ruby-core

* Author: kjtsanaktsidis (KJ Tsanaktsidis)
* Status: Open
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
I'm running the Fedora 38 beta on my machine, and the Ruby crash reporter is itself crashing while trying to print C-level backtraces, with an error like this:

```
-- C level backtrace information -------------------------------------------
1344: Abbrev Number 27547 not found
```

This seems to happen because the debuginfo provided by Fedora 38 (for e.g. the libc frames) is compressed with dwz. Some debginfo is shared between multiple files, and a reference to the shared file is put in the `.gnu_debugaltlink` attribute of the debug object. Then, the main debug object contains forms of type `DW_FORM_GNU_ref_alt` and `DW_FORM_GNU_strp_alt`, whose values will refer to offsets inside the debugaltlink file.

For example, libc symbols contain a `.gnu_debugaltlink` section
```
% readelf -x .gnu_debugaltlink /usr/lib/debug/lib64/libc.so.6-2.37-1.fc38.x86_64.debug

Hex dump of section '.gnu_debugaltlink':
0x00000000 2e2e2f2e 64777a2f 676c6962 632d322e ../.dwz/glibc-2.
0x00000010 33372d31 2e666333 382e7838 365f3634 37-1.fc38.x86_64
0x00000020 002a30a8 c71ff2fd aa58a637 226c0f56 .*0......X.7"l.V
0x00000030 3ffd674a de ?.gJ.
```

Some DWARF abbrev's contain references to this shared data:

```
% readelf --debug-dump /usr/lib/debug/lib64/libc.so.6-2.37-1.fc38.x86_64.debug
...
Contents of the .debug_abbrev section (loaded from /usr/lib/debug/lib64/libc.so.6-2.37-1.fc38.x86_64.debug):
...
50 DW_TAG_typedef [no children]
DW_AT_name DW_FORM_GNU_strp_alt
DW_AT_decl_file DW_FORM_data1
DW_AT_decl_line DW_FORM_data1
DW_AT_decl_column DW_FORM_data1
DW_AT_type DW_FORM_GNU_ref_alt
DW_AT value: 0 DW_FORM value: 0
...
```

Because addr2line.c doesn't know how to read these `DW_FORM_GNU_*` forms in `debug_info_reader_read_value`, it doesn't advance the reader at all and so the DWARF parsing becomes confused. This can lead to a garbage abbrev number being read, or even to just straight up segfaults.

I have a patch which simply skips over the right number of bytes for these forms, without actually reading any of the data: https://github.com/ruby/ruby/pull/7731. This was sufficient to make the crash reporter work again properly on my machine.

_Technically_ speaking, perhaps we should _actually_ open the `.gnu_debugaltlink` file and dig out the referenced attribute value - this would be required if filename strings were put into the shared dwz info with `DW_FORM_GNU_strp_apt`. If you think this is necessary I can try and put together a follow up patch to do this. However I've not actually seen a debuginfo file that does this, yet.

--
https://bugs.ruby-lang.org/
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