[Bug corefiles/31635] New: debuginfod cannot correctly fetch shared libraries without soname from server due to inconsistency

[Bug corefiles/31635] New: debuginfod cannot correctly fetch shared libraries without soname from server due to inconsistency

[pablogsal at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31635

            Bug ID: 31635
           Summary: debuginfod cannot correctly fetch shared libraries
                    without soname from server due to inconsistency
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: corefiles
          Assignee: unassigned at sourceware dot org
          Reporter: pablogsal at gmail dot com
  Target Milestone: ---

When gdb uses debuginfod to fetch missing sharedlibraries from a given
debuginfod server it fails for most libraries that don't have a soname due to
an inconsistency in how the buildids are registered and how they are queried.

In corelow.c in the build_file_mappings we can see how the build ids are
registered:

        /* If this is a bfd of a shared library, record its soname
           and build id.  */
        if (build_id != nullptr)
          {
            gdb::unique_xmalloc_ptr<char> soname
              = gdb_bfd_read_elf_soname (bfd->filename);
            if (soname != nullptr)
              set_cbfd_soname_build_id (current_program_space->cbfd,
                                        soname.get (), build_id);
          }
      });


Here, if a library was downloaded by a debuginfod server bfd->filename it's the
file that is in the debuginfod cache, (for example
/home/pablogsal/.cache/debuginfod_client/4a834042b43eec1f2556ef4979828ea3b0813adc/executable).
Notice that here there are two problems:

1) Libraries that do not have soname will never be registered in the map by
set_cbfd_soname_build_id. This is very unfortunate because most shared
libraries for dynamic languages such as Python do not have sonames set.
2) The registering into the map from library to buildid in
set_cbfd_soname_build_id it's made by SONAME (from the dynamic table).


But later, in solib.c in the get_cbfd_soname_build_id() function we can see the
following:

  soname_build_id_map *mapptr
    = cbfd_soname_build_id_data_key.get (abfd.get ());

  if (mapptr == nullptr)
    return {};

  auto it = mapptr->find (lbasename (soname));
  if (it == mapptr->end ())
    return {};


Here the query is made by the basename of the *soname* argument (lbasename
(soname)) but this function is NOT called with the soname but rather the full
path. Indeed, in solib_map_sections() we can observe:

  gdb::unique_xmalloc_ptr<char> build_id_hexstr
    = get_cbfd_soname_build_id (current_program_space->cbfd,
                                so.so_name.c_str ());

but so.so_name.c_str() is a full path from the linker map. Notice the
following:

1) This full path may be different from the one in the previous section because
one comes from the core and the other comes from the linker map. One it's an
absolute path and the other can be a symlink.
2) The query is made by full path but the map it's populate from DT_SONAME,
which is inconsistent.


All of this together means that gdb will never properly load executables that
are downloaded from the server for shared libraries that do not have sonames
set. This is very unfortunate because many extension modules for dynamic
languages such as Python don't have sonames and will never work for debuginfod:

$ python
Python 3.11.7 (main, Dec 24 2023, 14:12:08) [GCC 13.2.1 20230801] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import _ssl
>>> _ssl
<module '_ssl' from '/home/pablogsal/.pyenv/versions/3.11.7/lib/python3.11/lib-

$ readelf -d
/home/pablogsal/.pyenv/versions/3.11.7/lib/python3.11/lib-dynload/_ssl.cpython-311-x86_64-linux-gnu.so

Dynamic section at offset 0x2ad28 contains 27 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libssl.so.3]
 0x0000000000000001 (NEEDED)             Shared library: [libcrypto.so.3]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000001d (RUNPATH)            Library runpath:
[/home/pablogsal/.pyenv/versions/3.11.7/lib]
 0x000000000000000c (INIT)               0x12000
 0x000000000000000d (FINI)               0x1c23c
 0x0000000000000019 (INIT_ARRAY)         0x2bc10
 0x000000000000001b (INIT_ARRAYSZ)       8 (bytes)
 0x000000000000001a (FINI_ARRAY)         0x2bc18
 0x000000000000001c (FINI_ARRAYSZ)       8 (bytes)
 0x000000006ffffef5 (GNU_HASH)           0x300
 0x0000000000000005 (STRTAB)             0x22c0
 0x0000000000000006 (SYMTAB)             0x328
 0x000000000000000a (STRSZ)              6467 (bytes)
 0x000000000000000b (SYMENT)             24 (bytes)
 0x0000000000000003 (PLTGOT)             0x2bfe8
 0x0000000000000002 (PLTRELSZ)           7488 (bytes)
 0x0000000000000014 (PLTREL)             RELA
 0x0000000000000017 (JMPREL)             0xffd8
 0x0000000000000007 (RELA)               0x3f18
 0x0000000000000008 (RELASZ)             49344 (bytes)
 0x0000000000000009 (RELAENT)            24 (bytes)
 0x000000006ffffffe (VERNEED)            0x3ea8
 0x000000006fffffff (VERNEEDNUM)         3
 0x000000006ffffff0 (VERSYM)             0x3c04
 0x000000006ffffff9 (RELACOUNT)          2033
 0x0000000000000000 (NULL)               0x0

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug corefiles/31635] debuginfod cannot correctly fetch shared libraries without soname from server due to inconsistency

[pablogsal at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31635

--- Comment #1 from Pablo Galindo Salgado <pablogsal at gmail dot com> ---
Also related: https://sourceware.org/bugzilla/show_bug.cgi?id=31634

-- 
You are receiving this mail because:
You are on the CC list for the bug.

[Bug corefiles/31635] debuginfod cannot correctly fetch shared libraries without soname from server due to inconsistency

[amerey at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31635

Aaron Merey <amerey at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amerey at redhat dot com

--- Comment #2 from Aaron Merey <amerey at redhat dot com> ---
(In reply to Pablo Galindo Salgado from comment #0) 
> 1) Libraries that do not have soname will never be registered in the map by
> set_cbfd_soname_build_id. This is very unfortunate because most shared
> libraries for dynamic languages such as Python do not have sonames set.
> 2) The registering into the map from library to buildid in
> set_cbfd_soname_build_id it's made by SONAME (from the dynamic table).

One challenge for getting debuginfod to work with corefiles is figuring out 
which library buildid in the corefile corresponds to a particular library
that gdb is unable to open in solib_map_sections.  Sonames are used because
we can create a soname to buildid mapping from the corefile and then use 
this mapping to get the right buildid for the debuginfod query when a shared
library can't otherwise be found by gdb in solib_map_sections.

> [...]
> Here the query is made by the basename of the *soname* argument (lbasename
> (soname)) but this function is NOT called with the soname but rather the 
> full path. Indeed, in solib_map_sections() we can observe:
> 
>   gdb::unique_xmalloc_ptr<char> build_id_hexstr
>     = get_cbfd_soname_build_id (current_program_space->cbfd,
>                               so.so_name.c_str ());
> 
> but so.so_name.c_str() is a full path from the linker map. Notice the 
> following:
> 
> 1) This full path may be different from the one in the previous section
> because one comes from the core and the other comes from the linker map. One 
> it's an absolute path and the other can be a symlink.
> 2) The query is made by full path but the map it's populate from DT_SONAME,
> which is inconsistent.

AFAIK when the library has a soname, the basename in get_cbfd_soname_build_id
should generally match a soname in the map.  If this wasn't the case then gdb 
would fail to query debuginfod for libraries associated with corefiles much 
more often.

I'll look into what paths gdb uses in solib_map_sections for libraries with
no soname.  Maybe it's possible to create a path-to-buildid map from the
corefile
that we could use instead of the the soname-to-buildid map when a soname isn't
available.

-- 
You are receiving this mail because:
You are on the CC list for the bug.