sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 211 insertions, 0 deletions

diff --git a/ipc/syscall.c b/ipc/syscall.c
new file mode 100644
index 000000000..dfb0e988d
--- /dev/null
+++ b/ipc/syscall.c
@@ -0,0 +1,211 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * sys_ipc() is the old de-multiplexer for the SysV IPC calls.
+ *
+ * This is really horribly ugly, and new architectures should just wire up
+ * the individual syscalls instead.
+ */
+#include <linux/unistd.h>
+#include <linux/syscalls.h>
+#include <linux/security.h>
+#include <linux/ipc_namespace.h>
+#include "util.h"
+
+#ifdef __ARCH_WANT_SYS_IPC
+#include <linux/errno.h>
+#include <linux/ipc.h>
+#include <linux/shm.h>
+#include <linux/uaccess.h>
+
+int ksys_ipc(unsigned int call, int first, unsigned long second,
+	unsigned long third, void __user * ptr, long fifth)
+{
+	int version, ret;
+
+	version = call >> 16; /* hack for backward compatibility */
+	call &= 0xffff;
+
+	switch (call) {
+	case SEMOP:
+		return ksys_semtimedop(first, (struct sembuf __user *)ptr,
+				       second, NULL);
+	case SEMTIMEDOP:
+		if (IS_ENABLED(CONFIG_64BIT))
+			return ksys_semtimedop(first, ptr, second,
+			        (const struct __kernel_timespec __user *)fifth);
+		else if (IS_ENABLED(CONFIG_COMPAT_32BIT_TIME))
+			return compat_ksys_semtimedop(first, ptr, second,
+			        (const struct old_timespec32 __user *)fifth);
+		else
+			return -ENOSYS;
+
+	case SEMGET:
+		return ksys_semget(first, second, third);
+	case SEMCTL: {
+		unsigned long arg;
+		if (!ptr)
+			return -EINVAL;
+		if (get_user(arg, (unsigned long __user *) ptr))
+			return -EFAULT;
+		return ksys_old_semctl(first, second, third, arg);
+	}
+
+	case MSGSND:
+		return ksys_msgsnd(first, (struct msgbuf __user *) ptr,
+				  second, third);
+	case MSGRCV:
+		switch (version) {
+		case 0: {
+			struct ipc_kludge tmp;
+			if (!ptr)
+				return -EINVAL;
+
+			if (copy_from_user(&tmp,
+					   (struct ipc_kludge __user *) ptr,
+					   sizeof(tmp)))
+				return -EFAULT;
+			return ksys_msgrcv(first, tmp.msgp, second,
+					   tmp.msgtyp, third);
+		}
+		default:
+			return ksys_msgrcv(first,
+					   (struct msgbuf __user *) ptr,
+					   second, fifth, third);
+		}
+	case MSGGET:
+		return ksys_msgget((key_t) first, second);
+	case MSGCTL:
+		return ksys_old_msgctl(first, second,
+				   (struct msqid_ds __user *)ptr);
+
+	case SHMAT:
+		switch (version) {
+		default: {
+			unsigned long raddr;
+			ret = do_shmat(first, (char __user *)ptr,
+				       second, &raddr, SHMLBA);
+			if (ret)
+				return ret;
+			return put_user(raddr, (unsigned long __user *) third);
+		}
+		case 1:
+			/*
+			 * This was the entry point for kernel-originating calls
+			 * from iBCS2 in 2.2 days.
+			 */
+			return -EINVAL;
+		}
+	case SHMDT:
+		return ksys_shmdt((char __user *)ptr);
+	case SHMGET:
+		return ksys_shmget(first, second, third);
+	case SHMCTL:
+		return ksys_old_shmctl(first, second,
+				   (struct shmid_ds __user *) ptr);
+	default:
+		return -ENOSYS;
+	}
+}
+
+SYSCALL_DEFINE6(ipc, unsigned int, call, int, first, unsigned long, second,
+		unsigned long, third, void __user *, ptr, long, fifth)
+{
+	return ksys_ipc(call, first, second, third, ptr, fifth);
+}
+#endif
+
+#ifdef CONFIG_COMPAT
+#include <linux/compat.h>
+
+#ifndef COMPAT_SHMLBA
+#define COMPAT_SHMLBA	SHMLBA
+#endif
+
+struct compat_ipc_kludge {
+	compat_uptr_t msgp;
+	compat_long_t msgtyp;
+};
+
+#ifdef CONFIG_ARCH_WANT_OLD_COMPAT_IPC
+int compat_ksys_ipc(u32 call, int first, int second,
+	u32 third, compat_uptr_t ptr, u32 fifth)
+{
+	int version;
+	u32 pad;
+
+	version = call >> 16; /* hack for backward compatibility */
+	call &= 0xffff;
+
+	switch (call) {
+	case SEMOP:
+		/* struct sembuf is the same on 32 and 64bit :)) */
+		return ksys_semtimedop(first, compat_ptr(ptr), second, NULL);
+	case SEMTIMEDOP:
+		if (!IS_ENABLED(CONFIG_COMPAT_32BIT_TIME))
+			return -ENOSYS;
+		return compat_ksys_semtimedop(first, compat_ptr(ptr), second,
+						compat_ptr(fifth));
+	case SEMGET:
+		return ksys_semget(first, second, third);
+	case SEMCTL:
+		if (!ptr)
+			return -EINVAL;
+		if (get_user(pad, (u32 __user *) compat_ptr(ptr)))
+			return -EFAULT;
+		return compat_ksys_old_semctl(first, second, third, pad);
+
+	case MSGSND:
+		return compat_ksys_msgsnd(first, ptr, second, third);
+
+	case MSGRCV: {
+		void __user *uptr = compat_ptr(ptr);
+
+		if (first < 0 || second < 0)
+			return -EINVAL;
+
+		if (!version) {
+			struct compat_ipc_kludge ipck;
+			if (!uptr)
+				return -EINVAL;
+			if (copy_from_user(&ipck, uptr, sizeof(ipck)))
+				return -EFAULT;
+			return compat_ksys_msgrcv(first, ipck.msgp, second,
+						 ipck.msgtyp, third);
+		}
+		return compat_ksys_msgrcv(first, ptr, second, fifth, third);
+	}
+	case MSGGET:
+		return ksys_msgget(first, second);
+	case MSGCTL:
+		return compat_ksys_old_msgctl(first, second, compat_ptr(ptr));
+
+	case SHMAT: {
+		int err;
+		unsigned long raddr;
+
+		if (version == 1)
+			return -EINVAL;
+		err = do_shmat(first, compat_ptr(ptr), second, &raddr,
+			       COMPAT_SHMLBA);
+		if (err < 0)
+			return err;
+		return put_user(raddr, (compat_ulong_t __user *)compat_ptr(third));
+	}
+	case SHMDT:
+		return ksys_shmdt(compat_ptr(ptr));
+	case SHMGET:
+		return ksys_shmget(first, (unsigned int)second, third);
+	case SHMCTL:
+		return compat_ksys_old_shmctl(first, second, compat_ptr(ptr));
+	}
+
+	return -ENOSYS;
+}
+
+COMPAT_SYSCALL_DEFINE6(ipc, u32, call, int, first, int, second,
+	u32, third, compat_uptr_t, ptr, u32, fifth)
+{
+	return compat_ksys_ipc(call, first, second, third, ptr, fifth);
+}
+#endif
+#endif