Em Wed, Apr 03, 2019 at 04:05:06PM +0200, Mark Wielaard escreveu:
> Hi,
>
> On Wed, 2019-04-03 at 10:19 -0300, Arnaldo Carvalho de Melo wrote:
> > Most are false positives, as DWARF (let alone BTF) has explicit ways
> > to tell that these are all __attribute__((aligned(something)) and
> > pahole still don't infer those by realizing that its not a natural
> > alignment, a heuristic that would catch most of these bigger holes,
> > such as:
>
> DWARF does have DW_AT_alignment and GCC8 does output it.
Excellent! So it is GCC that first used it? I just checked with this
vmlinux:
<0><2d>: Abbrev Number: 121 (DW_TAG_compile_unit)
<2e> DW_AT_producer : (indirect string, offset: 0x5d0c): GNU C89 8.2.1 20181215 (Red Hat 8.2.1-6) -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -mno-80387 -mno-f
p-ret-in-387 -mpreferred-stack-boundary=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel -mindirect-branch=thunk-extern -mindirect-branch-register -mrecord-mcount
-mfentry -march=x86-64 -g -O2 -std=gnu90 -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -falign-jumps=1 -falign-loops=1 -fno-asynchronous-unwind-tables -fno-delete-nul
l-pointer-checks -fstack-protector-strong -fno-var-tracking-assignments -fno-strict-overflow -fno-merge-all-constants -fmerge-constants -fstack-check=no -fconserve-stack --param
allow-store-data-races=0
And indeed:
[acme@quaco pahole]$ readelf -wi vmlinux | grep DW_AT_alignment.*:*4096 | head
<451d> DW_AT_alignment : 4096
<453d> DW_AT_alignment : 4096
<4583> DW_AT_alignment : 4096
<d62d> DW_AT_alignment : 4096
<d6c8> DW_AT_alignment : 4096
<d715> DW_AT_alignment : 4096
<d737> DW_AT_alignment : 4096
<d748> DW_AT_alignment : 4096
<d766> DW_AT_alignment : 4096
<d7ef> DW_AT_alignment : 4096
[acme@quaco pahole]$
Thanks for the pointer, going to my TODO to use when rebuilding the data
structures, i.e. in class__fprintf() and also to be taken into account
in the reorganize code.
- Arnaldo
> DW_AT_alignment Non-default alignment of type, subprogram or
> variable
>
> 2.24 Alignment
>
> A debugging information entry may have a DW_AT_alignment attribute
> whose value of class constant is a positive, non-zero, integer
> describing the alignment of the entity.
>
> For example, an alignment attribute whose value is 8 indicates that
> the entity to which it applies occurs at an address that is a
> multiple of eight (not a multiple of 2^8 or 256).
>
> $ cat align.c
> struct tt {
> char *ptr;
> int __attribute__((__aligned__(64))) i;
> int n;
> } t;
>
> $ gcc -g -c align.c
> $ eu-readelf --debug-dump=info align.o
>
> DWARF section [ 4] '.debug_info' at offset 0x40:
> [Offset]
> Compilation unit at offset 0:
> Version: 4, Abbreviation section offset: 0, Address size: 8, Offset
> size: 4
> [ b] compile_unit abbrev: 1
> producer (strp) "GNU C17 8.2.1 20180905 (Red Hat
> 8.2.1-3) -mtune=generic -march=x86-64 -g"
> language (data1) C99 (12)
> name (strp) "align.c"
> comp_dir (strp) "/tmp"
> stmt_list (sec_offset) 0
> [ 1d] structure_type abbrev: 2
> name (string) "tt"
> byte_size (data1) 128
> alignment (data1) 64
> decl_file (data1) align.c (1)
> decl_line (data1) 1
> decl_column (data1) 8
> sibling (ref4) [ 4f]
> [ 2a] member abbrev: 3
> name (string) "ptr"
> decl_file (data1) align.c (1)
> decl_line (data1) 2
> decl_column (data1) 9
> type (ref4) [ 4f]
> data_member_location (data1) 0
> [ 37] member abbrev: 4
> name (string) "i"
> decl_file (data1) align.c (1)
> decl_line (data1) 3
> decl_column (data1) 40
> type (ref4) [ 5c]
> HERE => alignment (data1) 64
> data_member_location (data1) 40
> [ 43] member abbrev: 3
> name (string) "n"
> decl_file (data1) align.c (1)
> decl_line (data1) 4
> decl_column (data1) 7
> type (ref4) [ 5c]
> data_member_location (data1) 44
> [ 4f] pointer_type abbrev: 5
> byte_size (data1) 8
> type (ref4) [ 55]
> [ 55] base_type abbrev: 6
> byte_size (data1) 1
> encoding (data1) signed_char (6)
> name (strp) "char"
> [ 5c] base_type abbrev: 7
> byte_size (data1) 4
> encoding (data1) signed (5)
> name (string) "int"
> [ 63] variable abbrev: 8
> name (string) "t"
> decl_file (data1) align.c (1)
> decl_line (data1) 5
> decl_column (data1) 3
> type (ref4) [ 1d]
> external (flag_present) yes
> location (exprloc)
> [ 0] addr .bss+0
--
- Arnaldo
