[PATCH 5.10 110/406] samples/bpf: fix broken map lookup probe

Greg Kroah-Hartman
2023-09-17T19:25:14Z

From: Daniel T. Lee <danieltimlee@gmail.com>

[ Upstream commit d93a7cf6ca2cfcd7de5d06f753ce8d5e863316ac ]

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@gmail.com>
Fixes: 7c4cd051add3 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
samples/bpf/tracex6_kern.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/samples/bpf/tracex6_kern.c b/samples/bpf/tracex6_kern.c
index acad5712d8b4f..fd602c2774b8b 100644
--- a/samples/bpf/tracex6_kern.c
+++ b/samples/bpf/tracex6_kern.c
@@ -2,6 +2,8 @@
#include <linux/version.h>
#include <uapi/linux/bpf.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);
@@ -45,13 +47,24 @@ 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;
--
2.40.1