Commit d93a7cf6 authored by Daniel T. Lee's avatar Daniel T. Lee Committed by Alexei Starovoitov
Browse files

samples/bpf: fix broken map lookup probe 



In the commit 7c4cd051 ("bpf: Fix syscall's stackmap lookup
potential deadlock"), a potential deadlock issue was addressed, which
resulted in *_map_lookup_elem not triggering BPF programs.
(prior to lookup, bpf_disable_instrumentation() is used)

To resolve the broken map lookup probe using "htab_map_lookup_elem",
this commit introduces an alternative approach. Instead, it utilize
"bpf_map_copy_value" and apply a filter specifically for the hash table
with map_type.

Signed-off-by: default avatarDaniel T. Lee <danieltimlee@gmail.com>
Fixes: 7c4cd051 ("bpf: Fix syscall's stackmap lookup potential deadlock")
Link: https://lore.kernel.org/r/20230818090119.477441-8-danieltimlee@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 92632115

samples/bpf/tracex6.bpf.c

+15 −2
Original line number Diff line number Diff line 
#include "vmlinux.h"

#include <linux/version.h>

#include <bpf/bpf_helpers.h>

#include <bpf/bpf_tracing.h>

#include <bpf/bpf_core_read.h>



struct {

	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
@@ -44,13 +46,24 @@ int bpf_prog1(struct pt_regs *ctx) 
	return 0;

}



SEC("kprobe/htab_map_lookup_elem")

int bpf_prog2(struct pt_regs *ctx)

/*

 * Since *_map_lookup_elem can't be expected to trigger bpf programs

 * due to potential deadlocks (bpf_disable_instrumentation), this bpf

 * program will be attached to bpf_map_copy_value (which is called

 * from map_lookup_elem) and will only filter the hashtable type.

 */

SEC("kprobe/bpf_map_copy_value")

int BPF_KPROBE(bpf_prog2, struct bpf_map *map)

{

	u32 key = bpf_get_smp_processor_id();

	struct bpf_perf_event_value *val, buf;

	enum bpf_map_type type;

	int error;



	type = BPF_CORE_READ(map, map_type);

	if (type != BPF_MAP_TYPE_HASH)

		return 0;



	error = bpf_perf_event_read_value(&counters, key, &buf, sizeof(buf));

	if (error)

		return 0;