[RFC bpf-next 0/1] libbpf: add compile-time OOB warning to bpf_tail_call_static

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

This RFC adds 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.

Example:

    struct { /* BPF_MAP_TYPE_PROG_ARRAY = 3 */
        __uint(type, 3);             // int (*type)[3];
        __uint(max_entries, 100);    // int (*max_entries)[100];
        __type(key, __u32);          // typeof(__u32) *key;
        __type(value, __u32);        // typeof(__u32) *value;
    } progs SEC(".maps");

    bpf_tail_call_static(ctx, &progs, 111);

produces:

    bound.bpf.c:26:9: warning: bpf_tail_call: slot >= max_entries [-Wuser-defined-warnings]
       26 |         bpf_tail_call_static(ctx, &progs, 111);
          |         ^
    /usr/local/include/bpf/bpf_helpers.h:190:54: note: expanded from macro 'bpf_tail_call_static'
      190 |          __bpf_tail_call_warn(__slot >= BPF_MAP_ENTRIES(map));                  \
          |                                                             ^
    /usr/local/include/bpf/bpf_helpers.h:183:20: note: from 'diagnose_if' attribute on '__bpf_tail_call_warn':
      183 |     __attribute__((diagnose_if(oob, "bpf_tail_call: slot >= max_entries", "warning")));
          |                    ^           ~~~

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.

Hoyeon Lee (1):
  libbpf: add compile-time OOB warning to bpf_tail_call_static

 tools/lib/bpf/bpf_helpers.h | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

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2.51.0