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

From: Martin KaFai Lau <kafai@fb.com>
To: 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>,
	<kernel-team@fb.com>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf 1/1] bpf: btf: Fix bitfield extraction for big endian
Date: Mon, 9 Jul 2018 11:32:36 -0700	[thread overview]
Message-ID: <20180709183236.r4b7gzmev5h4lcbw@kafai-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20180709004002.440153594@fb.com>

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.

>  	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,
>