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, 167 insertions, 0 deletions

diff --git a/arch/arm/lib/getuser.S b/arch/arm/lib/getuser.S
new file mode 100644
index 000000000..c5e420750
--- /dev/null
+++ b/arch/arm/lib/getuser.S
@@ -0,0 +1,167 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ *  linux/arch/arm/lib/getuser.S
+ *
+ *  Copyright (C) 2001 Russell King
+ *
+ *  Idea from x86 version, (C) Copyright 1998 Linus Torvalds
+ *
+ * These functions have a non-standard call interface to make them more
+ * efficient, especially as they return an error value in addition to
+ * the "real" return value.
+ *
+ * __get_user_X
+ *
+ * Inputs:	r0 contains the address
+ *		r1 contains the address limit, which must be preserved
+ * Outputs:	r0 is the error code
+ *		r2, r3 contains the zero-extended value
+ *		lr corrupted
+ *
+ * No other registers must be altered.  (see <asm/uaccess.h>
+ * for specific ASM register usage).
+ *
+ * Note that ADDR_LIMIT is either 0 or 0xc0000000.
+ * Note also that it is intended that __get_user_bad is not global.
+ */
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+#include <asm/errno.h>
+#include <asm/domain.h>
+
+ENTRY(__get_user_1)
+	check_uaccess r0, 1, r1, r2, __get_user_bad
+1: TUSER(ldrb)	r2, [r0]
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_1)
+_ASM_NOKPROBE(__get_user_1)
+
+ENTRY(__get_user_2)
+	check_uaccess r0, 2, r1, r2, __get_user_bad
+#if __LINUX_ARM_ARCH__ >= 6
+
+2: TUSER(ldrh)	r2, [r0]
+
+#else
+
+#ifdef CONFIG_CPU_USE_DOMAINS
+rb	.req	ip
+2:	ldrbt	r2, [r0], #1
+3:	ldrbt	rb, [r0], #0
+#else
+rb	.req	r0
+2:	ldrb	r2, [r0]
+3:	ldrb	rb, [r0, #1]
+#endif
+#ifndef __ARMEB__
+	orr	r2, r2, rb, lsl #8
+#else
+	orr	r2, rb, r2, lsl #8
+#endif
+
+#endif /* __LINUX_ARM_ARCH__ >= 6 */
+
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_2)
+_ASM_NOKPROBE(__get_user_2)
+
+ENTRY(__get_user_4)
+	check_uaccess r0, 4, r1, r2, __get_user_bad
+4: TUSER(ldr)	r2, [r0]
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_4)
+_ASM_NOKPROBE(__get_user_4)
+
+ENTRY(__get_user_8)
+	check_uaccess r0, 8, r1, r2, __get_user_bad8
+#ifdef CONFIG_THUMB2_KERNEL
+5: TUSER(ldr)	r2, [r0]
+6: TUSER(ldr)	r3, [r0, #4]
+#else
+5: TUSER(ldr)	r2, [r0], #4
+6: TUSER(ldr)	r3, [r0]
+#endif
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_8)
+_ASM_NOKPROBE(__get_user_8)
+
+#ifdef __ARMEB__
+ENTRY(__get_user_32t_8)
+	check_uaccess r0, 8, r1, r2, __get_user_bad
+#ifdef CONFIG_CPU_USE_DOMAINS
+	add	r0, r0, #4
+7:	ldrt	r2, [r0]
+#else
+7:	ldr	r2, [r0, #4]
+#endif
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_32t_8)
+_ASM_NOKPROBE(__get_user_32t_8)
+
+ENTRY(__get_user_64t_1)
+	check_uaccess r0, 1, r1, r2, __get_user_bad8
+8: TUSER(ldrb)	r3, [r0]
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_64t_1)
+_ASM_NOKPROBE(__get_user_64t_1)
+
+ENTRY(__get_user_64t_2)
+	check_uaccess r0, 2, r1, r2, __get_user_bad8
+#ifdef CONFIG_CPU_USE_DOMAINS
+rb	.req	ip
+9:	ldrbt	r3, [r0], #1
+10:	ldrbt	rb, [r0], #0
+#else
+rb	.req	r0
+9:	ldrb	r3, [r0]
+10:	ldrb	rb, [r0, #1]
+#endif
+	orr	r3, rb, r3, lsl #8
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_64t_2)
+_ASM_NOKPROBE(__get_user_64t_2)
+
+ENTRY(__get_user_64t_4)
+	check_uaccess r0, 4, r1, r2, __get_user_bad8
+11: TUSER(ldr)	r3, [r0]
+	mov	r0, #0
+	ret	lr
+ENDPROC(__get_user_64t_4)
+_ASM_NOKPROBE(__get_user_64t_4)
+#endif
+
+__get_user_bad8:
+	mov	r3, #0
+__get_user_bad:
+	mov	r2, #0
+	mov	r0, #-EFAULT
+	ret	lr
+ENDPROC(__get_user_bad)
+ENDPROC(__get_user_bad8)
+_ASM_NOKPROBE(__get_user_bad)
+_ASM_NOKPROBE(__get_user_bad8)
+
+.pushsection __ex_table, "a"
+	.long	1b, __get_user_bad
+	.long	2b, __get_user_bad
+#if __LINUX_ARM_ARCH__ < 6
+	.long	3b, __get_user_bad
+#endif
+	.long	4b, __get_user_bad
+	.long	5b, __get_user_bad8
+	.long	6b, __get_user_bad8
+#ifdef __ARMEB__
+	.long   7b, __get_user_bad
+	.long	8b, __get_user_bad8
+	.long	9b, __get_user_bad8
+	.long	10b, __get_user_bad8
+	.long	11b, __get_user_bad8
+#endif
+.popsection