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kbuild: rpm-pkg: build a debuginfo RPM

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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@purestorage.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
This commit is contained in:
Uday Shankar 2025-03-31 16:46:32 -06:00 committed by Masahiro Yamada
parent a26fe287ee
commit a7c699d090
2 changed files with 54 additions and 2 deletions
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46
scripts/package/kernel.spec
View File

@ -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

10
scripts/package/mkspec
View File

@ -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}