Re: [PATCH v3] bpf.2: Use standard types and attributes

[Alejandro Colomar (man-pages)]

On 5/15/21 9:01 PM, Alejandro Colomar wrote:
> Some manual pages are already using C99 syntax for integral
> types 'uint32_t', but some aren't.  There are some using kernel
> syntax '__u32'.  Fix those.
>
> Both the kernel and the standard types are 100% binary compatible,
> and the source code differences between them are very small, and
> not important in a manual page:
>
> - Some of them are implemented with different underlying types
>    (e.g., s64 is always long long, while int64_t may be long long
>    or long, depending on the arch).  This causes the following
>    differences.
>
> - length modifiers required by printf are different, resulting in
>    a warning ('-Wformat=').
>
> - pointer assignment causes a warning:
>    ('-Wincompatible-pointer-types'), but there aren't any pointers
>    in this page.
>
> But, AFAIK, all of those warnings can be safely ignored, due to
> the binary compatibility between the types.
>
> ...
>
> Some pages also document attributes, using GNU syntax
> '__attribute__((xxx))'.  Update those to use the shorter and more
> portable C11 keywords such as 'alignas()' when possible, and C2x
> syntax '[[gnu::xxx]]' elsewhere, which hasn't been standardized
> yet, but is already implemented in GCC, and available through
> either --std=c2x or any of the --std=gnu... options.
>
> The standard isn't very clear on how to use alignas() or
> [[]]-style attributes, and the GNU documentation isn't better, so
> the following link is a useful experiment about the different
> alignment syntaxes:
> __attribute__((aligned())), alignas(), and [[gnu::aligned()]]:
> <https://stackoverflow.com/q/67271825/6872717>
>
> Signed-off-by: Alejandro Colomar <alx.manpages@xxxxxxxxx>
Discussion: 
<https://lore.kernel.org/linux-man/6740a229-842e-b368-86eb-defc786b3658@xxxxxxxxx/T/>
> Nacked-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> Nacked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Zack Weinberg <zackw@xxxxxxxxx>
> Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
> Cc: glibc <libc-alpha@xxxxxxxxxxxxxx>
> Cc: GCC <gcc-patches@xxxxxxxxxxx>
> Cc: bpf <bpf@xxxxxxxxxxxxxxx>
> Cc: David Laight <David.Laight@xxxxxxxxxx>
> Cc: Joseph Myers <joseph@xxxxxxxxxxxxxxxx>
> Cc: Florian Weimer <fweimer@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> ---
>   man2/bpf.2 | 49 ++++++++++++++++++++++++-------------------------
>   1 file changed, 24 insertions(+), 25 deletions(-)
>
> diff --git a/man2/bpf.2 b/man2/bpf.2
> index 6e1ffa198..04b8fbcef 100644
> --- a/man2/bpf.2
> +++ b/man2/bpf.2
> @@ -186,41 +186,40 @@ commands:
>   .PP
>   .in +4n
>   .EX
> -union bpf_attr {
> +union [[gnu::aligned(8)]] bpf_attr {
>       struct {    /* Used by BPF_MAP_CREATE */
> -        __u32         map_type;
> -        __u32         key_size;    /* size of key in bytes */
> -        __u32         value_size;  /* size of value in bytes */
> -        __u32         max_entries; /* maximum number of entries
> -                                      in a map */
> +        uint32_t    map_type;
> +        uint32_t    key_size;    /* size of key in bytes */
> +        uint32_t    value_size;  /* size of value in bytes */
> +        uint32_t    max_entries; /* maximum number of entries
> +                                    in a map */
>       };
>   
> -    struct {    /* Used by BPF_MAP_*_ELEM and BPF_MAP_GET_NEXT_KEY
> -                   commands */
> -        __u32         map_fd;
> -        __aligned_u64 key;
> +    struct {    /* Used by BPF_MAP_*_ELEM and BPF_MAP_GET_NEXT_KEY commands */
> +        uint32_t            map_fd;
> +        uint64_t alignas(8) key;
>           union {
> -            __aligned_u64 value;
> -            __aligned_u64 next_key;
> +            uint64_t alignas(8) value;
> +            uint64_t alignas(8) next_key;
>           };
> -        __u64         flags;
> +        uint64_t            flags;
>       };
>   
>       struct {    /* Used by BPF_PROG_LOAD */
> -        __u32         prog_type;
> -        __u32         insn_cnt;
> -        __aligned_u64 insns;      /* \(aqconst struct bpf_insn *\(aq */
> -        __aligned_u64 license;    /* \(aqconst char *\(aq */
> -        __u32         log_level;  /* verbosity level of verifier */
> -        __u32         log_size;   /* size of user buffer */
> -        __aligned_u64 log_buf;    /* user supplied \(aqchar *\(aq
> -                                     buffer */
> -        __u32         kern_version;
> -                                  /* checked when prog_type=kprobe
> -                                     (since Linux 4.1) */
> +        uint32_t            prog_type;
> +        uint32_t            insn_cnt;
> +        uint64_t alignas(8) insns;     /* \(aqconst struct bpf_insn *\(aq */
> +        uint64_t alignas(8) license;   /* \(aqconst char *\(aq */
> +        uint32_t            log_level; /* verbosity level of verifier */
> +        uint32_t            log_size;  /* size of user buffer */
> +        uint64_t alignas(8) log_buf;   /* user supplied \(aqchar *\(aq
> +                                          buffer */
> +        uint32_t            kern_version;
> +                                       /* checked when prog_type=kprobe
> +                                          (since Linux 4.1) */
>   .\"                 commit 2541517c32be2531e0da59dfd7efc1ce844644f5
>       };
> -} __attribute__((aligned(8)));
> +};
>   .EE
>   .in
>   .\"


--
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/