Re: [PATCH 1/2] kallsyms: show function parameter info in oops/WARN dumps

[Sasha Levin <sashal@kernel.org>]

On Tue, Mar 24, 2026 at 08:03:30AM -0700, Alexei Starovoitov wrote:
>On Mon, Mar 23, 2026 at 9:49 AM Sasha Levin <sashal@kernel.org> wrote:
>>
>> Embed DWARF-derived function parameter name and type information in the
>> kernel image so that oops and WARN dumps display the crashing function's
>> register-passed arguments with their names, types, and values.
>>
>> A new build-time tool (scripts/gen_paraminfo.c) parses DW_TAG_subprogram
>> and DW_TAG_formal_parameter entries from DWARF .debug_info, extracting
>> parameter names and human-readable type strings. The resulting tables are
>> stored in .rodata using the same two-phase link approach as lineinfo.
>>
>> At runtime, kallsyms_show_paraminfo() performs a binary search on the
>> paraminfo tables, maps parameters to x86-64 calling convention registers
>> (RDI, RSI, RDX, RCX, R8, R9), and prints each parameter's name, type,
>> and value from pt_regs. If a parameter value matches the page fault
>> address (CR2), it is highlighted with "<-- fault address".
>>
>> Integration at show_regs() means this works for both oops and WARN()
>> automatically, since both paths provide full pt_regs at the exception
>> point.
>>
>> Example output:
>>
>>   Function parameters (ext4_readdir):
>>     file     (struct file *)         = 0xffff888123456000
>>     ctx      (struct dir_context *)  = 0x0000000000001234  <-- fault address
>>
>> Gated behind CONFIG_KALLSYMS_PARAMINFO (depends on CONFIG_KALLSYMS_LINEINFO).
>> Adds approximately 1-2 MB to the kernel image for ~58K functions.
>>
>> Assisted-by: Claude:claude-opus-4-6
>> Signed-off-by: Sasha Levin <sashal@kernel.org>
>
>Nack.
>
>You asked claude to reinvent pahole and BTF and it did it
>completely missing years of fine tuning that pahole does.

Let's keep this on the technical side please.

>dwarf cannot be trusted as-is. pahole converts it carefully
>by analyzing optimized out arguments and dropping signatures

Fair point about pahole and optimized-out args. The problem is that BTF depends
on BPF_SYSCALL, and the environments I care about can't enable either.
Automotive, robotics, and safety configs all have DWARF and KALLSYMS but no
path to BTF.

>from BTF that are not accurate. This work is still ongoing.
>For example see this set:
>https://lore.kernel.org/bpf/20260320190917.1970524-1-yonghong.song@linux.dev/

Ack. I wasn't familiar with this, and looks like it makes the 2nd patch in this
series unnecessary.

>pahole isn't perfect, but what you attempted to do here
>is just broken.

I hear you that raw DWARF isn't perfect with optimized code, but I'd rather
show best-effort info than nothing. Happy to mark it as such in the output.
Open to suggestions on improving accuracy without the BTF dependency.

-- 
Thanks,
Sasha