bpf: btf: Fix endianness of bitfields
Subsystems (3)
v2 cover bpf: btf: Fix endianness of bitfields
Hi,
Following are the changes from v1:
- use compile time check for endianness
- factor out right_shift_bits from the check as it is same in both
- change size of bit shift variables to match length of bit offset in BTF specification
Please note that I have considered non-big endians to be little endians for our purposes. Let me know if you think that's a problem. We need to then decide what to do in case neither of macros are defined.
Thanks,
Okash
PATCH v2 1/1 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.
--- kernel/bpf/btf.c | 30 +++++++++++++----------------- 1 file changed, 13 insertions(+), 17 deletions(-)
kernel/bpf/btf.c +13 -17
@@ -991,16 +991,13 @@ static void btf_int_bits_seq_show(const 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 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,
Re: [PATCH bpf v2 1/1] bpf: btf: Fix bitfield extraction for big endian
Quoted 10 lines · Okash Khawaja
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 <[email protected]>
Re: [PATCH bpf v2 1/1] bpf: btf: Fix bitfield extraction for big endian
Quoted 6 lines · Martin KaFai Lau
Quoted 5 lines · Okash Khawaja
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.
Quoted 6 lines
Quoted 5 lines
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 <[email protected]>Acked-by Martin KaFai Lau <[email protected]>
Applied to bpf, thanks Okash!