Re: [PATCH for-next v2 0/4] riscv: Improvments for stacktrace

[Mark Rutland]

On Wed, Sep 21, 2022 at 08:51:23PM +0800, Chen Zhongjin wrote:
> Currently, the stacktrace with FRAME_POINTER on riscv has some problem:
> 
> 1. stacktrace will stop at irq so it can't get the stack frames before
> irq entry.
> 2. stacktrace can't unwind all the real stack frames when there is
> k{ret}probes or ftrace.
> 
> These are mainly becase when there is a pt_regs on stack, we can't unwind
> the stack frame as normal function.
> 
> Some architectures (e.g. arm64) create a extra stackframe inside pt_regs.
> However this doesn't work for riscv because the ra is not ensured to be
> pushed to stack. As explained in:
> commit f766f77a74f5("riscv/stacktrace: Fix stack output without ra on the stack top")

FWIW, this is also a latent problem on arm64, since we don't know whether the
LR is live at an exception boundary (and we currently always ignore it).

My plan to fix that on arm64 is to push an empty frame record to the stack upon
an exception, and use that to find the pt_regs, from which we can get the PC
and LR (and then we can report the later as unreliable). That should be roughly
equivalent to what you do in this series (where use use the LSB to identify
that the pointer is actually a pt_regs).

One important thing to note is that when crossing an exception boundary you
won't know whether RA is live, and so you might try to consume an address twice
(if it has also been pushed to the stack). That could be problematic for
unwinding ftrace or kretprobes. On arm64 we had to implement
HAVE_FUNCTION_GRAPH_RET_ADDR_PTR so that we could reliably unwind ftrace. See
commit:

  c6d3cd32fd0064af ("arm64: ftrace: use HAVE_FUNCTION_GRAPH_RET_ADDR_PTR")

... and we haven't yet come up with something similar for kretprobes (though I
suspect we'll need to).

Thanks,
Mark.

> So, I choosed the method of x86 that, if there is a pt_regs on stack,
> we encoded the frame pointer and save it. When unwinding stack frame,
> we can get pt_regs and registers required for unwinding stacks.
> 
> In addition, the patch set contains some refactoring of stacktrace.c to
> keep the stacktrace code on riscv consistent with other architectures.
> 
> Stacktrace before for kretprobes:
> 
>   Call Trace:
>   ...
>   [<ffffffff800d5d48>] __kretprobe_trampoline_handler+0xc2/0x13e
>   [<ffffffff808b766c>] trampoline_probe_handler+0x30/0x46
>   [<ffffffff800070de>] __kretprobe_trampoline+0x52/0x92
>   [<ffffffff0163809c>] kprobe_init+0x9c/0x1000 [kprobe_unwind]
>   [<ffffffff800027c8>] do_one_initcall+0x4c/0x1f2
>   ...
> 
> Stacktrace after:
> 
>   Call Trace:
>   ...
>   [<ffffffff800d5d48>] __kretprobe_trampoline_handler+0xc2/0x13e
>   [<ffffffff808b766c>] trampoline_probe_handler+0x30/0x46
>   [<ffffffff800070de>] __kretprobe_trampoline+0x52/0x92
> + [<ffffffff01633076>] the_caller+0x2c/0x38 [kprobe_unwind]
>   [<ffffffff0163809c>] kprobe_init+0x9c/0x1000 [kprobe_unwind]
>   [<ffffffff800027c8>] do_one_initcall+0x4c/0x1f2
>   ...
> 
> Stacktrace before for ftrace:
> 
>   Call Trace:
>   ...
>   [<ffffffff80006df0>] kprobe_ftrace_handler+0x13e/0x188
>   [<ffffffff80008e7e>] ftrace_regs_call+0x8/0x10
>   [<ffffffff80002540>] do_one_initcall+0x4c/0x1f2
>   [<ffffffff8008a4e6>] do_init_module+0x56/0x210
>   ...
> 
>   Stacktrace after:
> 
>   Call Trace:
>   ...
>   [<ffffffff016150e0>] handler_pre+0x30/0x4a [kprobe_unwind]
>   [<ffffffff800bce96>] aggr_pre_handler+0x60/0x94
>   [<ffffffff80006df0>] kprobe_ftrace_handler+0x13e/0x188
>   [<ffffffff80008e82>] ftrace_regs_call+0x8/0x10
> + [<ffffffff01615000>] traced_func+0x0/0x1e [kprobe_unwind]
>   [<ffffffff80002540>] do_one_initcall+0x4c/0x1f2
>   [<ffffffff8008a4ea>] do_init_module+0x56/0x210
>   ...
> 
> Noticed that the caller of ftrace and probed func of kretprobe
> cannot be unwind because they are inside function pro/epilogue.
> 
> ---
> v1 -> v2:
> - Merge three patches which add ENCODE_FRAME_POINTER together
> - Update commit message
> - Delete the KRETPORBES stuff added in unwind_state, we don't need them
> to recover the kretporbes ret_addr because we can get it in pt_regs
> ---
> Chen Zhongjin (4):
>   riscv: stacktrace: Replace walk_stackframe with arch_stack_walk
>   riscv: stacktrace: Introduce unwind functions
>   riscv: stacktrace: Save pt_regs in ENCODE_FRAME_POINTER
>   riscv: stacktrace: Implement stacktrace for irq
> 
>  arch/riscv/include/asm/frame.h                |  45 ++++++
>  arch/riscv/include/asm/stacktrace.h           |   9 +-
>  arch/riscv/kernel/entry.S                     |   3 +
>  arch/riscv/kernel/mcount-dyn.S                |   7 +
>  arch/riscv/kernel/perf_callchain.c            |   2 +-
>  arch/riscv/kernel/probes/kprobes_trampoline.S |   7 +
>  arch/riscv/kernel/stacktrace.c                | 150 ++++++++++++------
>  7 files changed, 174 insertions(+), 49 deletions(-)
>  create mode 100644 arch/riscv/include/asm/frame.h
> 
> -- 
> 2.17.1
>