Re: [PATCH v11 1/4] kallsyms: embed source file:line info in kernel stack traces

[Helge Deller]

On 11/6/26 14:24, Sasha Levin wrote:
> Add CONFIG_KALLSYMS_LINEINFO, which embeds a compact address-to-line
> lookup table in the kernel image so stack traces directly print source
> file and line number information:
>
>    root@localhost:~# echo c > /proc/sysrq-trigger
>    [   11.201987] sysrq: Trigger a crash
>    [   11.202831] Kernel panic - not syncing: sysrq triggered crash
>    [   11.206218] Call Trace:
>    [   11.206501]  <TASK>
>    [   11.206749]  dump_stack_lvl+0x5d/0x80 (lib/dump_stack.c:94)
>    [   11.207403]  vpanic+0x36e/0x620 (kernel/panic.c:650)
>    [   11.208565]  ? __lock_acquire+0x465/0x2240 (kernel/locking/lockdep.c:4674)
>    [   11.209324]  panic+0xc9/0xd0 (kernel/panic.c:787)
>    [   11.211873]  ? find_held_lock+0x2b/0x80 (kernel/locking/lockdep.c:5350)
>    [   11.212597]  ? lock_release+0xd3/0x300 (kernel/locking/lockdep.c:5535)
>    [   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)
>    [   11.223942]  entry_SYSCALL_64_after_hwframe+0x77/0x7f (arch/x86/entry/entry_64.S:121)
>
> At build time, a new host tool (scripts/gen_lineinfo) reads DWARF
> .debug_line from vmlinux using libdw (elfutils), extracts all
> address-to-file:line mappings, and generates an assembly file with
> sorted parallel arrays (offsets from _text, file IDs, and line
> numbers). These are linked into vmlinux as .rodata.
>
> At runtime, kallsyms_lookup_lineinfo() does a binary search on the
> table and __sprint_symbol() appends "(file:line)" to each stack frame.
> The lookup uses offsets from _text so it works with KASLR, requires no
> locks or allocations, and is safe in any context including panic.
>
> The suffix is appended for every symbol variant that already prints an
> offset, so %pS, %pSb and the %pB backtrace forms all carry it: arm64
> prints frames with %pSb and the generic stack_trace_print() uses %pS.
> sprint_symbol_no_offset(), which backs lowercase %ps, stays bare
> because callers suffix it with a literal "()".
>
> A line program emits several rows per address, and every row but the
> last covers zero bytes, so the generator keeps the last row at each
> address.  That is the row addr2line reports, and for an inlined call it
> names the inlined body rather than the call site.
>
> Rows the compiler marks with line 0 declare that no source location
> applies from there on (DWARF5 6.2.2), and the end of a line program's
> sequence says the same of the bytes past it.  Both are kept, as entries
> whose line is zero that the lookup reads as "no annotation", so an
> address the compiler deliberately left unattributed reports nothing
> instead of inheriting the line above it.
>
> The feature requires CONFIG_DEBUG_INFO (for DWARF data) and libelf
> and libdw (from elfutils) on the build host.
>
> Memory footprint, measured with:
>
>    make ARCH=x86_64 O=$B defconfig
>    ./scripts/config --file $B/.config -e DEBUG_INFO \
>                                       -e DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>    make ARCH=x86_64 O=$B olddefconfig && make ARCH=x86_64 O=$B -j$(nproc)
>    strip -g $B/vmlinux -o vmlinux.nodbg && stat -c %s vmlinux.nodbg
>
>    Table: 1,657,663 entries from 4,234 source files
>      lineinfo_addrs[]     1,657,663 x u32  =  6.3 MiB
>      lineinfo_file_ids[]  1,657,663 x u16  =  3.2 MiB
>      lineinfo_lines[]     1,657,663 x u32  =  6.3 MiB
>      file_offsets + filenames              =  0.1 MiB
>      Total .rodata increase:               = 15.9 MiB
>
>    vmlinux (stripped):  52.2 MiB -> 68.2 MiB  (+16.0 MiB / +30.6%)
>
> That is the cost of the uncompressed format introduced here; the delta
> compression added later in this series brings it down to +8.0 MiB
> (+15.3%) on the same config.
>
> Suggested-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>


Tested-by: Helge Deller <deller@xxxxxx>   # parisc
Acked-by: Helge Deller <deller@xxxxxx>

Example:

root@62:/# echo c > /proc/sysrq-trigger
[   95.740284] sysrq: Trigger a crash
[   95.745518] Kernel panic - not syncing: sysrq triggered crash
[   95.746724] CPU: 1 UID: 0 PID: 1 Comm: bash Not tainted 7.4.0-rc1+ #3263 VOLUNTARY
[   95.746724] Hardware name: 9000/778/B160L
[   95.746724] Backtrace:
[   95.746724]  [<1041d778>] show_stack+0x54/0x78 (arch/parisc/kernel/traps.c:212)
[   95.746724]  [<10413404>] dump_stack_lvl+0x80/0xc8 (lib/dump_stack.c:122)
[   95.746724]  [<10413468>] dump_stack+0x1c/0x2c (lib/dump_stack.c:130)
[   95.746724]  [<10401288>] vpanic+0x14c/0x340 (kernel/panic.c:561)
[   95.746724]  [<104014b0>] panic+0x34/0x48 (kernel/panic.c:787)
[   95.746724]  [<10e6cbcc>] sysrq_handle_crash+0x20/0x34 (drivers/tty/sysrq.c:154)
[   95.746724]  [<10e6d734>] __handle_sysrq+0xb4/0x1d8 (drivers/tty/sysrq.c:635)
[   95.746724]  [<10e6e1b4>] write_sysrq_trigger+0x90/0xe0 (drivers/tty/sysrq.c:1220)
[   95.746724]  [<10977d88>] proc_reg_write+0xd4/0x120 (fs/proc/inode.c:343)
[   95.746724]  [<1088c9cc>] vfs_write+0xbc/0x468 (fs/read_write.c:690)
[   95.746724]  [<1088cf5c>] ksys_write+0x7c/0x12c (fs/read_write.c:740)
[   95.746724]  [<1088d028>] sys_write+0x1c/0x3c (fs/read_write.c:747)
[   95.746724]  [<10418334>] syscall_exit+0x0/0x10 (arch/parisc/kernel/entry.S:1722)
[   95.746724]
[   95.746724] ---[ end Kernel panic - not syncing: sysrq triggered crash ]---