Em Sat, Jan 12, 2019 at 12:00:42AM +0000, Yonghong Song escreveu:
> On 1/10/19 3:03 PM, Andrii Nakryiko wrote:
> > There appears to be problems with bitfields in both DWARF data and
> > pahole's conversion from DWARF to BTF (see test file source code and
> > pahole output at the bottom).
> > 1. pahole believes unpacked.y1 has offset 7, which is incorrect
> > (should be 32?).
> > unpacked.y2 has offset 32, which also doesn't make sense.
> > DWARF info for unpacked.y1 and unpacked.y2 also seem wrong (how
> > do we get offset 18 for y1?).
> > <2><63>: Abbrev Number: 3 (DW_TAG_member)
> > <64> DW_AT_name : y1
> > <67> DW_AT_decl_file : 1
> > <68> DW_AT_decl_line : 5
> > <69> DW_AT_type : <0x87>
> > <6d> DW_AT_byte_size : 4
> > <6e> DW_AT_bit_size : 7
> > <6f> DW_AT_bit_offset : 18
> > <70> DW_AT_data_member_location: 0
> > <2><71>: Abbrev Number: 3 (DW_TAG_member)
> > <72> DW_AT_name : y2
> > <75> DW_AT_decl_file : 1
> > <76> DW_AT_decl_line : 6
> > <77> DW_AT_type : <0x87>
> > <7b> DW_AT_byte_size : 4
> > <7c> DW_AT_bit_size : 20
> > <7d> DW_AT_bit_offset : 12
> > <7e> DW_AT_data_member_location: 4
> >
> >
> > 2. Similar problems with packed.y1 and packed.y2. packed.y2 offset
> > is negative value even in DWARF:
> >
> > <2><d2>: Abbrev Number: 6 (DW_TAG_member)
> > <d3> DW_AT_name : y2
> > <d6> DW_AT_decl_file : 1
> > <d7> DW_AT_decl_line : 14
> > <d8> DW_AT_type : <0x87>
> > <dc> DW_AT_byte_size : 4
> > <dd> DW_AT_bit_size : 20
> > <de> DW_AT_bit_offset : -2
> > <df> DW_AT_data_member_location: 0
>
> Did some research on why we got -2 as bit_offset with the above packed
> data structure.
>
> First, the compiler generates correct thing. the negative
> DW_AT_bit_offset is allowed for dwarf2, but not dwarf4.
> The "-2" here means (1). the member is crossing two
> adjacent "4" bytes boundary (DW_AT_byte_size = 4), and
Ok, but I would expect that the DW_AT_data_member_location would be 4,
meaning, it is mostly on this one but starts two bits before. So when
seeing negative DW_AT_bit_offsets we should think it starts in this
location (0 in this case), but straddles to the next, so I should say
that it is at bit offset (DW_AT_byte_size * 8 - DW_AT_bit_offset), the
user, looking at this new pahole output will realize that it has 2 bits
at the end of the current DW_AT_data_member_location (0) but has 18 bits
in the next.
> (2). the -2 means the the starting bit is at the "4" byte boundary - 2
> bits. So basically it covers:
>
> | |
> V V
> [0 1 .....30 31][0 .. 17 18 ... 31]
>
> totally 20 bits.
>
> The following patch fixed the issue:
> -bash-4.4$ git diff
> 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;
> -bash-4.4$
>
> With the above change:
> -bash-4.4$ ~/work/github/pahole/build/pahole -JV bitfields.o
> File bitfields.o:
> [1] INT int size=4 bit_offset=0 nr_bits=32 encoding=SIGNED
> [2] STRUCT packed kind_flag=1 size=5 vlen=5
> x1 type_id=3 bitfield_size=1 bits_offset=0
> x2 type_id=3 bitfield_size=3 bits_offset=1
> x3 type_id=3 bitfield_size=3 bits_offset=4
> y1 type_id=1 bitfield_size=7 bits_offset=7
> y2 type_id=1 bitfield_size=20 bits_offset=14
> [3] INT char size=1 bit_offset=0 nr_bits=8 encoding=(none)
> [4] STRUCT unpacked kind_flag=1 size=8 vlen=5
> x1 type_id=3 bitfield_size=1 bits_offset=0
> x2 type_id=3 bitfield_size=3 bits_offset=1
> x3 type_id=3 bitfield_size=3 bits_offset=4
> y1 type_id=1 bitfield_size=7 bits_offset=7
> y2 type_id=1 bitfield_size=20 bits_offset=32
> [5] UNION packed_union kind_flag=0 size=8 vlen=4
> x1 type_id=3 bits_offset=0
> y2 type_id=1 bits_offset=0
> s1 type_id=2 bits_offset=0
> s2 type_id=4 bits_offset=0
> [6] UNION unpacked_union kind_flag=0 size=8 vlen=3
> x1 type_id=3 bits_offset=0
> y2 type_id=1 bits_offset=0
> s type_id=4 bits_offset=0
> -bash-4.4$
>
> Will send out a patch shortly.
>
> >
> > andriin@devvm$ cat bitfields.c
> > struct unpacked{
> > char x1: 1;
> > char x2: 3;
> > char x3: 3;
> > int y1: 7;
> > int y2: 20;
> > };
> >
> > struct packed{
> > char x1: 1;
> > char x2: 3;
> > char x3: 3;
> > int y1: 7;
> > int y2: 20;
> > } __attribute__((packed));
> >
> > union packed_union {
> > char x1;
> > int y2;
> > struct packed s1;
> > struct unpacked s2;
> > };
> >
> > union unpacked_union {
> > char x1;
> > int y2;
> > struct unpacked s;
> > } __attribute__((packed));
> >
> > int main() {
> > struct packed s1;
> > struct unpacked s2;
> > union packed_union u1;
> > union unpacked_union u2;
> > return 0;
> > }
> > andriin@devvm$ cc -g bitfields.c -o bitfields
> > andriin@devvm$ LLVM_OBJCOPY=objcopy ~/local/pahole/build/pahole -JV bitfields
> > File bitfields:
> > [1] STRUCT unpacked kind_flag=1 size=8 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=32
> > [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
> > [4] 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
> > [5] UNION packed_union kind_flag=0 size=8 vlen=4
> > x1 type_id=2 bits_offset=0
> > y2 type_id=3 bits_offset=0
> > s1 type_id=4 bits_offset=0
> > s2 type_id=1 bits_offset=0
> > [6] UNION unpacked_union kind_flag=0 size=8 vlen=3
> > x1 type_id=2 bits_offset=0
> > y2 type_id=3 bits_offset=0
> > s type_id=1 bits_offset=0
> > andriin@devvm$ readelf -wi bitfields
> > Contents of the .debug_info section:
> >
> > ...
> >
> > <1><2d>: Abbrev Number: 2 (DW_TAG_structure_type)
> > <2e> DW_AT_name : (indirect string, offset: 0x4e): unpacked
> > <32> DW_AT_byte_size : 8
> > <33> DW_AT_decl_file : 1
> > <34> DW_AT_decl_line : 1
> > <35> DW_AT_sibling : <0x80>
> > <2><39>: Abbrev Number: 3 (DW_TAG_member)
> > <3a> DW_AT_name : x1
> > <3d> DW_AT_decl_file : 1
> > <3e> DW_AT_decl_line : 2
> > <3f> DW_AT_type : <0x80>
> > <43> DW_AT_byte_size : 1
> > <44> DW_AT_bit_size : 1
> > <45> DW_AT_bit_offset : 7
> > <46> DW_AT_data_member_location: 0
> > <2><47>: Abbrev Number: 3 (DW_TAG_member)
> > <48> DW_AT_name : x2
> > <4b> DW_AT_decl_file : 1
> > <4c> DW_AT_decl_line : 3
> > <4d> DW_AT_type : <0x80>
> > <51> DW_AT_byte_size : 1
> > <52> DW_AT_bit_size : 3
> > <53> DW_AT_bit_offset : 4
> > <54> DW_AT_data_member_location: 0
> > <2><55>: Abbrev Number: 3 (DW_TAG_member)
> > <56> DW_AT_name : x3
> > <59> DW_AT_decl_file : 1
> > <5a> DW_AT_decl_line : 4
> > <5b> DW_AT_type : <0x80>
> > <5f> DW_AT_byte_size : 1
> > <60> DW_AT_bit_size : 3
> > <61> DW_AT_bit_offset : 1
> > <62> DW_AT_data_member_location: 0
> > <2><63>: Abbrev Number: 3 (DW_TAG_member)
> > <64> DW_AT_name : y1
> > <67> DW_AT_decl_file : 1
> > <68> DW_AT_decl_line : 5
> > <69> DW_AT_type : <0x87>
> > <6d> DW_AT_byte_size : 4
> > <6e> DW_AT_bit_size : 7
> > <6f> DW_AT_bit_offset : 18
> > <70> DW_AT_data_member_location: 0
> > <2><71>: Abbrev Number: 3 (DW_TAG_member)
> > <72> DW_AT_name : y2
> > <75> DW_AT_decl_file : 1
> > <76> DW_AT_decl_line : 6
> > <77> DW_AT_type : <0x87>
> > <7b> DW_AT_byte_size : 4
> > <7c> DW_AT_bit_size : 20
> > <7d> DW_AT_bit_offset : 12
> > <7e> DW_AT_data_member_location: 4
> > <2><7f>: Abbrev Number: 0
> > <1><80>: Abbrev Number: 4 (DW_TAG_base_type)
> > <81> DW_AT_byte_size : 1
> > <82> DW_AT_encoding : 6 (signed char)
> > <83> DW_AT_name : (indirect string, offset: 0xb3): char
> > <1><87>: Abbrev Number: 5 (DW_TAG_base_type)
> > <88> DW_AT_byte_size : 4
> > <89> DW_AT_encoding : 5 (signed)
> > <8a> DW_AT_name : int
> > <1><8e>: Abbrev Number: 2 (DW_TAG_structure_type)
> > <8f> DW_AT_name : (indirect string, offset: 0x50): packed
> > <93> DW_AT_byte_size : 5
> > <94> DW_AT_decl_file : 1
> > <95> DW_AT_decl_line : 9
> > <96> DW_AT_sibling : <0xe1>
> > <2><9a>: Abbrev Number: 3 (DW_TAG_member)
> > <9b> DW_AT_name : x1
> > <9e> DW_AT_decl_file : 1
> > <9f> DW_AT_decl_line : 10
> > <a0> DW_AT_type : <0x80>
> > <a4> DW_AT_byte_size : 1
> > <a5> DW_AT_bit_size : 1
> > <a6> DW_AT_bit_offset : 7
> > <a7> DW_AT_data_member_location: 0
> > <2><a8>: Abbrev Number: 3 (DW_TAG_member)
> > <a9> DW_AT_name : x2
> > <ac> DW_AT_decl_file : 1
> > <ad> DW_AT_decl_line : 11
> > <ae> DW_AT_type : <0x80>
> > <b2> DW_AT_byte_size : 1
> > <b3> DW_AT_bit_size : 3
> > <b4> DW_AT_bit_offset : 4
> > <b5> DW_AT_data_member_location: 0
> > <2><b6>: Abbrev Number: 3 (DW_TAG_member)
> > <b7> DW_AT_name : x3
> > <ba> DW_AT_decl_file : 1
> > <bb> DW_AT_decl_line : 12
> > <bc> DW_AT_type : <0x80>
> > <c0> DW_AT_byte_size : 1
> > <c1> DW_AT_bit_size : 3
> > <c2> DW_AT_bit_offset : 1
> > <c3> DW_AT_data_member_location: 0
> > <2><c4>: Abbrev Number: 3 (DW_TAG_member)
> > <c5> DW_AT_name : y1
> > <c8> DW_AT_decl_file : 1
> > <c9> DW_AT_decl_line : 13
> > <ca> DW_AT_type : <0x87>
> > <ce> DW_AT_byte_size : 4
> > <cf> DW_AT_bit_size : 7
> > <d0> DW_AT_bit_offset : 18
> > <d1> DW_AT_data_member_location: 0
> > <2><d2>: Abbrev Number: 6 (DW_TAG_member)
> > <d3> DW_AT_name : y2
> > <d6> DW_AT_decl_file : 1
> > <d7> DW_AT_decl_line : 14
> > <d8> DW_AT_type : <0x87>
> > <dc> DW_AT_byte_size : 4
> > <dd> DW_AT_bit_size : 20
> > <de> DW_AT_bit_offset : -2
> > <df> DW_AT_data_member_location: 0
> > <2><e0>: Abbrev Number: 0
> >
> > ...
> >
--
- Arnaldo
