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samples/bpf: fix broken map lookup probe

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In the commit 7c4cd051ad ("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: Daniel T. Lee <danieltimlee@gmail.com>
Fixes: 7c4cd051ad ("bpf: Fix syscall's stackmap lookup potential deadlock")
Link: https://lore.kernel.org/r/20230818090119.477441-8-danieltimlee@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Daniel T. Lee 2023-08-18 18:01:17 +09:00 committed by Alexei Starovoitov
parent 92632115fb
commit d93a7cf6ca
1 changed files with 15 additions and 2 deletions
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17
samples/bpf/tracex6.bpf.c
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@ -1,6 +1,8 @@
#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;