RE: [RFC] bpf.2: Use standard types and attributes

David Laight
2021-04-24T20:43:36Z

From: Alexei Starovoitov
> Sent: 24 April 2021 00:20
>
> On Fri, Apr 23, 2021 at 4:15 PM Alejandro Colomar
> <alx.manpages@gmail.com> 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.
> >
> > Some pages also document attributes, using GNU syntax
> > '__attribute__((xxx))'. Update those to use the shorter and more
> > portable C2x syntax, 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.
> >
> > Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
> > ---
> > man2/bpf.2 | 47 +++++++++++++++++++++++------------------------
> > 1 file changed, 23 insertions(+), 24 deletions(-)
> >
> > diff --git a/man2/bpf.2 b/man2/bpf.2
> > index 6e1ffa198..204f01bfc 100644
> > --- a/man2/bpf.2
> > +++ b/man2/bpf.2
> > @@ -188,39 +188,38 @@ commands:
> > .EX
> > union 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 */
>
> Nack.
> The man page should describe the kernel api the way it is in .h file.

And the code below is no more portable that a #pragma'.
It is probably worse than __attribute__((aligned(8)))
+ uint64_t [[gnu::aligned(8)]] value;
The standards committee are smoking dope again.
At least the '__aligned_u64 value;' form stands a reasonable
chance of being converted by cpp into whatever your compiler supports.

OTOH the bfp developers want shooting for defining a structure
with hidden padding fields.
It they ensured that all 64bit fields were aligned they wouldn't
need the __aligned_u64 at all.
And would be much less likely to leak kernel stack to userspace.

David

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