[PATCH v2 0/7] arm64: stack backtraces

Tue Jul 29 14:34:26 CEST 2025

On 7/25/25 17:23, Heinrich Schuchardt wrote:
> On 25.07.25 14:57, Casey Connolly wrote:
>> Implement support for printing stack backtraces on ARM64, make
>> framepointer support configurable by the build system with
>> CONFIG_FRAMEPOINTER and teach U-Boot to walk the framepointers and
>> unwind the stack when an exception occurs. Also show all 64 bits of the
>> exception syndrom register (ESR).
>>
>> Additionally, a new global unwind_stack() function is added, this can
>> be called from anywhere to print a backtrace which can be useful when
>> debugging certain problems.
>>
>> In the future, stack unwinding with symbol support could be extended
>> to for example only emit log messages when a function is called in a
>> specific code path. This would allow for less noise when debugging hot
>> paths under certain conditions, but this feature is left as future work.
>>
>> === Handling relocation ===
>>
>> Since U-Boot relocates itself at runtime, and can be built to be
>> position independent in the first place (effectively "relocating" itself
>> when it first starts too), we can't really rely on gd->reloc_off.
>>
>> The approach taken here is to subtract CONFIG_TEXT_BASE from the address
>> of each symbol in the lookup table (while it's being generated), then
>> when decoding we just subtract the address of the _start label since it
>> is always correct pre and post relocation. This allows us to avoid all
>> the awkward maths since the symbols are always relative to 0x0 in the
>> lookup table.
>>
>> Example output:
>>
>> "Synchronous Abort" handler, esr 0x0000000096000004, far 0xfffffffffffff
>> elr: 000000009fc6f768 lr : 000000009fc6f710 (reloc)
>> elr: 00000001ffe49768 lr : 00000001ffe49710
>> x0 : 000fffffffffffff x1 : 0000000000000009
>> x2 : 00000001efdbadd0 x3 : 0000000000000000
>> x4 : 00000001efd99a28 x5 : 00000001ffec7676
>> x6 : 00000001efdbac70 x7 : 0000000000000000
>> x8 : 0000000000000002 x9 : 0000000000000034
>> x10: 000000000000000d x11: 0000000000000006
>> x12: 0000000000001000 x13: 00000000c3400000
>> x14: 00000000fffffff8 x15: 00000001efd99837
>> x16: 00000001ffe146b8 x17: 0000000000000000
>> x18: 00000001efdbac70 x19: 000000000a6f8800
>> x20: 00000001efe06fd0 x21: 0000000000000001
>> x22: 00000001ffef5b80 x23: 00000001ffee08c0
>> x24: 0000000000000002 x25: 0000000000000000
>> x26: 0000000000000000 x27: 0000000000000000
>> x28: 00000001efe512c0 x29: 00000001efd99c80
>>
>> Code: 2a010000 b9001260 d5033fbf 92fffe00 (39400001)
>>
>> CONFIG_TEXT_BASE   : 0x9fc00000
>> Relocated base addr: 0x1ffdda000
>> Backtrace:
>>     <0x000001ffe496d0> dwc3_qcom_glue_configure+0x40
>>     <0x000001ffe494a4> dwc3_glue_probe+0x10c
>>     <0x000001ffe1757c> device_probe+0x254
>>     <0x000001ffe1757c> device_probe+0x114
>>     <0x000001ffe4ff70> usb_init+0x158
>>     <0x000001ffe00fe4> do_usb_start+0xc
>>     <0x000001ffe01018> do_usb+0x78
>>     <0x000001ffe0dec4> cmd_process+0x140
>>     <0x000001ffe04e24> run_list_real+0x6f0
>>     <0x000001ffe05554> parse_stream_outer+0x148
>>     <0x000001ffe04d30> parse_string_outer+0x90
>>     <0x000001ffe0d148> run_command_list+0x50
>>     <0x000001ffe02be0> main_loop+0x28
>>     <0x000001ffe065c0> board_init_r+0x3c8
>>     <0x000001ffddd00c> relocation_return+0x4
>>
>> Resetting CPU ...
> 
> Thank you Casey for picking this up.
> 
> It working fine but there are some points to think about:

Thanks for taking a look!

> 
> It would make sense to show both the relocated and the non-relocated 
> addresses like we do for elr and lr. The value after subtracting the 
> relocation address is what I need when looking at the objdump.

OK, I can see why that would be useful, I'll give it a whirl.

> 
> The format of addresses with and without symbols does not match:
> 
> Backtrace:
>          <0x0000007f6ba868> cmd_process+0x130
>          <0x0000007f6af900> run_list_real+0x718
>          <0x0000007f6b0058> parse_stream_outer+0x14c
>          <0x0000007f6b06b4> parse_file_outer+0x34
>          <0x0000007f6b9dfc> cli_loop+0x18
>          <0x0000007f6ad6b0> main_loop+0x50
>          <0x0000007f6b0f94> board_init_r+0x378
>          <0x0000007f68400c> relocation_return+0x4
> 
> Backtrace:
>          (0x0000007f6d394c)
>          (0x0000007f6c8fcc)
>          (0x0000007f6c9158)
>          (0x0000007f6c969c)
>          (0x0000007f6d2dc8)
>          (0x0000007f6c66b4)
>          (0x0000007f6ca2c0)
>          (0x0000007f69d010)
> 
> I don't think parentheses are needed. But if we use them they should 
> look the same for the same content.

I was trying to differentiate between the addresses being symbol 
addresses vs lr addresses, but yeah maybe the different parentheses 
doesn't add much here. I used the <> to follow the kernel style.

> 
> I guess symbol lookup could be used in other places in future. I don't 
> see a necessity to let it depend on frame-pointers. Instead frame- 
> pointers should imply symbol-lookup. 'imply' should also work on RISC-V 
> where symbol-lookup is still missing.

Makes sense to me.

> 
> There should be at least one defconfig for 32-bit and one defconfig for 
> 64-bit that builds with both.

Hmm, well I'll enable it in qcom_defconfig for sure, I don't have any 
32-bit boards to test though, can you help with this?

> 
> I am missing the function name and offset for the function where the 
> exception actually occurred. Could we treat the value of elr like a 
> framepointer and add it as first line?

Oh yeah, I messed this up actually we should pass elr in except_msg(), 
I'll fix that.

Kind regards,
// Casey (she/her)

> 
> Best regards
> 
> Heinrich
> 
>>
>> ---
>> Changes in v2:
>> - Include calling function in backtrace (current LR)
>> - Implement support for runtime symbol lookup
>> - Use symbol lookup to print more informative backtraces when available
>> - Adjust backtrace format and print U-Boot base address
>> - Link to v1: https://lore.kernel.org/u-boot/20250703051951.43372-1- 
>> heinrich.schuchardt at canonical.com
>>
>> To: Tom Rini <trini at konsulko.com>
>> To: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
>> Cc: Simon Glass <sjg at chromium.org>
>> Cc: Marek Vasut <marek.vasut+renesas at mailbox.org>
>> Cc: Ben Dooks <ben.dooks at codethink.co.uk>
>> Cc: Rick Chen <rick at andestech.com>
>> Cc: Leo <ycliang at andestech.com>
>> Cc: Ilias Apalodimas <ilias.apalodimas at linaro.org>
>> Cc: u-boot at lists.denx.de
>>
>> ---
>> Casey Connolly (3):
>>        drop unused kallsyms support
>>        add support for symbol lookups
>>        arm64: implement printing backtraces with symbols
>>
>> Heinrich Schuchardt (4):
>>        cmd/exception: missing include string.h
>>        Kconfig: make CONFIG_FRAMEPOINTER available on arm64
>>        arm64: initialize the frame pointer register
>>        arm64: simplify interrupt code
>>
>>   Kconfig                      |  25 ++
>>   Makefile                     |  24 +-
>>   arch/arm/Makefile            |   9 +-
>>   arch/arm/lib/crt0_64.S       |   1 +
>>   arch/arm/lib/interrupts_64.c | 133 ++++++---
>>   arch/riscv/Kconfig           |  21 --
>>   common/Makefile              |   1 -
>>   common/kallsyms.c            |  42 ---
>>   common/system_map.c          |   8 -
>>   include/exception.h          |   1 +
>>   include/symbols.h            |  22 ++
>>   lib/Kconfig                  |   9 +
>>   lib/symbols.c                | 141 ++++++++++
>>   tools/Makefile               |   3 +
>>   tools/symbols.c              | 646 +++++++++++++++++++++++++++++++++ 
>> ++++++++++
>>   15 files changed, 965 insertions(+), 121 deletions(-)
>> ---
>> base-commit: f2f451d9d6d3756458d5037e3e5135e5211582d7
>>
>> // Caleb (they/them)
>>
>>
> 
.