tjh.dev
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@purestorage.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
authored by