bpf: btf: Clean up BTF_INT_BITS() in uapi btf.h

This patch shrinks the BTF_INT_BITS() mask.  The current
btf_int_check_meta() ensures the nr_bits of an integer
cannot exceed 64.  Hence, it is mostly an uapi cleanup.

The actual btf usage (i.e. seq_show()) is also modified
to use u8 instead of u16.  The verification (e.g. btf_int_check_meta())
path stays as is to deal with invalid BTF situation.

Fixes: 69b693f0 ("bpf: btf: Introduce BPF Type Format (BTF)")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/uapi/linux/btf.h

@@ -76,7 +76,7 @@ struct btf_type {
  */
 #define BTF_INT_ENCODING(VAL)	(((VAL) & 0x0f000000) >> 24)
 #define BTF_INT_OFFSET(VAL)	(((VAL  & 0x00ff0000)) >> 16)
-#define BTF_INT_BITS(VAL)	((VAL)  & 0x0000ffff)
+#define BTF_INT_BITS(VAL)	((VAL)  & 0x000000ff)
 
 /* Attributes stored in the BTF_INT_ENCODING */
 #define BTF_INT_SIGNED	(1 << 0)

kernel/bpf/btf.c

@@ -450,7 +450,7 @@ static const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id)
  */
 static bool btf_type_int_is_regular(const struct btf_type *t)
 {
-	u16 nr_bits, nr_bytes;
+	u8 nr_bits, nr_bytes;
 	u32 int_data;
 
 	int_data = btf_type_int(t);
@@ -993,12 +993,16 @@ static void btf_int_bits_seq_show(const struct btf *btf,
 {
 	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_bits = BTF_INT_BITS(int_data);
+	u8 total_bits_offset;
+	u8 nr_copy_bytes;
+	u8 nr_copy_bits;
 	u64 print_num;
 
+	/*
+	 * bits_offset is at most 7.
+	 * BTF_INT_OFFSET() cannot exceed 64 bits.
+	 */
 	total_bits_offset = bits_offset + BTF_INT_OFFSET(int_data);
 	data += BITS_ROUNDDOWN_BYTES(total_bits_offset);
 	bits_offset = BITS_PER_BYTE_MASKED(total_bits_offset);
@@ -1028,7 +1032,7 @@ static void btf_int_seq_show(const struct btf *btf, const struct btf_type *t,
 	u32 int_data = btf_type_int(t);
 	u8 encoding = BTF_INT_ENCODING(int_data);
 	bool sign = encoding & BTF_INT_SIGNED;
-	u32 nr_bits = BTF_INT_BITS(int_data);
+	u8 nr_bits = BTF_INT_BITS(int_data);
 
 	if (bits_offset || BTF_INT_OFFSET(int_data) ||
 	    BITS_PER_BYTE_MASKED(nr_bits)) {