[PATCH 00/10] arm64: stacktrace: improve unwind reporting

[Miroslav Benes]

Hi,

On Thu, 10 Oct 2024, Mark Rutland wrote:

> This series improves arm64's unwinder to explicitly identify exception
> boundaries, reporting both pt_regs::pc and pt_regs::lr and explicitly
> identifying the source of elements in the stacktrace. This is useful to
> humans when reviewing a stacktrace, and serves as infrastructure that
> can be used for RELIABLE_STACKTRACE in future.
> 
> The first 6 patches are preparatory work that are not intended to have
> any functional impact, with patches 7 to 10 making the key changes.
> Largely this involves teaching the unwinder to track metadata for each
> unwind step, and modifying the way we manage pt_regs::stackframe so that
> exception boundaries can be identifier explcitily.
> 
> With this series applied, the unwinder will report when unwind elements are not
> simply the result of a frame pointer based unwind, e.g.
> 
> | Call trace:
> |  show_stack+0x20/0x38 (CF)
> |  dump_stack_lvl+0x60/0x80 (F)
> |  dump_stack+0x18/0x28
> |  nmi_cpu_backtrace+0xfc/0x140
> |  nmi_trigger_cpumask_backtrace+0x1c8/0x200
> |  arch_trigger_cpumask_backtrace+0x20/0x40
> |  sysrq_handle_showallcpus+0x24/0x38 (F)
> |  __handle_sysrq+0xa8/0x1b0 (F)
> |  handle_sysrq+0x38/0x50 (F)
> |  pl011_int+0x420/0x570 (F)
> |  __handle_irq_event_percpu+0x60/0x220 (F)
> |  handle_irq_event+0x54/0xc0 (F)
> |  handle_fasteoi_irq+0xa8/0x1d0 (F)
> |  generic_handle_domain_irq+0x34/0x58 (F)
> |  gic_handle_irq+0x54/0x140 (FK)
> |  call_on_irq_stack+0x24/0x58 (F)
> |  do_interrupt_handler+0x88/0xa0
> |  el1_interrupt+0x34/0x68 (F)
> |  el1h_64_irq_handler+0x18/0x28
> |  el1h_64_irq+0x6c/0x70
> |  default_idle_call+0x34/0x180 (P)
> |  default_idle_call+0x28/0x180 (L)
> |  do_idle+0x204/0x268
> |  cpu_startup_entry+0x3c/0x50 (F)
> |  rest_init+0xe4/0xf0
> |  start_kernel+0x738/0x740
> |  __primary_switched+0x88/0x98
> 
> ... where:
> 
> * "C" indicates that the first element of the trace was the caller of an unwind
>   function (vs "T" for a blocked task's stave PC, or "P" for a pt_regs::pc).
> 
> * "F" indicates that the element was recovered from fgraph (and
>   would otherwise have been reported as return_to_handler).
> 
> * "K" indicates that the element was recovered from kretprobes (and
>   would otherwise have been reported as __kretprobe_trampoline).
> 
> * "P" indicates that the element was recovered from pt_regs::pc, and therefore
>   this is the first element after an exception boundary.
> 
> * "L" indidates that the element was recovered from pt_regs::lr, and therefore
>   this may or may not be reliable depending on whether the LR was live at the
>   moment the exception was taken.
> 
> Mark.
> 
> Mark Rutland (10):
>   arm64: pt_regs: assert pt_regs is a multiple of 16 bytes
>   arm64: pt_regs: remove stale big-endian layout
>   arm64: pt_regs: rename "pmr_save" -> "pmr"
>   arm64: pt_regs: swap 'unused' and 'pmr' fields
>   arm64: use a common struct frame_record
>   arm64: stacktrace: move dump_backtrace() to kunwind_stack_walk()
>   arm64: stacktrace: report source of unwind data
>   arm64: stacktrace: report recovered PCs
>   arm64: stacktrace: split unwind_consume_stack()
>   arm64: stacktrace: unwind exception boundaries
> 
>  arch/arm64/include/asm/daifflags.h         |   2 +-
>  arch/arm64/include/asm/processor.h         |   2 +-
>  arch/arm64/include/asm/ptrace.h            |  22 ++-
>  arch/arm64/include/asm/stacktrace/common.h |  74 +++++----
>  arch/arm64/include/asm/stacktrace/frame.h  |  48 ++++++
>  arch/arm64/kernel/asm-offsets.c            |   3 +-
>  arch/arm64/kernel/entry.S                  |  16 +-
>  arch/arm64/kernel/head.S                   |   3 +
>  arch/arm64/kernel/probes/stBV5U5j          |   0
>  arch/arm64/kernel/process.c                |   5 +-
>  arch/arm64/kernel/signal.c                 |   5 -
>  arch/arm64/kernel/stacktrace.c             | 176 +++++++++++++++++++--
>  12 files changed, 287 insertions(+), 69 deletions(-)
>  create mode 100644 arch/arm64/include/asm/stacktrace/frame.h

Very nice!

Reviewed-by: Miroslav Benes <mbenes at suse.cz>

>  create mode 100644 arch/arm64/kernel/probes/stBV5U5j

but this should probably disappear :)

M