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

Alejandro Colomar
2021-05-15T19:15:02Z

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>
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
.\"
--
2.31.1