Commit c3e34f88 authored by Puranjay Mohan's avatar Puranjay Mohan Committed by Alexei Starovoitov
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bpf: arm64: Optimize recursion detection by not using atomics 



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: default avatarYonghong Song <yonghong.song@linux.dev>
Signed-off-by: default avatarPuranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20251219184422.2899902-3-puranjay@kernel.org


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 93f0d096

include/linux/bpf.h

+29 −3
Original line number Diff line number Diff line
@@ -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)

kernel/bpf/core.c

+2 −1
Original line number Diff line number Diff line
@@ -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);