Re: [PATCH v2] kbuild: rpm-pkg: build a debuginfo RPM

[Masahiro Yamada]

On Tue, Apr 1, 2025 at 7:46 AM Uday Shankar <ushankar@xxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxxxx
> ---

Applied to linux-kbuild.
Thanks.


>  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@xxxxxxxxxxxxxxx>
>
>


-- 
Best Regards
Masahiro Yamada