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

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Date:   Tue, 10 Jul 2018 22:02:07 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Okash Khawaja <osk@...com>
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 07/10/2018 07:49 PM, Okash Khawaja wrote:
> 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?

Given this deals with bitfields, should be __{BIG,LITTLE}_ENDIAN_BITFIELD.