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

Alejandro Colomar (man-pages)
2021-05-16T09:16:34Z

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@gmail.com>
Discussion:
<https://lore.kernel.org/linux-man/6740a229-842e-b368-86eb-defc786b3658@gmail.com/T/>
> Nacked-by: Alexei Starovoitov <ast@kernel.org>
> Nacked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Acked-by: Zack Weinberg <zackw@panix.com>
> Cc: LKML <linux-kernel@vger.kernel.org>
> Cc: glibc <libc-alpha@sourceware.org>
> Cc: GCC <gcc-patches@gcc.gnu.org>
> Cc: bpf <bpf@vger.kernel.org>
> Cc: David Laight <David.Laight@ACULAB.COM>
> Cc: Joseph Myers <joseph@codesourcery.com>
> Cc: Florian Weimer <fweimer@redhat.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> ---
> 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/