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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Martin KaFai Lau' <kafai@fb.com>, Okash Khawaja <osk@fb.com>
CC:     Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Yonghong Song <yhs@fb.com>,
        Jakub Kicinski <jakub.kicinski@netronome.com>,
        "David S. Miller" <davem@davemloft.net>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "kernel-team@fb.com" <kernel-team@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian
Thread-Topic: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian
Thread-Index: AQHUF7OfQA7M1iwM3EWyyOlGSAzC+KSIqAuw
Date:   Tue, 10 Jul 2018 16:35:04 +0000
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From: Martin KaFai Lau
> Sent: 09 July 2018 19:33
> 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@fb.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)
> >
> >  #define BTF_INFO_MASK 0x0f00ffff
> >  #define BTF_INT_MASK 0x0fffffff
> > @@ -991,16 +993,13 @@ static void btf_int_bits_seq_show(const
> >  				  void *data, u8 bits_offset,
> >  				  struct seq_file *m)
> >  {
> > +	u8 left_shift_bits, right_shift_bits;
> Nit.
> Although only max 64 bit int is allowed now (ensured by btf_int_check_meta),
> it is better to use u16 such that it will be consistent to BTF_INT_BITS.

Double-nit.

Use 'int' or 'unsigned int'.
Sub-word arithmetic will require extra instructions on almost everything
except x86.

	David