Re: [PATCH v2] Search for DWZ files in debug-file-directories as well

[Simon Marchi <simark@simark.ca>]

On 2020-11-18 9:27 p.m., Sergio Durigan Junior via Gdb-patches wrote:
> Changes from v1:
>
> - Addressed Simon's comments (new comment explaining how we try to
>   match for the current debug-file-directory; properly use
>   .erase/.insert methods -- keeping in mind that they modify the
>   string in-place).
>
>
> When Debian (and Ubuntu) builds its binaries, it (still) doesn't use
> dwz's "--relative" option.  This causes their debuginfo files to
> carry a .gnu_debugaltlink section containing a full pathname to the
> DWZ alt debug file, like this:
>
>   $ readelf -wk /usr/bin/cat
>   Contents of the .gnu_debugaltlink section:
>
>     Separate debug info file: /usr/lib/debug/.dwz/x86_64-linux-gnu/coreutils.debug
>     Build-ID (0x14 bytes):
>    ee 76 5d 71 97 37 ce 46 99 44 32 bb e8 a9 1a ef 99 96 88 db
>
>   Contents of the .gnu_debuglink section:
>
>     Separate debug info file: 06d3bee37b8c7e67b31cb2689cb351102ae73b.debug
>     CRC value: 0x53267655
>
> This usually works OK, because most of the debuginfo files installed
> via apt will be present in /usr/lib/debug anyway.  However, imagine
> the following scenario:
>
> - You are using /usr/bin/cat, it crashes on you and generates a
>   corefile.
>
> - You don't want/need to "apt install" the debuginfo file for
>   coreutils from the repositories.  Instead, you already have the
>   debuginfo files in a separate directory (e.g., $HOME/dbgsym).
>
> - You start GDB and "set debug-file-directory $HOME/dbgsym".

Should the above read $HOME/dbgsym/usr/lib/debug to be consistent with
the example below?

> diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
> index 3c59826291..c462b9bb2c 100644
> --- a/gdb/dwarf2/read.c
> +++ b/gdb/dwarf2/read.c
> @@ -2190,7 +2190,7 @@ locate_dwz_sections (bfd *abfd, asection *sectp, dwz_file *dwz_file)
>  struct dwz_file *
>  dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
>  {
> -  const char *filename;
> +  std::string filename;
>    bfd_size_type buildid_len_arg;
>    size_t buildid_len;
>    bfd_byte *buildid;
> @@ -2216,19 +2216,17 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
>
>    filename = data.get ();

Declare the filename variable here.

>
> -  std::string abs_storage;
> -  if (!IS_ABSOLUTE_PATH (filename))
> +  if (!IS_ABSOLUTE_PATH (filename.c_str ()))
>      {
>        gdb::unique_xmalloc_ptr<char> abs
>  	= gdb_realpath (bfd_get_filename (per_bfd->obfd));
>
> -      abs_storage = ldirname (abs.get ()) + SLASH_STRING + filename;
> -      filename = abs_storage.c_str ();
> +      filename = ldirname (abs.get ()) + SLASH_STRING + filename;
>      }
>
>    /* First try the file name given in the section.  If that doesn't
>       work, try to use the build-id instead.  */
> -  gdb_bfd_ref_ptr dwz_bfd (gdb_bfd_open (filename, gnutarget));
> +  gdb_bfd_ref_ptr dwz_bfd (gdb_bfd_open (filename.c_str (), gnutarget));
>    if (dwz_bfd != NULL)
>      {
>        if (!build_id_verify (dwz_bfd.get (), buildid_len, buildid))
> @@ -2238,6 +2236,69 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
>    if (dwz_bfd == NULL)
>      dwz_bfd = build_id_to_debug_bfd (buildid_len, buildid);
>
> +  if (dwz_bfd == nullptr)
> +    {
> +      /* If the user has provided us with different
> +	 debug-file-directories, we can try them in order.  */
> +      size_t dwz_pos = filename.find ("/.dwz/");
> +
> +      if (dwz_pos != std::string::npos)
> +	{
> +	  std::vector<gdb::unique_xmalloc_ptr<char>> debugdir_vec
> +	    = dirnames_to_char_ptr_vec (debug_file_directory);
> +
> +	  for (const gdb::unique_xmalloc_ptr<char> &debugdir : debugdir_vec)
> +	    {
> +	      /* The idea is to iterate over the
> +		 debug-file-directories provided by the user and
> +		 replace the hard-coded path in the "filename" by each
> +		 debug-file-directory.
> +
> +		 For example, suppose that filename is:
> +
> +		   /usr/lib/debug/.dwz/foo.dwz
> +
> +		 And suppose that we have "$HOME/bar" as the
> +		 debug-file-directory.  We would then adjust filename
> +		 to look like:
> +
> +		   $HOME/bar/.dwz/foo.dwz
> +
> +		 which would hopefully allow us to find the alt debug
> +		 file.  */
> +	      std::string ddir = debugdir.get ();
> +
> +	      /* Check whether the beginning of FILENAME is DDIR.  If
> +		 it is, then we are dealing with a file which we
> +		 already attempted to open before, so we just skip it
> +		 and continue processing the reamining
> +		 debug-file-directories.  */
> +	      if (filename.size () > ddir.size ()
> +		  && filename.compare (0, ddir.size (), ddir) == 0)
> +		continue;

Corner case, but what if file name is /usr/lib/abcde/.dwz/foo.dwz and
ddir is /usr/lib/abc?  We will wrongfully skip that debug dir I guess?

Filesystem paths are hard :(.  Is there some "is parent of" function we
can use?  Worst case, we skip that check and do an unnecessary attempt.

> +
> +	      /* Replace FILENAME's default debug-file-directory with
> +		 DDIR.  */
> +	      std::string new_filename = filename;
> +	      new_filename.erase (0, dwz_pos);
> +	      new_filename.insert (0, ddir);

I think it would be more readable and efficient (less bytes copied
around) to do just build the new_filename string with something like:

  std::string new_filename = debugdir + &filename[dwz_pos];

This is untested, but I think you'll get the point.  &filename[dwz_pos]
could also be put into a variable with a meaningful name, for clarity
(though I can't think of a good name right now).

And we probably don't need the allocated `ddir` std::string, you
construct it but never really use it (you could just refer to debugdir).

> +
> +	      dwz_bfd = gdb_bfd_open (new_filename.c_str (), gnutarget);
> +
> +	      if (dwz_bfd != nullptr)
> +		{
> +		  if (!build_id_verify (dwz_bfd.get (), buildid_len, buildid))
> +		    {
> +		      dwz_bfd.reset (nullptr);

Just ".reset ()".

> +		      continue;
> +		    }
> +		  /* Found it.  */
> +		  break;
> +		}

Try to minimize the indentation levels where possible, by using early
returns or continues.  For example, here:

  if (dwz_bfd == nullptr)
    continue;

  if (!build_id_verify (...))
    {
      dwz_bfd.reset ();
      continue;
    }

  /* Found it.  */
  break;

Simon