Re: [PATCH -tip v6 00/13] kprobes: Fix stacktrace with kretprobes on x86

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, May 26, 2021 at 1:02 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Hello,
>
> Here is the 6th version of the series to fix the stacktrace with kretprobe
> on x86.
>
> The previous version is;
>
>  https://lore.kernel.org/bpf/161676170650.330141.6214727134265514123.stgit@devnote2/
>
> This version is rebased on the latest tip tree and add some patches for
> improving stacktrace[13/13].
>
> Changes from v5:
> [02/13]:
>   - Use dereference_symbol_descriptor() instead of dereference_function_descriptor()
> [04/13]:
>   - Replace BUG_ON() with WARN_ON_ONCE() in __kretprobe_trampoline_handler().
> [13/13]:
>   - Add a new patch to fix return address in earlier stage.
>
>
> With this series, unwinder can unwind stack correctly from ftrace as below;
>
>   # cd /sys/kernel/debug/tracing
>   # echo > trace
>   # echo 1 > options/sym-offset
>   # echo r vfs_read >> kprobe_events
>   # echo r full_proxy_read >> kprobe_events
>   # echo traceoff:1 > events/kprobes/r_vfs_read_0/trigger
>   # echo stacktrace:1 > events/kprobes/r_full_proxy_read_0/trigger
>   # echo 1 > events/kprobes/enable
>   # cat /sys/kernel/debug/kprobes/list
> ffffffff8133b740  r  full_proxy_read+0x0    [FTRACE]
> ffffffff812560b0  r  vfs_read+0x0    [FTRACE]
>   # echo 0 > events/kprobes/enable
>   # cat trace
> # tracer: nop
> #
> # entries-in-buffer/entries-written: 3/3   #P:8
> #
> #                                _-----=> irqs-off
> #                               / _----=> need-resched
> #                              | / _---=> hardirq/softirq
> #                              || / _--=> preempt-depth
> #                              ||| /     delay
> #           TASK-PID     CPU#  ||||   TIMESTAMP  FUNCTION
> #              | |         |   ||||      |         |
>            <...>-134     [007] ...1    16.185877: r_full_proxy_read_0: (vfs_read+0x98/0x180 <- full_proxy_read)
>            <...>-134     [007] ...1    16.185901: <stack trace>
>  => kretprobe_trace_func+0x209/0x300
>  => kretprobe_dispatcher+0x4a/0x70
>  => __kretprobe_trampoline_handler+0xd4/0x170
>  => trampoline_handler+0x43/0x60
>  => kretprobe_trampoline+0x2a/0x50
>  => vfs_read+0x98/0x180
>  => ksys_read+0x5f/0xe0
>  => do_syscall_64+0x37/0x90
>  => entry_SYSCALL_64_after_hwframe+0x44/0xae
>            <...>-134     [007] ...1    16.185902: r_vfs_read_0: (ksys_read+0x5f/0xe0 <- vfs_read)
>
> This shows the double return probes (vfs_read and full_proxy_read) on the stack
> correctly unwinded. (vfs_read will return to ksys_read+0x5f and full_proxy_read
> will return to vfs_read+0x98)
>
> This actually changes the kretprobe behavisor a bit, now the instraction pointer in
> the pt_regs passed to kretprobe user handler is correctly set the real return
> address. So user handlers can get it via instruction_pointer() API.
>
> You can also get this series from
>  git://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git kprobes/kretprobe-stackfix-v6
>
>
> Thank you,
>
> ---
>

Thanks for following up on this! I've applied this patch set on top of
bpf-next and tested with my local BPF-based tool that uses stack
traces in kretprobes heavily. It all works now and I'm getting
meaningful and correctly looking stacktraces. Thanks a lot!

Tested-by: Andrii Nakryik <andrii@kernel.org>


> Josh Poimboeuf (1):
>       x86/kprobes: Add UNWIND_HINT_FUNC on kretprobe_trampoline code
>
> Masami Hiramatsu (12):
>       ia64: kprobes: Fix to pass correct trampoline address to the handler
>       kprobes: treewide: Replace arch_deref_entry_point() with dereference_symbol_descriptor()
>       kprobes: treewide: Remove trampoline_address from kretprobe_trampoline_handler()
>       kprobes: Add kretprobe_find_ret_addr() for searching return address
>       ARC: Add instruction_pointer_set() API
>       ia64: Add instruction_pointer_set() API
>       arm: kprobes: Make a space for regs->ARM_pc at kretprobe_trampoline
>       kprobes: Setup instruction pointer in __kretprobe_trampoline_handler
>       x86/kprobes: Push a fake return address at kretprobe_trampoline
>       x86/unwind: Recover kretprobe trampoline entry
>       tracing: Show kretprobe unknown indicator only for kretprobe_trampoline
>       x86/kprobes: Fixup return address in generic trampoline handler
>
>
>  arch/arc/include/asm/ptrace.h       |    5 ++
>  arch/arc/kernel/kprobes.c           |    2 -
>  arch/arm/probes/kprobes/core.c      |    5 +-
>  arch/arm64/kernel/probes/kprobes.c  |    3 -
>  arch/csky/kernel/probes/kprobes.c   |    2 -
>  arch/ia64/include/asm/ptrace.h      |    5 ++
>  arch/ia64/kernel/kprobes.c          |   15 ++---
>  arch/mips/kernel/kprobes.c          |    3 -
>  arch/parisc/kernel/kprobes.c        |    4 +
>  arch/powerpc/kernel/kprobes.c       |   13 ----
>  arch/riscv/kernel/probes/kprobes.c  |    2 -
>  arch/s390/kernel/kprobes.c          |    2 -
>  arch/sh/kernel/kprobes.c            |    2 -
>  arch/sparc/kernel/kprobes.c         |    2 -
>  arch/x86/include/asm/kprobes.h      |    1
>  arch/x86/include/asm/unwind.h       |   23 +++++++
>  arch/x86/include/asm/unwind_hints.h |    5 ++
>  arch/x86/kernel/kprobes/core.c      |   53 +++++++++++++++--
>  arch/x86/kernel/unwind_frame.c      |    4 +
>  arch/x86/kernel/unwind_guess.c      |    3 -
>  arch/x86/kernel/unwind_orc.c        |   19 +++++-
>  include/linux/kprobes.h             |   41 +++++++++++--
>  kernel/kprobes.c                    |  108 +++++++++++++++++++++++++----------
>  kernel/trace/trace_output.c         |   17 +-----
>  lib/error-inject.c                  |    3 +
>  25 files changed, 237 insertions(+), 105 deletions(-)
>
> --
> Masami Hiramatsu (Linaro) <mhiramat@kernel.org>