> On Fri, Nov 18, 2022 at 10:30:50AM +0000, Per Sundström XP wrote:
>> Hi,
>>
>> I don't know if this is the channel for reporting issues with the
>> "bpftool dump .. format c" function.
>> If this is not the one, please help me find the correct one.
>>
>> This bash script illustrates a problem where 'bpftool btf dump <file>
>> format c': produces an incorrect 'h' file.
>> I looked into it a bit, and the problem seem to be in the
>> "libbpf/btfdump.c : btf_dump_emit_bit_padding()" function.
>>
>> I can dig into it more if you like, but first I want to report it as a
>> bug.
>>
>> Regards,
>> /Per
>>
>> ---- bad_padding bash script ---
>> ----------------------------------------------------
>> #
>> # Reproduction bash script for wrong offsets
>> #
>> cat >foo.h <<EOF
>> #pragma clang attribute push (__attribute__((preserve_access_index)),
>> apply_to = record)
>> struct foo {
>> struct {
>> int aa;
>> char ab;
>> } a;
>> long :64;
>> int :4;
>> char b;
>> short c;
>> };
>> #pragma clang attribute pop
>> EOF
>>
>> cat >foo.c <<EOF
>> #include "foo.h"
>>
>> #define offsetof(TYPE, MEMBER) ((long) &((TYPE*)0)->MEMBER)
>>
>> long foo()
>> {
>> long ret = 0;
>> //ret += ((struct foo*)0)->a.ab;
>> ret += ((struct foo*)0)->b;
>> ret += ((struct foo*)0)->c;
>> return ret;
>> }
>> EOF
>>
>> cat >main.c <<EOF
>> #include <stdio.h>
>> #include "foo.h"
>>
>> #define offsetof(TYPE, MEMBER) ((long) &((TYPE*)0)->MEMBER)
>>
>> void main(){
>> printf("offsetof(struct foo, c)=%ld\n", offsetof(struct foo, c));
>> }
>> EOF
>>
>> # Vanilla header case
>> printf "============ Vanilla ==========\n"
>> cat foo.h | awk '/^struct foo/,/^}/'
>> gcc -O0 -g -I. -o main main.c; ./main
>>
>> # Proudce a custom [minimized] header
>> CFLAGS="-I. -ggdb -gdwarf -O2 -Wall -fpie -target bpf
>> -D__TARGET_ARCH_x86"
>> clang $CFLAGS -DBOOTSTRAP -c foo.c -o foo.o
>> pahole --btf_encode_detached full.btf foo.o
>> bpftool gen min_core_btf full.btf custom.btf foo.o
>> bpftool btf dump file custom.btf format c > foo.h
>>
>> printf "\n============ Custom ==========\n"
>> cat foo.h | awk '/^struct foo/,/^}/'
>> gcc -O0 -g -I. -o main main.c; ./main
>>
>> printf "\n=== BTF offsets ===\n"
>> printf "full : "
>> /usr/sbin/bpftool btf dump file full.btf | grep "'c'"
>> printf "custom : "
>> /usr/sbin/bpftool btf dump file custom.btf | grep "'c'"
>>
>> #---------------------end of script -------------------------------
>>
>>
>> Output of ./bad_padding.sh:
>> ---
>> ============ Vanilla ==========
>> struct foo {
>> struct {
>> int aa;
>> char ab;
>> } a;
>> long :64;
>> int :4;
>> char b;
>> short c;
>> };
>> offsetof(struct foo, c)=18
>>
>> ============ Custom ==========
>> struct foo {
>> long: 8;
>> long: 64;
>> long: 64;
>> char b;
>> short c;
>> };
>
> so I guess the issue is that the first 'long: 8' is padded to full long: 64 ?
>
> looks like btf_dump_emit_bit_padding did not take into accout the gap on the
> begining of the struct
>
> on the other hand you generated that header file from 'min_core_btf' btf data,
> which takes away all the unused fields.. it might not beeen considered as a
> use case before
>
> jirka
>
That could be the case, but I think the 'emit_bit_padding()' will not really have a
lot to do for the non sparse headers ..
/Per
>
>> offsetof(struct foo, c)=26
>>
>> === BTF offsets ===
>> full : 'c' type_id=6 bits_offset=144
>> custom : 'c' type_id=3 bits_offset=144
>>
