Bad padding with bpftool btf dump .. format c

["Per Sundström XP" <per.xp.sundstrom@ericsson.com>]

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; 
}; 
offsetof(struct foo, c)=26 

=== BTF offsets === 
full   :        'c' type_id=6 bits_offset=144 
custom :        'c' type_id=3 bits_offset=144