Re: [PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[Florian Weimer <fweimer@redhat.com>]

* Florian Weimer:

> * Fangrui Song via Libc-alpha:
>
>> Original author is Ambrose Feinstein while working at Google.
>>
>> _dl_map_object iterates over loaded objects and calls _dl_name_match_p.
>> If l->l_soname_added is 0, we incur two costs.
>>
>> First, loading l->l_info[DT_SONAME]->d_un.d_val has TLB pressure as
>> every library's string table is in a different page.  The cost will be
>> avoided if the string is on the heap.
>>
>> Second, add_name_to_object repeats the l_libname comparison already done
>> by the _dl_name_match_p call.
>>
>> To remove these costs, we eagerly add the SONAME to the libname_list.
>> l_soname_added is typically 1, so laziness doesn't provide savings.
>> ---
>>  elf/dl-load.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/elf/dl-load.c b/elf/dl-load.c
>> index 9a0e40c0e9..1b17410ce0 100644
>> --- a/elf/dl-load.c
>> +++ b/elf/dl-load.c
>> @@ -1451,10 +1451,11 @@ cannot enable executable stack as shared object requires");
>>  
>>    /* When we profile the SONAME might be needed for something else but
>>       loading.  Add it right away.  */
>> -  if (__glibc_unlikely (GLRO(dl_profile) != NULL)
>> -      && l->l_info[DT_SONAME] != NULL)
>> +  if (l->l_info[DT_SONAME] != NULL) {
>>      add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
>>  			    + l->l_info[DT_SONAME]->d_un.d_val));
>> +    l->l_soname_added = 1;
>> +  }
>>  
>>    /* If we have newly loaded libc.so, update the namespace
>>       description.  */
>
> The comment is now outdated.
>
> Not sure if this kind of micro-optimization makes sense.  Maybe until we
> add a hash table here …

The hash table is implemented here:

  [PATCH 32/33] elf: Add hash tables to speed up DT_NEEDED, dlopen lookups
  <https://sourceware.org/pipermail/libc-alpha/2023-July/149669.html>

Thanks,
Florian