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@xxxxxxxxxx>
> ---
> 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) {
