Re: [PATCH dwarves 2/3] dwarves_fprintf: support skipping modifier

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2023-03-10 at 14:50 +0000, Alan Maguire wrote:
> When doing BTF comparisons between functions defined in multiple
> CUs, it was noticed a few critical functions failed prototype
> comparisons due to multiple "const" modifiers; for example:
> 
> function mismatch for 'memchr_inv'('memchr_inv'): 'void * ()(const const void  * , int, size_t)' != 'void * ()(const void  *, int, size_t)'
> 
> function mismatch for 'strnlen'('strnlen'): '__kernel_size_t ()(const const char  * , __kernel_size_t)' != '__kernel_size_t ()(const char  *, size_t)'
> 
> (note the "const const" in the first parameter.)

Hi Alan,

Could you please share which command/flags do you use to generate the
'memchr_inv' with 'const const'?
I tried the ones used in 'btfdiff':
- pahole -F dwarf  --flat_arrays --sort --jobs --suppress_aligned_attribute \
  --suppress_force_paddings --suppress_packed --lang_exclude rust \
  --show_private_classes ./vmlinux
- pahole -F btf --sort --suppress_aligned_attribute --suppress_packed ./vmlinux

But don't see any function prototypes generated with 'const const'.

On the other hand, I see it in a few structure definitions, e.g. here
is original C code (include/linux/sysrq.h:32):

    struct sysrq_key_op {
    	void (* const handler)(int);
    	const char * const help_msg;
    	const char * const action_msg;
    	const int enable_mask;
    };

And here is how it is reconstructed from DWARF (same happens when
reconstructed from BTF):

    struct sysrq_key_op {
            const void                 (*handler)(int);      /*     0     8 */
            const const char  *        help_msg;             /*     8     8 */
            const const char  *        action_msg;           /*    16     8 */
            const int                  enable_mask;          /*    24     4 */
    
            /* size: 32, cachelines: 1, members: 4 */
            /* padding: 4 */
            /* last cacheline: 32 bytes */
    };

So it seems to be a general issue with modifiers printing.

Thanks,
Eduard
> 
> As such it would be useful to omit modifiers for comparison
> purposes.  Also noted was the fact that for the "no_parm_names"
> case, an extra space was being emitted in some cases, also
> throwing off string comparisons of prototypes.
> 
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  dwarves.h         |  1 +
>  dwarves_fprintf.c | 26 ++++++++++++++++----------
>  2 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/dwarves.h b/dwarves.h
> index d04a36d..7a319d1 100644
> --- a/dwarves.h
> +++ b/dwarves.h
> @@ -134,6 +134,7 @@ struct conf_fprintf {
>  	uint8_t	   strip_inline:1;
>  	uint8_t	   skip_emitting_atomic_typedefs:1;
>  	uint8_t	   skip_emitting_errors:1;
> +	uint8_t    skip_emitting_modifier:1;
>  };
>  
>  struct cus;
> diff --git a/dwarves_fprintf.c b/dwarves_fprintf.c
> index 5c6bf9c..b20a473 100644
> --- a/dwarves_fprintf.c
> +++ b/dwarves_fprintf.c
> @@ -506,7 +506,8 @@ static const char *tag__ptr_name(const struct tag *tag, const struct cu *cu,
>  				struct tag *next_type = cu__type(cu, type->type);
>  
>  				if (next_type && tag__is_pointer(next_type)) {
> -					const_pointer = "const ";
> +					if (!conf->skip_emitting_modifier)
> +						const_pointer = "const ";
>  					type = next_type;
>  				}
>  			}
> @@ -580,13 +581,16 @@ static const char *__tag__name(const struct tag *tag, const struct cu *cu,
>  				   *type_str = __tag__name(type, cu, tmpbf,
>  							   sizeof(tmpbf),
>  							   pconf);
> -			switch (tag->tag) {
> -			case DW_TAG_volatile_type: prefix = "volatile "; break;
> -			case DW_TAG_const_type:    prefix = "const ";	 break;
> -			case DW_TAG_restrict_type: suffix = " restrict"; break;
> -			case DW_TAG_atomic_type:   prefix = "_Atomic ";  break;
> +			if (!conf->skip_emitting_modifier) {
> +				switch (tag->tag) {
> +				case DW_TAG_volatile_type: prefix = "volatile "; break;
> +				case DW_TAG_const_type: prefix = "const"; break;
> +				case DW_TAG_restrict_type: suffix = " restrict"; break;
> +				case DW_TAG_atomic_type:   prefix = "_Atomic ";  break;
> +				}
>  			}
> -			snprintf(bf, len, "%s%s%s ", prefix, type_str, suffix);
> +			snprintf(bf, len, "%s%s%s%s", prefix, type_str, suffix,
> +				 conf->no_parm_names ? "" : " ");
>  		}
>  		break;
>  	case DW_TAG_array_type:
> @@ -818,9 +822,11 @@ print_default:
>  	case DW_TAG_const_type:
>  		modifier = "const";
>  print_modifier: {
> -		size_t modifier_printed = fprintf(fp, "%s ", modifier);
> -		tconf.type_spacing -= modifier_printed;
> -		printed		   += modifier_printed;
> +		if (!conf->skip_emitting_modifier) {
> +			size_t modifier_printed = fprintf(fp, "%s ", modifier);
> +			tconf.type_spacing -= modifier_printed;
> +			printed		   += modifier_printed;
> +		}
>  
>  		struct tag *ttype = cu__type(cu, type->type);
>  		if (ttype) {