[PATCH v2 0/7] arm64: stack backtraces

Fri Jul 25 17:23:38 CEST 2025

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:

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.

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 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.

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

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?

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@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)
> 
> 