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bpf: btf: Fix bitfield extraction for big endian 

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: b00b8daec8 ("bpf: btf: Add pretty print capability for data with BTF type info")
Signed-off-by: Okash Khawaja <osk@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Okash Khawaja 2018-07-10 14:33:07 -07:00 committed by Daniel Borkmann
parent 61d769807f
commit b65f370d06
1 changed files with 13 additions and 17 deletions
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30
kernel/bpf/btf.c
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@ -991,16 +991,13 @@ static void btf_int_bits_seq_show(const struct btf *btf,
void *data, u8 bits_offset,
struct seq_file *m)
{
u16 left_shift_bits, right_shift_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 +1005,20 @@ static void btf_int_bits_seq_show(const struct btf *btf,
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);
print_num = 0;
memcpy(&print_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;
#ifdef __BIG_ENDIAN_BITFIELD
left_shift_bits = bits_offset;
#else
left_shift_bits = BITS_PER_U64 - nr_copy_bits;
#endif
right_shift_bits = BITS_PER_U64 - nr_bits;
print_num.u8_nums[nr_copy_bytes - 1] &= mask;
}
print_num <<= left_shift_bits;
print_num >>= right_shift_bits;
print_num.u64_num >>= bits_offset;
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,