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

Masami Hiramatsu
2021-08-24T05:32:49Z

On Mon, 23 Aug 2021 22:12:06 -0700
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> On Thu, Jul 29, 2021 at 4:35 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > On Thu, 29 Jul 2021 23:05:56 +0900
> > Masami Hiramatsu <mhiramat@kernel.org> 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.

No, not yet as far as I know.
I'm waiting for any comment on this series. Since this is basically
x86 ORC unwinder improvement, this series should be merged to -tip tree.

Ingo and Josh,

Could you give me any comment, please?

Thank you,

> Thanks!
>
> >
> > Thank you,
> >
> > --
> > Masami Hiramatsu <mhiramat@kernel.org>

--
Masami Hiramatsu <mhiramat@kernel.org>