On Sat, Jul 6, 2019 at 5:00 PM Aleksa Sarai [off-list ref] wrote:

My plan here was to add new syscalls in the same order as everwhere else,
just with the number 110 higher. In the long run, I hope we can automate
this.

434 is already used in linux-next, I suggest you use 437 (Palmer
just submitted fchmodat4, which could become 436).

+/**
+ * Arguments for how openat2(2) should open the target path. If @extra is zero,
+ * then openat2(2) is identical to openat(2).
+ *
+ * @flags: O_* flags (unknown flags ignored).
+ * @mode: O_CREAT file mode (ignored otherwise).
+ * @upgrade_mask: restrict how the O_PATH may be re-opened (ignored otherwise).
+ * @resolve: RESOLVE_* flags (-EINVAL on unknown flags).
+ * @reserved: reserved for future extensions, must be zeroed.
+ */
+struct open_how {
+       __u32 flags;
+       union {
+               __u16 mode;
+               __u16 upgrade_mask;
+       };
+       __u16 resolve;
+       __u64 reserved[7]; /* must be zeroed */
+};

We can have system calls with up to six arguments on all architectures, so
this could still be done more conventionally without the indirection: like

long openat2(int dfd, const char __user * filename, int flags, mode_t
mode_mask, __u16 resolve);

In fact, that seems similar enough to the existing openat() that I think
you could also just add the fifth argument to the existing call when
a newly defined flag is set, similarly to how we only use the 'mode'
argument when O_CREAT or O_TMPFILE are set.

This file seems to lack a declaration for the system call, which means it
will cause a build failure on some architectures, e.g. arch/arc/kernel/sys.c:

#define __SYSCALL(nr, call) [nr] = (call),
void *sys_call_table[NR_syscalls] = {
        [0 ... NR_syscalls-1] = sys_ni_syscall,
#include <asm/unistd.h>
};

        Arnd