Re: [PATCH 22/28] elf: Add extension mechanism to ld.so.cache

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella via Libc-alpha:

>> +/* Print the extension information at the cache at start address
>> +   FILE_BASE, of ltength FILE_SIZE bytes.  The new-format cache header
>
> s/ltength/length

Fixed.

>> +   is at CACHE, and the file name for diagnostics is CACHE_NAME.  */
>> +static void
>> +print_extensions (struct cache_extension_all_loaded *ext)
>> +{
>> +  if (ext->sections[cache_extension_tag_generator].base != NULL)
>> +    {
>> +      fputs (_("Cache generated by: "), stdout);
>> +      fwrite (ext->sections[cache_extension_tag_generator].base, 1,
>> +	      ext->sections[cache_extension_tag_generator].size, stdout);
>> +      putchar ('\n');
>> +    }
>> +}
>> +
>
> Ok.  Will be the extension tag data always comprised of ascii printable
> characters?

For the generator extension: Yes.  But not for other extensions.
This code only deals with the generator extension.

>> +/* Size of the cache extension directory.  All tags are assumed to be
>> +   present.  */
>> +enum
>> +  {
>> +   cache_extension_size = (offsetof (struct cache_extension, sections)
>> +			   + (cache_extension_count
>> +			      * sizeof (struct cache_extension_section)))
>> +  };
>> +
>> +/* Write the cache extensions to FD.  The extension directory is
>> +   assumed to be located at CACHE_EXTENSION_OFFSET.  */
>> +static void
>> +write_extensions (int fd, uint32_t cache_extension_offset)
>> +{
>> +  assert ((cache_extension_offset % 4) == 0);
>
> Maybe a proper error msg instead of an assert here?

This is in the cache generator, so it's a code bug if the assert fires,
not corrupted input.

>> @@ -435,6 +497,25 @@ save_cache (const char *cache_name)
>>        && idx_old < cache_entry_old_count)
>>      file_entries->libs[idx_old] = file_entries->libs[idx_old - 1];
>>  
>> +  /* Compute the location of the extension directory.  This
>> +     implementation puts the directory after the string table.  The
>> +     size computation matches the write calls below.  The extension
>> +     directory does not exist with format 0, so the value does not
>> +     matter.  */
>> +  uint32_t extension_offset = 0;
>> +  if (opt_format != 2)
>> +    extension_offset += file_entries_size;
>> +  if (opt_format != 0)
>> +    {
>> +      if (opt_format != 2)
>> +	extension_offset += pad;
>> +      extension_offset += file_entries_new_size;
>> +    }
>
> Ok, although I think we should be good move the 'opt_format' definition to
> a proper enumeration.

Can this wait?  Something for a future patch?

>> +static bool __attribute__ ((unused))
>
> Maybe use inline and let the compiler decide? Or the function is
> really duplicate in a lot of places?

The compiler already decides for static functions and will inline them
if they are only used once.  Given the size of the function, I think
that's the appropriate approach here.

>> +cache_extension_load (const struct cache_file_new *cache,
>> +		      const void *file_base, size_t file_size,
>> +		      struct cache_extension_all_loaded *loaded)
>> +{
>> +  memset (loaded, 0, sizeof (*loaded));
>> +  if (cache->extension_offset == 0)
>> +    /* No extensions present.  This is not a format error.  */
>> +    return true;
>> +  if ((cache->extension_offset % 4) != 0)
>> +    /* Extension offset is misaligned.  */
>> +    return false;
>> +  size_t size_tmp;
>> +  if (__builtin_add_overflow (cache->extension_offset,
>> +			      sizeof (struct cache_extension), &size_tmp)
>> +      || size_tmp > file_size)
>> +    /* Extension extends beyond the end of the file.  */
>> +    return false;
>> +  const struct cache_extension *ext = file_base + cache->extension_offset;
>
> Maybe we should add an alignment check for 'file_base' as well (to
> avoid unaligned struct member deference)?

This pointer comes from mmap, so it's not something that can be wrong as
the result of the file data.  I could add an assert, but I don't think
this is likely to go wrong.

Thanks,
Florian
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