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

[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 <osk@xxxxxx>

---
 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;
 	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;
 	}
 
-	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,