The epoll_wait() system call wrapper is one of the remaining users of
the set_fs() infrasturcture for Arm. Changing it to not require set_fs()
is rather complex unfortunately.

The approach I'm taking here is to allow architectures to override
the code that copies the output to user space, and let the oabi-compat
implementation check whether it is getting called from an EABI or OABI
system call based on the thread_info->syscall value.

The in_oabi_syscall() check here mirrors the in_compat_syscall() and
in_x32_syscall() helpers for 32-bit compat implementations on other
architectures.

Overall, the amount of code goes down, at least with the newly added
sys_oabi_epoll_pwait() helper getting removed again. The downside
is added complexity in the source code for the native implementation.
There should be no difference in runtime performance except for Arm
kernels with CONFIG_OABI_COMPAT enabled that now have to go through
an external function call to check which of the two variants to use.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/arm/include/asm/syscall.h    | 11 +++++
 arch/arm/kernel/sys_oabi-compat.c | 72 +++++++------------------------
 arch/arm/tools/syscall.tbl        |  4 +-
 fs/eventpoll.c                    |  5 +--
 include/linux/eventpoll.h         | 16 +++++++
 5 files changed, 46 insertions(+), 62 deletions(-)

diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
index ff6cc365eaf7..0d8afceeefd9 100644
--- a/arch/arm/include/asm/syscall.h
+++ b/arch/arm/include/asm/syscall.h

@@ -28,6 +28,17 @@ static inline int syscall_get_nr(struct task_struct *task,
 	return task_thread_info(task)->syscall;
 }
 
+static inline bool __in_oabi_syscall(struct task_struct *task)
+{
+	return IS_ENABLED(CONFIG_OABI_COMPAT) &&
+		(task_thread_info(task)->syscall & __NR_OABI_SYSCALL_BASE);
+}
+
+static inline bool in_oabi_syscall(void)
+{
+	return __in_oabi_syscall(current);
+}
+
 static inline void syscall_rollback(struct task_struct *task,
 				    struct pt_regs *regs)
 {

diff --git a/arch/arm/kernel/sys_oabi-compat.c b/arch/arm/kernel/sys_oabi-compat.c
index 2ce3e8c6ca91..abf1153c5315 100644
--- a/arch/arm/kernel/sys_oabi-compat.c
+++ b/arch/arm/kernel/sys_oabi-compat.c

@@ -83,6 +83,8 @@
 #include <linux/uaccess.h>
 #include <linux/slab.h>
 
+#include <asm/syscall.h>
+
 struct oldabi_stat64 {
 	unsigned long long st_dev;
 	unsigned int	__pad1;

@@ -264,68 +266,24 @@ asmlinkage long sys_oabi_epoll_ctl(int epfd, int op, int fd,
 	return do_epoll_ctl(epfd, op, fd, &kernel, false);
 }
 
-static long do_oabi_epoll_wait(int epfd, struct oabi_epoll_event __user *events,
-			       int maxevents, int timeout)
+struct epoll_event __user *
+epoll_put_uevent(__poll_t revents, __u64 data, struct epoll_event __user *uevent)
 {
+	if (in_oabi_syscall()) {
+		struct oabi_epoll_event *oevent = (void __user *)uevent;
 
+		if (__put_user(revents, &oevent->events) ||
+		    __put_user(data, &oevent->data))
+			return NULL;
 
+		return (void __user *)uevent+1;
+	}