This is an early RFC to not rewrite stuff one more time later on if the implementation is not acceptable or any major design changes are required. For the TODO list, please scroll to the end.
Make kallsyms independent of symbols positions in vmlinux (or module) by including relative filepath in each symbol's kallsyms value. I.e.
dev_gro_receive -> net/core/gro.c:dev_gro_receive
For the implementation details, please look at the patch 3/3. Patch 2/3 is just a stub, I plan to reuse kallsyms enhancement from the Rust series for it. Patch 1/3 is a fix of one modpost macro straight from 2.6.12-rc2.
A nice side effect is that it's now easier to debug the kernel, as stacktraces will now tell every call's place in the file tree:
[ 0.733191] Call Trace: [ 0.733668] <TASK> [ 0.733980] lib/dump_stack.c:dump_stack_lvl+0x48/0x68 [ 0.734689] kernel/panic.c:panic+0xf8/0x2ae [ 0.735291] init/do_mounts.c:mount_block_root+0x143/0x1ea [ 0.736046] init/do_mounts.c:prepare_namespace+0x13f/0x16e [ 0.736798] init/main.c:kernel_init_freeable+0x240/0x24f [ 0.737549] ? init/main.c:rest_init+0xc0/0xc0 [ 0.738171] init/main.c:kernel_init+0x1a/0x140 [ 0.738765] arch/x86/entry/entry_64.S:ret_from_fork+0x1f/0x30 [ 0.739529] </TASK>
Here are some stats:
Despite running a small utility on each object file and a script on each built-in.a plus one at the kallsyms generation process, it adds only 3 seconds to the whole clean build time:
make -j$(($(nproc) + 1)) all compile_commands.json 19071.12s user 3481.97s system 4587% cpu 8:11.64 total make -j$(($(nproc) + 1)) all compile_commands.json 19202.88s user 3598.80s system 4607% cpu 8:14.85 total
Compressed kallsyms become bigger by 1.4 Mbytes on x86_64 standard distroconfig - 60% of the kallsyms and 2.4% of the decompressed vmlinux in the memory:
ffffffff8259ab30 D kallsyms_offsets ffffffff82624ed0 D kallsyms_relative_base ffffffff82624ed8 D kallsyms_num_syms ffffffff82624ee0 D kallsyms_names ffffffff827e9c68 D kallsyms_markers ffffffff827ea510 D kallsyms_token_table ffffffff827ea8c0 D kallsyms_token_index ffffffff827eaac0 d .LC1
->
ffffffff8259ac30 D kallsyms_offsets ffffffff82624fb8 D kallsyms_relative_base ffffffff82624fc0 D kallsyms_num_syms ffffffff82624fc8 D kallsyms_names ffffffff8294de50 D kallsyms_markers ffffffff8294e6f8 D kallsyms_token_table ffffffff8294eac8 D kallsyms_token_index ffffffff8294ecc8 d .LC1
TODO: * ELF rel and MIPS relocation support (only rela currently, just to test on x86_64); * modules support. Currently, the kernel reuses standard ELF strtab for module kallsyms. My plan is to create a new section which will have the same symbol order as symtab, but point to new complete strings with filepaths, and use this section solely for kallsyms (leaving symtab alone); * LTO support (now relies on that object files are ELFs); * the actual kallsyms/livepatching/probes code which will use filepaths instead of indexes/positions.
Have fun!
Alexander Lobakin (3): modpost: fix TO_NATIVE() with expressions and consts [STUB] increase kallsyms length limit kallsyms: add option to include relative filepaths into kallsyms
.gitignore | 1 + Makefile | 2 +- include/linux/kallsyms.h | 2 +- init/Kconfig | 26 ++- kernel/livepatch/core.c | 4 +- scripts/Makefile.build | 7 +- scripts/Makefile.lib | 10 +- scripts/Makefile.modfinal | 3 +- scripts/gen_sympaths.pl | 270 ++++++++++++++++++++++++++ scripts/kallsyms.c | 89 +++++++-- scripts/link-vmlinux.sh | 14 +- scripts/mod/.gitignore | 1 + scripts/mod/Makefile | 9 + scripts/mod/modpost.h | 7 +- scripts/mod/sympath.c | 385 ++++++++++++++++++++++++++++++++++++++ 15 files changed, 796 insertions(+), 34 deletions(-) create mode 100755 scripts/gen_sympaths.pl create mode 100644 scripts/mod/sympath.c