Re: [PATCH] ctf-reader: Lookup debug info for symbols in a non default archive member

["Guillermo E. Martinez" <guillermo.e.martinez@oracle.com>]

On 9/6/22 07:49, Dodji Seketeli wrote:
> Hello Guillermo,

Hello Dodji,

> Thanks for the patch.  I have tested and it seems to pass regression
> testing on my system.  However, there are some things that I don't
> understand so I have some questions below.  The questions are just for
> my own understanding.  I don't have anything major against the patch,
> obviously.
> 
> [...]
> 
> "Guillermo E. Martinez via Libabigail" <libabigail@sourceware.org> a
> écrit:
> 
> 
> [...]
> 
>> +/// Given a symbol name, lookup the corresponding CTF information in
>> +/// the default dictionary (CTF archive member provided by the caller)
>> +/// If the search is not success, the  looks for the symbol name
>> +/// in _all_ archive members.
>> +///
>> +/// @param ctfa the CTF archive.
>> +/// @param dict the default dictionary to looks for.
>> +/// @param sym_name the symbol name.
>> +/// @param corp the IR corpus.
>> +///
>> +/// Note that if @ref sym_name is found in other than default dictionary
>> +/// @ref ctf_dict will be updated and it must be explicate closed by its
>> +/// caller.
>> +///
>> +/// @return a valid CTF type id, if @ref sym_name was found, -1 otherwise.
>> +
>> +static ctf_id_t
>> +lookup_symbol_in_ctf_archive(ctf_archive_t *ctfa, ctf_dict_t **ctf_dict,
>> +                             const char *sym_name, corpus_sptr corp)
>> +{
>> +  int ctf_err;
>> +  ctf_dict_t *dict = *ctf_dict;
>> +  ctf_id_t ctf_type = ctf_lookup_variable(dict, sym_name);
> 
> So, here, we begin by looking for a variable (using ctf_lookup_variable)
> which ELF symbol is sym_name, is that correct?

That's correct, `sym_name' is the symbol name.

>> +
>> +  /* lookup CTF type for a given symbol in its default
>> +     dictionary */
>> +  if (ctf_type == (ctf_id_t) -1
> 
> So, I guess the variable lookup failed, right?

Correct, libctf `ctf_lookup_*' functions return CTF_ERR when fails,
so I'm goinf to changed it for clarity.

>> +      && !(corp->get_origin() & corpus::LINUX_KERNEL_BINARY_ORIGIN))
> 
> Why this condition?  Why only considering cases where we are not looking
> at a Linux Kernel binary?  I would think that we would want to consider
> the case where the variable lookup failed, even in the case of a Linux
> Kernel binary, wouldn't we? If not why?  Maybe we should add a comment
> to explain this.

OK. The linker (ld) in the Kenel build mechanism uses: `--ctf-variables',
then it emits the symbols type definitions using just the CTF Variable
ection:

$ objdump --ctf foo

   ...

   Labels:

   Data objects:

   Function objects:

   Variables:
     main -> 0x2: (kind 5) int (*) () (aligned at 0x8)
     main_func -> 0x4: (kind 5) void (*) () (aligned at 0x8)
     okkk -> 0x1: (kind 1) int (format 0x1) (size 0x4) (aligned at 0x4)


Otherwise, it must be splitted across CTF Data, Function and Variable
sections:

$ objdump --ctf foo.o

  Data objects:
     okkk -> 0x1: (kind 1) int (format 0x1) (size 0x4) (aligned at 0x4)

   Function objects:
     main -> 0x2: (kind 5) int (*) () (aligned at 0x8)
     main_func -> 0x4: (kind 5) void (*) () (aligned at 0x8)

   Variables:
     okkk -> 0x1: (kind 1) int (format 0x1) (size 0x4) (aligned at 0x4)


Since, vmlinux + *.ko, is *big* binary, I arranged the order of CTF
lookup functions invoking at first: 'ctf_lookup_variable` and then,
if it fails `ctf_lookup_by_symbol_name' by performance reasons.
But I'm agree to remove `!(corp->get_origin() & corpus::LINUX_KERNEL_BINARY_ORIGIN))'
changing the invocation order for those functions, the penalty performance
was less than 10s building the ABI representation for the kernel,
I consider it as acceptable.


>> +    ctf_type = ctf_lookup_by_symbol_name(dict, sym_name);
> 
> So I am guessing that ctf_lookup_by_symbol_name looks up both variable
> and function symbols from the same dictionary, is that correct?

True.

> Also, I don't understand why we don't just use ctf_lookup_by_symbol_name
> rather than starting with ctf_lookup_variable first.  Is it a
> performance things?

Exactly. Performance when we are processing a Linux tree directory.

> Incidentally, I haven't found documentation for the lookup functions
> other than by looking at the code, in say:
> https://sourceware.org/git/?p=binutils-gdb.git;a=blob_plain;f=libctf/ctf-lookup.c;hb=refs/heads/master.

I'm afraid that the documentation is just in the source code.

> If there is documentation for it somewhere else, maybe we can link that
> place in the code here in a comment somewhere, or we can just point to
> that link above.  Both would be fine by me.
> 
>> +
>> +  /* Not lucky, then, search in whole archive */
>> +  if (ctf_type == (ctf_id_t) -1)
>> +    {
>> +      ctf_dict_t *fp;
>> +      ctf_next_t *i = NULL;
>> +      const char *arcname;
>> +
>> +      while ((fp = ctf_archive_next(ctfa, &i, &arcname, 1, &ctf_err)) != NULL)
>> +        {
>> +          ctf_type = ctf_lookup_variable (fp, sym_name);
>> +          if (ctf_type == (ctf_id_t) -1
>> +              && !(corp->get_origin() & corpus::LINUX_KERNEL_BINARY_ORIGIN))
> 
> The same questions as above.
> 
>> +            ctf_type = ctf_lookup_by_symbol_name(fp, sym_name);
>> +
>> +          if (ctf_type != (ctf_id_t) -1)
>> +            {
>> +              *ctf_dict = fp;
>> +              break;
>> +            }
>> +          ctf_dict_close(fp);
>> +        }
>> +    }
>> +
>> +  return ctf_type;
>> +}
>> +
> 
> Cheers,
> 
> [...]
> 
> 

Really thanks for your comments!,
I will prepare the v2

Kind regards,
guillermo