Patch "samples/bpf: fix broken map lookup probe" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    samples/bpf: fix broken map lookup probe

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     samples-bpf-fix-broken-map-lookup-probe.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 29c99fa352ec24fbdd0e0d9a8907ba7bf0bd27db
Author: Daniel T. Lee <danieltimlee@xxxxxxxxx>
Date:   Fri Aug 18 18:01:17 2023 +0900

    samples/bpf: fix broken map lookup probe
    
    [ 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@xxxxxxxxx>
    Fixes: 7c4cd051add3 ("bpf: Fix syscall's stackmap lookup potential deadlock")
    Link: https://lore.kernel.org/r/20230818090119.477441-8-danieltimlee@xxxxxxxxx
    Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

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;