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    SubjectRE: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian
    Date
    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

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