Em Thu, Jan 17, 2019 at 01:01:58PM -0300, Arnaldo Carvalho de Melo escreveu:
> 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?
That was just a distraction, the BTF encoding is done right, the problem
is when loading the BTF section, in class__fixup_btf_bitfields(), first
we set it right:
pos->byte_offset = pos->bit_offset / 8;
But then, later, this is recalculated as:
pos->byte_offset = (((pos->bit_offset / integral_bit_size) * integral_bit_size) / 8);
Now trying to recall why this last recalc...
- Arnaldo
