->syscall field is not just the number any more, it
>> is now renamed to abi_syscall. In kernels that enable both OABI and EABI,
>> the upper bits of this field encode 0x900000 (__NR_OABI_SYSCALL_BASE)
>> for OABI tasks, while normal EABI tasks do not set the upper bits. This
>> makes it possible to implement the in_oabi_syscall() helper later.
>> 
>> All other users of thread_info->syscall go through the syscall_get_nr()
>> helper, which in turn filters out the ABI bits.
>
> While I've reproducing the bisect done by mediatek, I'm still poking
> around in here to figure out what's gone wrong. There was a recent patch
> to fix this, but it looks like it's not complete:
> https://lore.kernel.org/all/20230724121655.7894-1-lecopzer.chen@mediatek.com/
>
> With the above applied, syscall_errno and syscall_faked start working
> again, but not the syscall_restart test.

Right, I also see you addressed this better in your follow-up patch,
I'll comment there.

>> Note that the ABI information is lost with PTRACE_SET_SYSCALL, so one
>> cannot set the internal number to a particular version, but this was
>> already the case. We could change it to let gdb encode the ABI type along
>> with the syscall in a CONFIG_OABI_COMPAT-enabled kernel, but that itself
>> would be a (backwards-compatible) ABI change, so I don't do it here.
>> 
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>
> Another issue of note, which may just be "by design" for arm32, is that
> an invalid syscall (or, at least, a negative syscall) results in SIGILL,
> rather than a errno=ENOSYS failure. This seems to have been true at least
> as far back as v5.8 (where this was cleaned up for at least arm64 and
> s390). There was a seccomp test added for it in v5.9, but it has been
> failing for arm32 since then. :(
>
> I mention this because the behavior of the syscall_restart test looks
> like an invalid syscall: on restart a SIGILL is caught instead of the
> syscall correctly continuing.


The odd arm behavior came up on IRC recently, and I saw that this
was what arm has always done, but I could not figure out why this
is done. I tried to see where s390 and arm64 changed the behavior
but can't find it. Do you have the commit IDs?

      Arnd