This series adds CONFIG_KALLSYMS_LINEINFO, which embeds source file:line information directly in the kernel image so that stack traces annotate every frame with the originating source location - no external tools, no debug symbols at runtime, and safe to use in NMI/panic context.
Motivation ========== The recent "slowly decommission bugzilla?" thread surfaced a recurring problem: when users encounter kernel crashes they see stack traces like `func+0x1ec/0x240` but have no way to identify which subsystem or maintainer to contact. Richard Weinberger proposed building a database mapping symbols to source files using nm/DWARF. Linus pointed to scripts/decode_stacktrace.sh as the existing solution. But as the discussion progressed, it became clear that decode_stacktrace.sh has significant practical barriers that prevent it from being useful in the common case. Problems with scripts/decode_stacktrace.sh ========================================== - Requires debug symbols: the script needs vmlinux with DWARF debug info. Many distros don't retain debug symbols for older or security kernels, and even when available, asking users to obtain matching debuginfo packages is a significant hurdle. - Requires toolchain: users need addr2line and nm installed. - Version-matching requirement: debug symbols must exactly match the running kernel binary. What this series does ===================== Patch 1: CONFIG_KALLSYMS_LINEINFO At build time, a host tool (scripts/gen_lineinfo) reads DWARF .debug_line from vmlinux, extracts address-to-file:line mappings, and embeds them as sorted lookup tables in .rodata. At runtime, kallsyms_lookup_lineinfo() binary-searches the table and __sprint_symbol() appends "(file:line)" to each stack frame. NMI/panic- safe (no locks, no allocations), KASLR-compatible. Patch 2: CONFIG_KALLSYMS_LINEINFO_MODULES Extends lineinfo to loadable modules. Each .ko gets a .mod_lineinfo section embedded at build time. The module loader picks it up at load time. Same zero-allocation, NMI-safe lookup. Patch 3: delta compression Block-indexed delta-encoding with LEB128 varints, implementing the approach suggested by Juergen Gross in the RFC review. Reduces overhead from ~44 MiB to ~11 MiB (~3.7 bytes/entry), addressing the primary size concern from the RFC. Patch 4: KUnit tests 30 KUnit tests covering the lineinfo lookup paths, delta-decode logic, boundary conditions, and integration with the backtrace formatting APIs. Example output ============== [ 11.206749] dump_stack_lvl+0x5d/0x80 (lib/dump_stack.c:94) [ 11.207403] vpanic+0x36e/0x620 (kernel/panic.c:650) [ 11.209324] panic+0xc9/0xd0 (kernel/panic.c:787) [ 11.213312] sysrq_handle_crash+0x1a/0x20 (drivers/tty/sysrq.c:154) [ 11.214005] __handle_sysrq.cold+0x66/0x256 (drivers/tty/sysrq.c:611) [ 11.214712] write_sysrq_trigger+0x65/0x80 (drivers/tty/sysrq.c:1221) [ 11.215424] proc_reg_write+0x1bd/0x3c0 (fs/proc/inode.c:330) [ 11.216061] vfs_write+0x1c6/0xff0 (fs/read_write.c:686) [ 11.218848] ksys_write+0xfa/0x200 (fs/read_write.c:740) [ 11.222394] do_syscall_64+0xf3/0x690 (arch/x86/entry/syscall_64.c:63) Size impact =========== Measured with a Debian kernel config: - bzImage: +3.6 MiB (14 MiB -> 18 MiB, +26%) - Runtime memory: +5.9 MiB (text+data+bss) - Code overhead: +5.0 KiB (.text, lookup functions only) - Data overhead: +5.9 MiB (.data, lineinfo tables) Lineinfo data breakdown: - lineinfo_data (delta-compressed): 5,728 KiB (97%) - lineinfo_block_addrs: 99 KiB - lineinfo_block_offsets: 99 KiB - lineinfo_filenames: 111 KiB - lineinfo_file_offsets: 17 KiB The ~5.9 MiB is after 2.7x delta compression; uncompressed would be ~16 MiB. This is a fraction of the cost of shipping full DWARF debug info (hundreds of MiB), which distros must store and serve for every kernel version. For distros, maintaining debug symbol repositories is expensive: storage, mirrors, and CDN bandwidth for hundreds of MiB per kernel build add up quickly. A ~5.9 MiB increase in the kernel image itself is a modest cost that eliminates the need for users to find, download, and version-match debuginfo packages just to make a crash report useful. For developers, the file:line annotations appear immediately in crash traces - no post-processing with decode_stacktrace.sh needed. Changes since v2 ================= - Replace #ifdef CONFIG_KALLSYMS_LINEINFO with IS_ENABLED() throughout, so the compiler checks the code for syntax errors regardless of configuration. (Suggested by Helge Deller) - Replace zigzag + ULEB128 encoding of signed deltas with native SLEB128, removing the unnecessary zigzag transform layer. (Suggested by Vivian Wang) - Deduplicate the binary search and delta-decode logic: extract shared struct lineinfo_table and lineinfo_search() into mod_lineinfo.h instead of maintaining near-identical copies in kernel/kallsyms.c and kernel/module/kallsyms.c. (Suggested by Vivian Wang) - Use .uleb128 / .sleb128 assembler directives in gen_lineinfo output instead of encoding varints in C and emitting .byte hex literals. (Suggested by Vivian Wang) - Redesign module mod_lineinfo_header to use explicit (offset, size) pairs for each sub-array, similar to flattened devicetree layout. This makes bounds validation straightforward: offset + size <= section_size. (Suggested by Vivian Wang) - Remove dead sym_start parameter from kallsyms_lookup_lineinfo() and module_lookup_lineinfo(). Changes since v1 ================= - Fix path stripping regression on architectures where DWARF comp_dir is a subdirectory (e.g. arch/parisc/kernel) rather than the source tree root: paths now correctly show "kernel/traps.c:212" instead of bare "traps.c:212". Added kernel_dirs[] fallback scan and bare-filename recovery via comp_dir. (Reported by Helge Deller) - Fix RST heading: overline/underline must be at least as long as the heading text in kallsyms-lineinfo.rst. (Reported by Randy Dunlap) - Fix MAINTAINERS alphabetical ordering: move KALLSYMS LINEINFO entry before KASAN. (Reported by Randy Dunlap) - Fix arch-portability of .debug_line relocation handling: replace hardcoded R_X86_64_32 with r_type_abs32() supporting x86, arm, arm64, riscv, s390, mips, ppc, loongarch, and parisc. - Fix vmlinux compressed-path data_end for the last block: use lineinfo_data_size instead of UINT_MAX. - Add file_offsets[] and filenames_size bounds checks in vmlinux lookup path (the module path already had them). - Add alignment padding for file_offsets[] in module .mod_lineinfo binary format (data[] is variable-length u8, followed by u32[]). - Remove sym_start cross-validation check that incorrectly rejected valid lineinfo entries for assembly-adjacent functions. - Add KUnit test suite (new patch 4/4): 30 tests covering vmlinux lookup, module lookup, delta decode, boundary conditions, and backtrace formatting integration. Changes since RFC ================== - Added module support (patch 2) - Added delta compression (patch 3), reducing size from ~44 MiB to ~11 MiB, addressing the primary concern from RFC review - Added documentation (Documentation/admin-guide/kallsyms-lineinfo.rst) - Added MAINTAINERS entry Sasha Levin (4): kallsyms: embed source file:line info in kernel stack traces kallsyms: extend lineinfo to loadable modules kallsyms: delta-compress lineinfo tables for ~2.7x size reduction kallsyms: add KUnit tests for lineinfo feature Documentation/admin-guide/index.rst | 1 + .../admin-guide/kallsyms-lineinfo.rst | 97 ++ MAINTAINERS | 9 + include/linux/kallsyms.h | 30 +- include/linux/mod_lineinfo.h | 243 +++++ include/linux/module.h | 16 + init/Kconfig | 35 + kernel/kallsyms.c | 59 ++ kernel/kallsyms_internal.h | 13 + kernel/module/kallsyms.c | 75 ++ kernel/module/main.c | 4 + lib/Kconfig.debug | 10 + lib/tests/Makefile | 3 + lib/tests/lineinfo_kunit.c | 813 +++++++++++++++++ scripts/.gitignore | 1 + scripts/Makefile | 4 + scripts/Makefile.modfinal | 6 + scripts/empty_lineinfo.S | 38 + scripts/gen-mod-lineinfo.sh | 48 + scripts/gen_lineinfo.c | 850 ++++++++++++++++++ scripts/kallsyms.c | 17 + scripts/link-vmlinux.sh | 43 +- 22 files changed, 2411 insertions(+), 4 deletions(-) create mode 100644 Documentation/admin-guide/kallsyms-lineinfo.rst create mode 100644 include/linux/mod_lineinfo.h create mode 100644 lib/tests/lineinfo_kunit.c create mode 100644 scripts/empty_lineinfo.S create mode 100755 scripts/gen-mod-lineinfo.sh create mode 100644 scripts/gen_lineinfo.c -- 2.51.0
