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    Subject[PATCH AUTOSEL 6.0 58/73] libbpf: Avoid enum forward-declarations in public API in C++ mode
    Date
    From: Andrii Nakryiko <andrii@kernel.org>

    [ Upstream commit b42693415b86f608049cf1b4870adc1dc65e58b0 ]

    C++ enum forward declarations are fundamentally not compatible with pure
    C enum definitions, and so libbpf's use of `enum bpf_stats_type;`
    forward declaration in libbpf/bpf.h public API header is causing C++
    compilation issues.

    More details can be found in [0], but it comes down to C++ supporting
    enum forward declaration only with explicitly specified backing type:

    enum bpf_stats_type: int;

    In C (and I believe it's a GCC extension also), such forward declaration
    is simply:

    enum bpf_stats_type;

    Further, in Linux UAPI this enum is defined in pure C way:

    enum bpf_stats_type { BPF_STATS_RUN_TIME = 0; }

    And even though in both cases backing type is int, which can be
    confirmed by looking at DWARF information, for C++ compiler actual enum
    definition and forward declaration are incompatible.

    To eliminate this problem, for C++ mode define input argument as int,
    which makes enum unnecessary in libbpf public header. This solves the
    issue and as demonstrated by next patch doesn't cause any unwanted
    compiler warnings, at least with default warnings setting.

    [0] https://stackoverflow.com/questions/42766839/c11-enum-forward-causes-underlying-type-mismatch
    [1] Closes: https://github.com/libbpf/libbpf/issues/249

    Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
    Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
    Link: https://lore.kernel.org/bpf/20221130200013.2997831-1-andrii@kernel.org
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    tools/lib/bpf/bpf.h | 7 +++++++
    1 file changed, 7 insertions(+)

    diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
    index 9c50beabdd14..fddc05c667b5 100644
    --- a/tools/lib/bpf/bpf.h
    +++ b/tools/lib/bpf/bpf.h
    @@ -393,8 +393,15 @@ LIBBPF_API int bpf_task_fd_query(int pid, int fd, __u32 flags, char *buf,
    __u32 *buf_len, __u32 *prog_id, __u32 *fd_type,
    __u64 *probe_offset, __u64 *probe_addr);

    +#ifdef __cplusplus
    +/* forward-declaring enums in C++ isn't compatible with pure C enums, so
    + * instead define bpf_enable_stats() as accepting int as an input
    + */
    +LIBBPF_API int bpf_enable_stats(int type);
    +#else
    enum bpf_stats_type; /* defined in up-to-date linux/bpf.h */
    LIBBPF_API int bpf_enable_stats(enum bpf_stats_type type);
    +#endif

    struct bpf_prog_bind_opts {
    size_t sz; /* size of this struct for forward/backward compatibility */
    --
    2.35.1
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