On 9/4/25 7:53 PM, Hoyeon Lee wrote:
Add a compile-time check to bpf_tail_call_static() to warn when a
constant slot(index) is >= map->max_entries. This uses a small
BPF_MAP_ENTRIES() macro together with Clang's diagnose_if attribute.
Clang front-end keeps the map type with a '(*max_entries)[N]' field,
so the expression
sizeof(*(m)->max_entries) / sizeof(**(m)->max_entries)
is resolved to N entirely at compile time. This allows diagnose_if()
to emit a warning when a constant slot index is out of range.
Out-of-bounds tail call checkup is no-ops at runtime. Emitting a
compile-time warning can help developers detect mistakes earlier. The
check is currently limited to Clang (due to diagnose_if) and constant
indices, but should catch common errors.
Signed-off-by: Hoyeon Lee <hoyeon.lee@xxxxxxxx>
---
tools/lib/bpf/bpf_helpers.h | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 80c028540656..0d9551bb90c0 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -173,6 +173,26 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
:: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
: "r0", "r1", "r2", "r3", "r4", "r5");
}
+
+#if __has_attribute(diagnose_if)
+static __always_inline void __bpf_tail_call_warn(int oob)
+ __attribute__((diagnose_if(oob, "bpf_tail_call: slot >= max_entries", "warning")));
+
+#define BPF_MAP_ENTRIES(m) \
+ ((__u32)(sizeof(*(m)->max_entries) / sizeof(**(m)->max_entries)))
+
+#ifndef bpf_tail_call_static
+#define bpf_tail_call_static(ctx, map, slot) \
+({ \
+ /* wrapped to avoid double evaluation. */ \
+ const __u32 __slot = (slot); \
+ __bpf_tail_call_warn(__slot >= BPF_MAP_ENTRIES(map)); \
+ /* Avoid re-expand & invoke original as (bpf_tail_call_static)(..) */ \
+ (bpf_tail_call_static)(ctx, map, __slot); \
+})
+#endif /* bpf_tail_call_static */
+#endif
+
#endif
#endif
I got compilation failures with llvm21.
In file included from progs/bpf_flow.c:19:
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include/bpf/bpf_helpers.h:178:29: error: function '__bpf_tail_call_warn' has internal
linkage but is not defined [-Werror,-Wundefined-internal]
178 | static __always_inline void __bpf_tail_call_warn(int oob)
| CLNG-BPF [test_progs] btf_type_tag_percpu.bpf.o
^
progs/bpf_flow.c:122:3: note: used here
122 | bpf_tail_call_static(skb, &jmp_table, IP);
| ^
/home/yhs/work/bpf-next/tools/testing/selftests/bpf/tools/include/bpf/bpf_helpers.h:189:2: note: expanded from macro 'bpf_tail_call_static'
189 | __bpf_tail_c CLNG-BPF [test_progs] btf_type_tag_user.bpf.o
all_warn(__slot >= BPF_MAP_ENTRIES(map)); CLNG-BPF [test_progs] cb_refs.bpf.o
\
| ^
--
2.51.0
