Re: [PATCH -tip v10 00/16] kprobes: Fix stacktrace with kretprobes on x86

[Andrii Nakryiko]

On Thu, Jul 29, 2021 at 4:35 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Thu, 29 Jul 2021 23:05:56 +0900
> Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> > Hello,
> >
> > This is the 10th version of the series to fix the stacktrace with kretprobe on x86.
> >
> > The previous version is here;
> >
> >  https://lore.kernel.org/bpf/162601048053.1318837.1550594515476777588.stgit@devnote2/
> >
> > This version is rebased on top of new kprobes cleanup series(*1) and merging
> > Josh's objtool update series (*2)(*3) as [6/16] and [7/16].
> >
> > (*1) https://lore.kernel.org/bpf/162748615977.59465.13262421617578791515.stgit@devnote2/
> > (*2) https://lore.kernel.org/bpf/20210710192433.x5cgjsq2ksvaqnss@treble/
> > (*3) https://lore.kernel.org/bpf/20210710192514.ghvksi3ozhez4lvb@treble/
> >
> > Changes from v9:
> >  - Add Josh's objtool update patches with a build error fix as [6/16] and [7/16].
> >  - Add a API document for kretprobe_find_ret_addr() and check cur != NULL in [5/16].
> >
> > 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() returns to 'ksys_read+0x5f' and full_proxy_read()
> > returns to 'vfs_read+0x98')
> >
> > This also 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, and can use
> > stack_trace_save_regs().
> >
> > You can also get this series from
> >  git://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git kprobes/kretprobe-stackfix-v9
>
> Oops, this is of course 'kprobes/kretprobe-stackfix-v10'. And this branch includes above (*1) series.

Hi Masami,

Was this ever merged/applied? This is a very important functionality
for BPF kretprobes, so I hope this won't slip through the cracks.
Thanks!

>
> Thank you,
>
> --
> Masami Hiramatsu <mhiramat@xxxxxxxxxx>