[RFC bpf-next v2 1/1] libbpf: add compile-time OOB warning to bpf_tail_call_static

[Hoyeon Lee <hoyeon.lee@suse.com>]

Add a compile-time check to bpf_tail_call_static() to warn when a
constant slot(index) >= 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 calls are currently silent no-ops at runtime, so
emitting a compile-time warning helps detect logic errors earlier.
This is currently limited to Clang (due to diagnose_if) and only for
constant indices, but should still catch the common cases.

Signed-off-by: Hoyeon Lee <hoyeon.lee@suse.com>
---
Changes in V2:
- add function definition for __bpf_tail_call_warn for compile error

 tools/lib/bpf/bpf_helpers.h | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 80c028540656..98bc1536c497 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -173,6 +173,27 @@ 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

--
2.51.0