Message-Id: <20250331-debuginfo-v2-1-fb460999a5b3@purestorage.com>
Date: Mon, 31 Mar 2025 16:46:32 -0600
From: Uday Shankar <ushankar@...estorage.com>
To: Masahiro Yamada <masahiroy@...nel.org>, 
 Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Uday Shankar <ushankar@...estorage.com>
Subject: [PATCH v2] kbuild: rpm-pkg: build a debuginfo RPM

The rpm-pkg make target currently suffers from a few issues related to
debuginfo:
1. debuginfo for things built into the kernel (vmlinux) is not available
   in any RPM produced by make rpm-pkg. This makes using tools like
   systemtap against a make rpm-pkg kernel impossible.
2. debug source for the kernel is not available. This means that
   commands like 'disas /s' in gdb, which display source intermixed with
   assembly, can only print file names/line numbers which then must be
   painstakingly resolved to actual source in a separate editor.
3. debuginfo for modules is available, but it remains bundled with the
   .ko files that contain module code, in the main kernel RPM. This is a
   waste of space for users who do not need to debug the kernel (i.e.
   most users).

Address all of these issues by additionally building a debuginfo RPM
when the kernel configuration allows for it, in line with standard
patterns followed by RPM distributors. With these changes:
1. systemtap now works (when these changes are backported to 6.11, since
   systemtap lags a bit behind in compatibility), as verified by the
   following simple test script:

   # stap -e 'probe kernel.function("do_sys_open").call { printf("%s\n", $$parms); }'
   dfd=0xffffffffffffff9c filename=0x7fe18800b160 flags=0x88800 mode=0x0
   ...

2. disas /s works correctly in gdb, with source and disassembly
   interspersed:

   # gdb vmlinux --batch -ex 'disas /s blk_op_str'
   Dump of assembler code for function blk_op_str:
   block/blk-core.c:
   125     {
      0xffffffff814c8740 <+0>:     endbr64

   127
   128             if (op < ARRAY_SIZE(blk_op_name) && blk_op_name[op])
      0xffffffff814c8744 <+4>:     mov    $0xffffffff824a7378,%rax
      0xffffffff814c874b <+11>:    cmp    $0x23,%edi
      0xffffffff814c874e <+14>:    ja     0xffffffff814c8768 <blk_op_str+40>
      0xffffffff814c8750 <+16>:    mov    %edi,%edi

   126             const char *op_str = "UNKNOWN";
      0xffffffff814c8752 <+18>:    mov    $0xffffffff824a7378,%rdx

   127
   128             if (op < ARRAY_SIZE(blk_op_name) && blk_op_name[op])
      0xffffffff814c8759 <+25>:    mov    -0x7dfa0160(,%rdi,8),%rax

   126             const char *op_str = "UNKNOWN";
      0xffffffff814c8761 <+33>:    test   %rax,%rax
      0xffffffff814c8764 <+36>:    cmove  %rdx,%rax

   129                     op_str = blk_op_name[op];
   130
   131             return op_str;
   132     }
      0xffffffff814c8768 <+40>:    jmp    0xffffffff81d01360 <__x86_return_thunk>
   End of assembler dump.

3. The size of the main kernel package goes down substantially,
   especially if many modules are built (quite typical). Here is a
   comparison of installed size of the kernel package (configured with
   allmodconfig, dwarf4 debuginfo, and module compression turned off)
   before and after this patch:

   # rpm -qi kernel-6.13* | grep -E '^(Version|Size)'
   Version     : 6.13.0postpatch+
   Size        : 1382874089
   Version     : 6.13.0prepatch+
   Size        : 17870795887

   This is a ~92% size reduction.

Note that a debuginfo package can only be produced if the following
configs are set:
- CONFIG_DEBUG_INFO=y
- CONFIG_MODULE_COMPRESS=n
- CONFIG_DEBUG_INFO_SPLIT=n

The first of these is obvious - we can't produce debuginfo if the build
does not generate it. The second two requirements can in principle be
removed, but doing so is difficult with the current approach, which uses
a generic rpmbuild script find-debuginfo.sh that processes all packaged
executables. If we want to remove those requirements the best path
forward is likely to add some debuginfo extraction/installation logic to
the modules_install target (controllable by flags). That way, it's
easier to operate on modules before they're compressed, and the logic
can be reused by all packaging targets.

Signed-off-by: Uday Shankar <ushankar@...estorage.com>
---
Changes in v2:
- Check config requirements more explicitly (Masahiro Yamada)
- Ensure modules stay non-executable (Masahiro Yamada)
- Always combine debuginfo and debugsource package
- Link to v1: https://lore.kernel.org/r/20250210-debuginfo-v1-0-368feb58292a@purestorage.com
---
 scripts/package/kernel.spec | 46 +++++++++++++++++++++++++++++++++++++++++++--
 scripts/package/mkspec      | 10 ++++++++++
 2 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/scripts/package/kernel.spec b/scripts/package/kernel.spec
index ac3e5ac01d8a4daa031bc9e70b792a68f74c388b..726f34e1196018165adf350933a5f816faeeef0b 100644
--- a/scripts/package/kernel.spec
+++ b/scripts/package/kernel.spec
@@ -2,8 +2,6 @@
 %{!?_arch: %define _arch dummy}
 %{!?make: %define make make}
 %define makeflags %{?_smp_mflags} ARCH=%{ARCH}
-%define __spec_install_post /usr/lib/rpm/brp-compress || :
-%define debug_package %{nil}
 
 Name: kernel
 Summary: The Linux Kernel
@@ -46,6 +44,36 @@ This package provides kernel headers and makefiles sufficient to build modules
 against the %{version} kernel package.
 %endif
 
+%if %{with_debuginfo}
+# list of debuginfo-related options taken from distribution kernel.spec
+# files
+%undefine _include_minidebuginfo
+%undefine _find_debuginfo_dwz_opts
+%undefine _unique_build_ids
+%undefine _unique_debug_names
+%undefine _unique_debug_srcs
+%undefine _debugsource_packages
+%undefine _debuginfo_subpackages
+%global _find_debuginfo_opts -r
+%global _missing_build_ids_terminate_build 1
+%global _no_recompute_build_ids 1
+%{debug_package}
+%endif
+# some (but not all) versions of rpmbuild emit %%debug_package with
+# %%install. since we've already emitted it manually, that would cause
+# a package redefinition error. ensure that doesn't happen
+%define debug_package %{nil}
+
+# later, we make all modules executable so that find-debuginfo.sh strips
+# them up. but they don't actually need to be executable, so remove the
+# executable bit, taking care to do it _after_ find-debuginfo.sh has run
+%define __spec_install_post \
+	%{?__debug_package:%{__debug_install_post}} \
+	%{__arch_install_post} \
+	%{__os_install_post} \
+	find %{buildroot}/lib/modules/%{KERNELRELEASE} -name "*.ko" -type f \\\
+		| xargs --no-run-if-empty chmod u-x
+
 %prep
 %setup -q -n linux
 cp %{SOURCE1} .config
@@ -89,8 +117,22 @@ ln -fns /usr/src/kernels/%{KERNELRELEASE} %{buildroot}/lib/modules/%{KERNELRELEA
 	echo "%exclude /lib/modules/%{KERNELRELEASE}/build"
 } > %{buildroot}/kernel.list
 
+# make modules executable so that find-debuginfo.sh strips them. this
+# will be undone later in %%__spec_install_post
+find %{buildroot}/lib/modules/%{KERNELRELEASE} -name "*.ko" -type f \
+	| xargs --no-run-if-empty chmod u+x
+
+%if %{with_debuginfo}
+# copying vmlinux directly to the debug directory means it will not get
+# stripped (but its source paths will still be collected + fixed up)
+mkdir -p %{buildroot}/usr/lib/debug/lib/modules/%{KERNELRELEASE}
+cp vmlinux %{buildroot}/usr/lib/debug/lib/modules/%{KERNELRELEASE}
+%endif
+
 %clean
 rm -rf %{buildroot}
+rm -f debugfiles.list debuglinks.list debugsourcefiles.list debugsources.list \
+	elfbins.list
 
 %post
 if [ -x /usr/bin/kernel-install ]; then
diff --git a/scripts/package/mkspec b/scripts/package/mkspec
index 4dc1466dfc815c110eb7206f83dd874b17f5170f..c7375bfc25a9ad3ae98c088273bd76375ea6962e 100755
--- a/scripts/package/mkspec
+++ b/scripts/package/mkspec
@@ -23,6 +23,16 @@ else
 echo '%define with_devel 0'
 fi
 
+# debuginfo package generation uses find-debuginfo.sh under the hood,
+# which only works on uncompressed modules that contain debuginfo
+if grep -q CONFIG_DEBUG_INFO=y include/config/auto.conf &&
+   (! grep -q CONFIG_MODULE_COMPRESS=y include/config/auto.conf) &&
+   (! grep -q CONFIG_DEBUG_INFO_SPLIT=y include/config/auto.conf); then
+echo '%define with_debuginfo %{?_without_debuginfo: 0} %{?!_without_debuginfo: 1}'
+else
+echo '%define with_debuginfo 0'
+fi
+
 cat<<EOF
 %define ARCH ${ARCH}
 %define KERNELRELEASE ${KERNELRELEASE}

---
base-commit: 2c8725c1dca3de043670b38592b1b43105322496
change-id: 20250209-debuginfo-cd3d9417af21

Best regards,
-- 
Uday Shankar <ushankar@...estorage.com>

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