On Mon, Jul 26, 2021 at 04:11:35PM +0200, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@xxxxxxxx> > > 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. > 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@xxxxxxxxxxxx/ With the above applied, syscall_errno and syscall_faked start working again, but not the syscall_restart test. > 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@xxxxxxxx> 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. Anyway, I'll keep debugging this, but figured I'd mention it in case anyone else had been seeing issues in here. -Kees -- Kees Cook
