The dwarves class_member field bitfield_offset represents the
dwarf tag DW_AT_bit_offset. For dwarf2, this field can be negative
for little endian for bitfields in packed data structures
which cross type boundary.
-bash-4.4$ cat bitfield.c
struct packed {
char x1: 1;
char x2: 3;
char x3: 3;
int y1: 7;
int y2: 20;
} __attribute__((packed));
struct packed g;
-bash-4.4$ gcc -O2 -c -g bitfield.c
-bash-4.4$ pahole -JV bitfield.o
File bitfield.o:
[1] STRUCT packed kind_flag=1 size=5 vlen=5
x1 type_id=2 bitfield_size=1 bits_offset=0
x2 type_id=2 bitfield_size=3 bits_offset=1
x3 type_id=2 bitfield_size=3 bits_offset=4
y1 type_id=3 bitfield_size=7 bits_offset=7
y2 type_id=3 bitfield_size=255 bits_offset=16776974
[2] INT char size=1 bit_offset=0 nr_bits=8 encoding=(none)
[3] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
-bash-4.4$
The above large negative bits_offset and bitfield_size=255
results from negative bitfield_offset which is interpreted
as positive value in btf encoding.
With this fix, the pahole works properly for BTF:
-bash-4.4$ pahole -JV bitfield.o
File bitfield.o:
[1] STRUCT packed kind_flag=1 size=5 vlen=5
x1 type_id=2 bitfield_size=1 bits_offset=0
x2 type_id=2 bitfield_size=3 bits_offset=1
x3 type_id=2 bitfield_size=3 bits_offset=4
y1 type_id=3 bitfield_size=7 bits_offset=7
y2 type_id=3 bitfield_size=20 bits_offset=14
[2] INT char size=1 bit_offset=0 nr_bits=8 encoding=(none)
[3] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
-bash-4.4$
Note that change bitfield_offset from uint8_t to int8_t is safe
as the maximum int type we support is __int128 and maximum
bitfield_offset is 127.
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
dwarves.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/dwarves.h b/dwarves.h
index e6bffe8..e5f8347 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -831,7 +831,7 @@ struct class_member {
uint32_t bit_size;
uint32_t byte_offset;
size_t byte_size;
- uint8_t bitfield_offset;
+ int8_t bitfield_offset;
uint8_t bitfield_size;
uint8_t bit_hole;
uint8_t bitfield_end:1;
--
2.17.1
