In the commit 7c4cd051add3 ("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@xxxxxxxxx>
Fixes: 7c4cd051add3 ("bpf: Fix syscall's stackmap lookup potential deadlock")
---
samples/bpf/tracex6.bpf.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/samples/bpf/tracex6.bpf.c b/samples/bpf/tracex6.bpf.c
index 6ad82e68f998..9b23b4737cfb 100644
--- a/samples/bpf/tracex6.bpf.c
+++ b/samples/bpf/tracex6.bpf.c
@@ -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;
--
2.34.1
