[PATCH bpf-next v3 2/2] bpf: arm64: Optimize recursion detection by not using atomics

[Puranjay Mohan]

BPF programs detect recursion using a per-CPU 'active' flag in struct
bpf_prog. The trampoline currently sets/clears this flag with atomic
operations.

On some arm64 platforms (e.g., Neoverse V2 with LSE), per-CPU atomic
operations are relatively slow. Unlike x86_64 - where per-CPU updates
can avoid cross-core atomicity, arm64 LSE atomics are always atomic
across all cores, which is unnecessary overhead for strictly per-CPU
state.

This patch removes atomics from the recursion detection path on arm64 by
changing 'active' to a per-CPU array of four u8 counters, one per
context: {NMI, hard-irq, soft-irq, normal}. The running context uses a
non-atomic increment/decrement on its element.  After increment,
recursion is detected by reading the array as a u32 and verifying that
only the expected element changed; any change in another element
indicates inter-context recursion, and a value > 1 in the same element
indicates same-context recursion.

For example, starting from {0,0,0,0}, a normal-context trigger changes
the array to {0,0,0,1}.  If an NMI arrives on the same CPU and triggers
the program, the array becomes {1,0,0,1}. When the NMI context checks
the u32 against the expected mask for normal (0x00000001), it observes
0x01000001 and correctly reports recursion. Same-context recursion is
detected analogously.

Acked-by: Yonghong Song <yonghong.song at linux.dev>
Signed-off-by: Puranjay Mohan <puranjay at kernel.org>
---
 include/linux/bpf.h | 32 +++++++++++++++++++++++++++++---
 kernel/bpf/core.c   |  3 ++-
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2da986136d26..da6a00dd313f 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1746,6 +1746,8 @@ struct bpf_prog_aux {
 	struct bpf_map __rcu *st_ops_assoc;
 };
 
+#define BPF_NR_CONTEXTS        4       /* normal, softirq, hardirq, NMI */
+
 struct bpf_prog {
 	u16			pages;		/* Number of allocated pages */
 	u16			jited:1,	/* Is our filter JIT'ed? */
@@ -1772,7 +1774,7 @@ struct bpf_prog {
 		u8 tag[BPF_TAG_SIZE];
 	};
 	struct bpf_prog_stats __percpu *stats;
-	int __percpu		*active;
+	u8 __percpu		*active;	/* u8[BPF_NR_CONTEXTS] for recursion protection */
 	unsigned int		(*bpf_func)(const void *ctx,
 					    const struct bpf_insn *insn);
 	struct bpf_prog_aux	*aux;		/* Auxiliary fields */
@@ -2006,12 +2008,36 @@ struct bpf_struct_ops_common_value {
 
 static inline bool bpf_prog_get_recursion_context(struct bpf_prog *prog)
 {
-	return this_cpu_inc_return(*(prog->active)) == 1;
+#ifdef CONFIG_ARM64
+	u8 rctx = interrupt_context_level();
+	u8 *active = this_cpu_ptr(prog->active);
+	u32 val;
+
+	preempt_disable();
+	active[rctx]++;
+	val = le32_to_cpu(*(__le32 *)active);
+	preempt_enable();
+	if (val != BIT(rctx * 8))
+		return false;
+
+	return true;
+#else
+	return this_cpu_inc_return(*(int __percpu *)(prog->active)) == 1;
+#endif
 }
 
 static inline void bpf_prog_put_recursion_context(struct bpf_prog *prog)
 {
-	this_cpu_dec(*(prog->active));
+#ifdef CONFIG_ARM64
+	u8 rctx = interrupt_context_level();
+	u8 *active = this_cpu_ptr(prog->active);
+
+	preempt_disable();
+	active[rctx]--;
+	preempt_enable();
+#else
+	this_cpu_dec(*(int __percpu *)(prog->active));
+#endif
 }
 
 #if defined(CONFIG_BPF_JIT) && defined(CONFIG_BPF_SYSCALL)
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index c66316e32563..e0b8a8a5aaa9 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -112,7 +112,8 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
 		vfree(fp);
 		return NULL;
 	}
-	fp->active = alloc_percpu_gfp(int, bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags));
+	fp->active = __alloc_percpu_gfp(sizeof(u8[BPF_NR_CONTEXTS]), 4,
+					bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags));
 	if (!fp->active) {
 		vfree(fp);
 		kfree(aux);
-- 
2.47.3