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Date: Mon, 9 Jul 2018 11:32:36 -0700 From: Martin KaFai Lau <kafai@...com> To: Okash Khawaja <osk@...com> CC: Daniel Borkmann <daniel@...earbox.net>, 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 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) > > #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. > u32 int_data = btf_type_int(t); > u16 nr_bits = BTF_INT_BITS(int_data); > u16 total_bits_offset; > u16 nr_copy_bytes; > u16 nr_copy_bits; > - u8 nr_upper_bits; > - union { > - u64 u64_num; > - u8 u8_nums[8]; > - } print_num; > + u64 print_num; > > total_bits_offset = bits_offset + BTF_INT_OFFSET(int_data); > data += BITS_ROUNDDOWN_BYTES(total_bits_offset); > @@ -1008,21 +1007,20 @@ static void btf_int_bits_seq_show(const > nr_copy_bits = nr_bits + bits_offset; > nr_copy_bytes = BITS_ROUNDUP_BYTES(nr_copy_bits); > > - print_num.u64_num = 0; > - memcpy(&print_num.u64_num, data, nr_copy_bytes); > - > - /* Ditch the higher order bits */ > - nr_upper_bits = BITS_PER_BYTE_MASKED(nr_copy_bits); > - if (nr_upper_bits) { > - /* We need to mask out some bits of the upper byte. */ > - u8 mask = (1 << nr_upper_bits) - 1; > - > - print_num.u8_nums[nr_copy_bytes - 1] &= mask; > + print_num = 0; > + memcpy(&print_num, data, nr_copy_bytes); > + if (is_big_endian()) { > + left_shift_bits = bits_offset; > + right_shift_bits = BITS_PER_U64 - nr_bits; > + } else { > + left_shift_bits = BITS_PER_U64 - nr_copy_bits; > + right_shift_bits = BITS_PER_U64 - nr_bits; Nit. right_shift_bits is the same for both cases. Lets simplify it. > } > > - print_num.u64_num >>= bits_offset; > + print_num <<= left_shift_bits; > + print_num >>= right_shift_bits; > > - seq_printf(m, "0x%llx", print_num.u64_num); > + seq_printf(m, "0x%llx", print_num); > } > > static void btf_int_seq_show(const struct btf *btf, const struct btf_type *t, >
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