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

[Luis Machado via Gdb-patches <gdb-patches@sourceware.org>]

Hi,

I like the idea of GDB trying harder to find a particular file. 
Sometimes it is a bit frustrating to deal with GDB and hardcoded paths.

With that said, the code is a bit confusing as-is, maybe because there 
is a lot of fiddling with the paths etc.

I think putting the new code into a function with the appropriate name 
would help here. Your choice.

In any case, it looks OK for me as-is. Just a nit below...

On 11/18/20 11:27 PM, 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".
>    You then get the following message:
> 
>    $ gdb -ex 'set debug-file-directory ./dbgsym/usr/lib/debug' -ex 'file /bin/cat' -ex 'core-file ./cat.core'
>    GNU gdb (Ubuntu 10.1-0ubuntu1) 10.1
>    ...
>    Reading symbols from /bin/cat...
>    Reading symbols from /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug...
>    could not find '.gnu_debugaltlink' file for /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug
> 
> This error happens because GDB is trying to locate the build-id
> link (inside /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id) for the
> DWZ alt debug file, which doesn't exist.  Arguably, this is a problem
> with how dh_dwz works in Debian, and it's something I'm also planning
> to tackle.  But, back at the problem at hand.
> 
> Besides not being able to find the build-id link in the directory
> mentioned above, GDB also tried to open the DWZ alt file using its
> filename.  The problem here is that, since we don't have the distro's
> debuginfo installed, it can't find anything under /usr/lib/debug that
> satisfies it.
> 
> It occurred to me that a good way to workaround this problem is to
> actually try to locate the DWZ alt debug file inside the
> debug-file-directories (that were likely provided by the user).  So
> this is what the proposed patch does.
> 
> The idea here is simple: get the filename extracted from the
> .gnu_debugaltlink section, and manipulate it in order to replace the
> initial part of the path (everything before "/.dwz/") by whatever
> debug-file-directories the user might have provided.
> 
> I talked with Mark Wielaard and he agrees this is a sensible approach.
> In fact, apparently this is something that eu-readelf also does.
> 
> I regtested this code, and no regressions were found.
> 
> 2020-11-14  Sergio Durigan Junior  <sergiodj@sergiodj.net>
> 
> 	* dwarf2/read.c (dwarf2_get_dwz_file): Convert 'filename' to a
> 	std::string.  Implement ability to search for DWZ files in the
> 	debug-file-directories provided by the user as well.
> ---
>   gdb/dwarf2/read.c | 73 +++++++++++++++++++++++++++++++++++++++++++----
>   1 file changed, 67 insertions(+), 6 deletions(-)
> 
> 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 ();
>   
> -  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

remaining?

> +		 debug-file-directories.  */

debug-file-directories doesn't exist in the code. I suppose this should 
be "debug file directories" or "debug-file-directory's"?

> +	      if (filename.size () > ddir.size ()
> +		  && filename.compare (0, ddir.size (), ddir) == 0)
> +		continue;
> +
> +	      /* 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);
> +
> +	      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);
> +		      continue;
> +		    }
> +		  /* Found it.  */
> +		  break;
> +		}
> +	    }
> +	}
> +    }
> +
>     if (dwz_bfd == nullptr)
>       {
>         gdb::unique_xmalloc_ptr<char> alt_filename;
>