netdev - Re: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian

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Message-ID: <20180710174904.GA3247@w1t1fb>
Date:   Tue, 10 Jul 2018 18:49:05 +0100
From:   Okash Khawaja <osk@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     Martin KaFai Lau <kafai@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian

On Tue, Jul 10, 2018 at 10:21:02AM +0200, Daniel Borkmann wrote:
> On 07/09/2018 08:32 PM, Martin KaFai Lau wrote:
> > On Sun, Jul 08, 2018 at 05:22:03PM -0700, Okash Khawaja wrote:
> >> When extracting bitfield from a number, btf_int_bits_seq_show() builds
> >> a mask and accesses least significant byte of the number in a way
> >> specific to little-endian. This patch fixes that by checking endianness
> >> of the machine and then shifting left and right the unneeded bits.
> >>
> >> Thanks to Martin Lau for the help in navigating potential pitfalls when
> >> dealing with endianess and for the final solution.
> >>
> >> Fixes: b00b8daec828 ("bpf: btf: Add pretty print capability for data with BTF type info")
> >> Signed-off-by: Okash Khawaja <osk@...com>
> >>
> >> ---
> >>  kernel/bpf/btf.c |   32 +++++++++++++++-----------------
> >>  1 file changed, 15 insertions(+), 17 deletions(-)
> >>
> >> --- a/kernel/bpf/btf.c
> >> +++ b/kernel/bpf/btf.c
> >> @@ -162,6 +162,8 @@
> >>  #define BITS_ROUNDDOWN_BYTES(bits) ((bits) >> 3)
> >>  #define BITS_ROUNDUP_BYTES(bits) \
> >>  	(BITS_ROUNDDOWN_BYTES(bits) + !!BITS_PER_BYTE_MASKED(bits))
> >> +const int one = 1;
> >> +#define is_big_endian() ((*(char *)&one) == 0)
> 
> Also here, in the kernel archs provide proper definitions.
Is this the __BIG_ENDIAN #define or are there better ways to check that?

Thanks,
Okash