Note: The article will likely get frequent updates in the next few days.
This article describes some approaches to distribute debug information. Commands below will use two simple C files for demonstration.
1 | cat > a.c <<eof |
Debug information in the executable or shared object
This is the simplest model. Debug information resides in the executable or shared object.
1 | gcc -c -g a.c b.c |
The linker collects input debug sections, resolves relocations, does
minimum merging (SHF_STRING merge for
.debug_str and .debug_line_str), and combines
them into output debug sections.
1 | 0 0 a7 1 .debug_abbrev |
Separate debug files
Debug information is large and not needed by many people. As a general size optimization, many distributions don't provide debug information in main software packages. For debugging needs, distributions may provide debug information in separate packages, leveraging a debugger feature that debug information may reside in a separate file. See https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html.
objcopy from binutils-gdb can create a separate debug file (since
2003). 1
2objcopy --only-keep-debug a a.debug
strip -S a -o a.stripped
eu-strip from elfutils can generate two output files in one
invocation. 1
eu-strip -f a.debug a -o a.stripped
The elfutils way is convenient for simple use cases and is adopted by
rpm. But there is general ambiguity whether an operation applies to one
output file or both. The --only-keep-debug way is
orthogonal and integrates well with other features (e.g.
--compress-debug-sections, --remove-section).
I favor --only-keep-debug and implemented it in
llvm-objcopy.
When debugging a.stripped in gdb, use
add-symbol-file -o xxx a.debug to load the separate debug
file.
Solaris 11 Update 1 introduced a similar feature called Ancillary
Object. See Ancillary
Objects: Separate Debug ELF Files For Solaris. They made a nice
choice that ld -z ancilliary creates two output files,
saving one objcopy command in the GNU linking model.
.gnu_debuglink
objcopy --add-gnu-debuglink=a.debug a.stripped adds a
non-SHF_ALLOC section .gnu_debuglink to
a.stripped. The section contains a filename (no directory
information) and a four-byte CRC checksum. 1
2
3
4
5
6% objdump -g a.stripped
...
Contents of the .gnu_debuglink section (loaded from a.stripped):
Separate debug info file: a.debug
CRC value: 0x4d1e2a66
gdb has supported
.gnu_debuglink since 2003. It finds a debug file in the
same directory of the executable and .debug/ relative to
the directory. (I filed a feature
request for supporting zstd in 2022-09.) 1
2
3% gdb -ex q a.stripped
Reading symbols from a.stripped...
Reading symbols from /tmp/c/a.debug...
Directories specified by debug-file-directory are used
as well. This option needs to be set before loading the inferior.
1
2
3
4
5
6
7% pwd
/tmp/c
% install -D -t debug/tmp/c a.debug
% rm a.debug
% gdb -iex 'set debug-file-directory debug' -ex q a.stripped
Reading symbols from a.stripped...
Reading symbols from debug//tmp/c/a.debug...
A debug file can be found by build ID. A build ID resides in an ELF
note section. Many Linux distributions configure GCC with
--enable-linker-build-id to generate a build ID by default.
See --build-id
for the option.
1 | % readelf -Wn a.stripped |
lldb uses target.debug-file-search-paths to locate a
separate debug file. (TODO)
On Debian, if we install hello-dbgsym, a debug file will
be available in /usr/lib/debug. 1
2
3
4
5% gdb -batch -ex 'show debug-file-directory'
The directory where separate debug symbols are searched for is "/usr/lib/debug".
% gdb -ex q =hello
Reading symbols from /usr/bin/hello...
Reading symbols from /usr/lib/debug/.build-id/ff/29703f105c66821e9b10149db8cff3b2e4043a.debug...
Debian uses dh_strip
for packaging commands. dh_strip uses
objcopy --only-keep-debug --compress-debug-sections to compress debug
sections.
MiniDebugInfo
See https://sourceware.org/gdb/onlinedocs/gdb/MiniDebugInfo.html
(implemented
in 2012). When a binary contains .gnu_debugdata, gdb
decompresses it with xz and loads it. 1
2
3objcopy --only-keep-debug a a.debug
xz a.debug
objcopy -S --add-section=.gnu_debugdata=a.debug.xz a a.stripped1
2
3% gdb -ex q a.stripped
Reading symbols from a.stripped...
Reading symbols from .gnu_debugdata for /tmp/c/a.stripped...
Fedora uses the feature to improve symbolization of stack traces in
the absence of debug information. A Fedora MiniDebugInfo file mostly
just provides note sections and a .symtab with symbols not
in .dynsym. Non-SHF_ALLOC
SHT_PROGBITS/SHT_NOTE/SHT_NOBITS sections (e.g.
.comment) are removed. Note sections are duplicated in the
original binary and the MiniDebugInfo file due to a small missing
optimization in eu-strip -f.
See Support
MiniDebugInfo in rpm for the original implementation. The new
implementation is in scripts/find-debuginfo.in in the debugedit
repository. Support
for mini-debuginfo in LLDB introduces the lldb implementation.
Here is a simplified demonstration of what
scripts/find-debuginfo.in does: 1
2
3
4
5
6
7eu-strip --remove-comment -f a.mini a -o a.stripped
nm a -f sysv --defined-only | awk -F \| '$4 ~ "FUNC" {print $1}' | sort > a.symtab
nm a -f sysv --defined-only -D | sort > a.dynsym
comm -13 a.dynsym a.symtab > a.keepsyms
objcopy -S --keep-symbols=a.keepsyms a.mini
xz a.mini
objcopy -S --add-section=.gnu_debugdata=a.mini.xz a a.stripped
DWARF supplementary object files
A DWARF supplementary object file contains debug sections which can
be referenced by multiple executable and shared object files. dwz
provides -m file to extracts duplicate debug information
into a supplementary object file and rewrite input files to reference
the supplementary object file. The supplementary object file may contain
debugging information entries, strings, and macro descriptions.
1 | print '#include <stdio.h>\nint main() { puts("hello"); }' | gcc -g -gdwarf-5 -xc - -o 1 |
3 is a supplementary object file with a few
.debug_* sections. Its .debug_sup uses
is_supplementary=1.
1 gets a new section .debug_sup with
is_supplementary=0. It may use
DW_FORM_ref_sup4 (DW_FORM_GNU_ref_alt), DW_FORM_strp_sup (DW_FORM_GNU_strp_alt), DW_MACRO_define_sup, DW_MACRO_undef_sup, DW_MACRO_import_sup
attributes to reference the supplementary object file. In this simple
example only DW_FORM_ref_sup4 and
DW_FORM_strp_sup attributes are used.
Before standardization in DWARF v5, the special section name
.gnu_debugaltlink is used.
Split DWARF object files
This was originally proposed as GCC debug fission. Later in
DWARF version 5 this feature was standardized as split DWARF object
files, commonly abbreviated as "split DWARF". The idea is to move the
bulk of .debug_* sections into a separate file
(.dwo) and leave just a small amount in the relocatable
object file (.o). .dwo files are not handled
by the linker: this reduces the input section combining work and
relocation work for the linker, leading to smaller link time and lower
memory usage. The smaller input has advantages for a distributed build
farm.
1 | % clang -c -g -gsplit-dwarf a.c b.c |
(Note: don't perform compiling and linking in one action: the DWO files' location may be surprising.)
-ggnu-pubnames implied by -gsplit-dwarf
generates .debug_gnu_pubnames and
.debug_gnu_pubtypes sections. (-ggnu-pubnames
can be used without -gsplit-dwarf, but it is uncommon.)
--gdb-index supported by some ELF linkers (gold, ld.lld,
mold) causes the linker to read .debug_gnu_pubnames,
.debug_gnu_pubtypes, and other debug sections
(.debug_info, .debug_addr,
.debug_ranges, .debug_rnglists, etc) to
construct .gdb_index. .debug_gnu_pubnames and
.debug_gnu_pubtypes are then discarded.
When gdb loads a symbol file, it constructs an internal symbol table.
.gdb_index
can improve startup time. With split DWARF, the .dwo files
as referenced by the executable or shared object will be fully parsed on
demand.
Clang supports -gsplit-dwarf=single to embed
.dwo sections in the relocatable object file. The
compilation will not produce a .dwo file. This mode is
convenient for single-machine linking.
lldb uses target.debug-file-search-paths for
.dwo searching but does not use a directory structure.
(TODO)
Distributing a number of .dwo files can be inconvenient.
A DWARF package
file (typically given the extension .dwp) can be used
in replace of .dwo files. (Note: currently DWP has incomplete
DWARF64 support.) dwp from binutils-gdb/gold can build a DWARF
package file. llvm-dwp
is another implementation in llvm-project, but there are some memory
usage scaling problems. 1
dwp a.dwo b.dwo -o a.dwp
It is a TODO to integrate DWARF compressing (e.g. dwz) into dwp.
gold is under-maintained in recent years. dwp does not support DWARF v5 yet.
1 | # Note: older Bazel needs --features=per_object_debug_info |
debuginfod
While gdb can hint that debug information is missing (e.g.
Missing separate debuginfos, use: dnf debuginfo-install xxx),
the manual step of installing the relevant debug information package is
considered by many as inconvenient. In 2019, elfutils introduced a new
program debuginfod.
Some distributions have hosted debug information in public servers. See
https://www.redhat.com/en/blog/how-debuginfod-project-evolved-2021.
Here is an example demonstrating what debuginfod does:
1 | objcopy --only-keep-debug a a.debug |
debuginfod listens on port 8002 by default. -F causes it
to scan archives in the specified directory.
-Z .tar.zst=zstdcat tells it to use zstdcat to handle a
.tar.zst file (e.g. an Arch Linux package).
For every archive member, debuginfod classifies the file as an
regular executable/shared object (with at least one
SHF_ALLOC SHT_PROGBITS section) or a debug file (with a
.debug_* section). By default debuginfod parses a
.debug_line
debuginfod-find is a client. 1
2
3
4buildid=$(readelf -n a | awk '/Build ID:/ {print $3}')
DEBUGINFOD_URLS='http://localhost:8002/' debuginfod-find debuginfo $buildid
DEBUGINFOD_URLS='http://localhost:8002/' debuginfod-find executable $buildid
DEBUGINFOD_URLS='http://localhost:8002/' debuginfod-find source $buildid /tmp/c/a.c
We can query the server manually: 1
2
3curl -s localhost:8002/buildid/$buildid/debuginfo -o output && cmp a.debug output
curl -s localhost:8002/buildid/$buildid/executable -o output && cmp a.stripped output
curl -s localhost:8002/buildid/a3b3f0788440fd94/source/$(<<</tmp/c/a.c sed 's,/,%2F,g') | diff /tmp/c/a.c -
With set debuginfod enabled on,
gdb can query a debuginfod server if a symbol file is not found. As of
today, a few programs support debuginfod, e.g. valgrind.
Main change for Arch Linux: debuginfod: Implement role
In llvm-project, llvm-debuginfod is an alternative implementation. It
requires LLVM_ENABLE_HTTPLIB=on.
Microsoft SymbolStore
This is similar to debuginfod. See https://github.com/dotnet/symstore/blob/main/docs/specs/SSQP_Key_Conventions.md.
Apple's "lazy" DWARF scheme
See http://wiki.dwarfstd.org/index.php?title=Apple%27s_%22Lazy%22_DWARF_Scheme.
Apple platforms use a model different from Linux distributions. Apple
ld64 does not combine input __debug_* sections into output
sections. Instead, unless -S is specified, debug map
entries are emitted into the symbol table to record source files and
relocatable object files. Among the entries, N_OS and
N_OSO record source files and relocatable object files.
N_FUN gives value/size for a function.
N_GSYM/N_STSYM describe a global/static
variable symbol.
When debugging a program with lldb, lldb parses DWARF from dSYM
bundles and (for N_OSO entries) relocatable object
files.
1 | clang -c -g a.c b.c |
dsymutil can create a dSYM bundle from relocatable object files. It
takes an executable name or if -y is specified a YAML debug
map. dsymutil finds two relocatable object files a.o and
b.o, combines and optimizes their DWARF information into a
dSYM file. In addition, dsymutil builds an accelerator table
(.apple_* or .debug_names).
Windows PDB
TODO
Appendix: debug information size
https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html#index-feliminate-unused-debug-types