Investigating this one now:
struct desc_ptr {
short unsigned int size; /* 0 2 */
- long unsigned int address; /* 2 8 */
+ long unsigned int address; /* 0 8 */
/* size: 10, cachelines: 1, members: 2 */
+ /* padding: 2 */
/* last cacheline: 10 bytes */
+
+ /* BRAIN FART ALERT! 10 != 2 + 0(holes), diff = 8 */
};
The original struct is this, at arch/x86/include/asm/desc_defs.h:
struct desc_ptr {
unsigned short size;
unsigned long address;
} __attribute__((packed)) ;
So both DWARF and BTF have the size right, 10 bytes, but BTF gets the
'address' member offset wrong, from readelf:
<1><28ed>: Abbrev Number: 17 (DW_TAG_structure_type)
<28ee> DW_AT_name : (indirect string, offset: 0x112bf): desc_ptr
<28f2> DW_AT_byte_size : 10
<28f3> DW_AT_decl_file : 101
<28f4> DW_AT_decl_line : 105
<28f5> DW_AT_decl_column : 8
<28f6> DW_AT_sibling : <0x2915>
<2><28fa>: Abbrev Number: 1 (DW_TAG_member)
<28fb> DW_AT_name : (indirect string, offset: 0x8b12): size
<28ff> DW_AT_decl_file : 101
<2900> DW_AT_decl_line : 106
<2901> DW_AT_decl_column : 17
<2902> DW_AT_type : <0xd0>
<2906> DW_AT_data_member_location: 0
<2><2907>: Abbrev Number: 1 (DW_TAG_member)
<2908> DW_AT_name : (indirect string, offset: 0xfa9): address
<290c> DW_AT_decl_file : 101
<290d> DW_AT_decl_line : 107
<290e> DW_AT_decl_column : 16
<290f> DW_AT_type : <0x2a>
<2913> DW_AT_data_member_location: 2
If I isolate this in a file and then do the BTF encoding:
[acme@quaco pahole]$ cat examples/desc_ptr.c
struct desc_ptr {
unsigned short size;
unsigned long address;
} __attribute__((packed)) ;
struct desc_ptr foo;
[acme@quaco pahole]$ gcc -c -g examples/desc_ptr.c -o examples/desc_ptr.o
[acme@quaco pahole]$ pahole -JV examples/desc_ptr.o
File examples/desc_ptr.o:
[1] STRUCT desc_ptr kind_flag=0 size=10 vlen=2
size type_id=2 bits_offset=0
address type_id=3 bits_offset=16
[2] INT short unsigned int size=2 bit_offset=0 nr_bits=16 encoding=(none)
[3] INT long unsigned int size=8 bit_offset=0 nr_bits=64 encoding=(none)
[4] INT (anon) size=4 bit_offset=0 nr_bits=32 encoding=(none)
[acme@quaco pahole]$
Now going thru the code to see why [3] get that bit_offset=0, should be
16, no?
- Arnaldo
