Re: [PATCH 0/3] probes: Use trace_printk() in probe handlers

[Steven Rostedt]

[ Cc'ing John Ogness ]

On Mon,  2 Nov 2026 16:10:54 +0900
"Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx> wrote:

> Hi,
> 
> Here are patches to update the probe sample code to use trace_printk
> instead of normal printk. Since the fprobe and kprobe/kretprobe handlers
> can be called from somewhere where console is locked, using printk() is
> basically unsafe. Only the unoptimized kprobe on x86 is safe because it
> is ran with NMI context (in this case, the kernel uses printk_deferred()
> automatically), but optimized kprobe/kretprobe/fprobe are not.

I don't think printk() is unsafe in these situations anymore.

John?

-- Steve

> 
> Thus, this series replaces those printk() (pr_*()) with trace_printk()
> which should be safe to output the result into trace buffer, and use
> trace_dump_stack() instead of stack_trace_print().
> 
> The similar issue had been reported by Tiezhu Yang [1] in 2023, but
> it was pointed different reason, and not applicable to upstream.
> I investigated it again.
> 
> [1] https://lore.kernel.org/all/d179086d-78d8-d0e3-e113-9072cffa55f4@xxxxxxxxxxx/
> 
> Thank you,
> ---
> base-commit: bc71431221f5ba8bb33918399552636e059c2c45
> 
> Masami Hiramatsu (Google) (3):
>       samples/kprobes: Use trace_printk() in probe handlers
>       samples/fprobe: Use trace_printk() and trace_dump_stack()
>       docs: trace: Warn against using printk() in probe handlers
> 
> 
>  Documentation/trace/fprobe.rst      |    5 +++
>  Documentation/trace/kprobes.rst     |   11 +++++-
>  samples/Kconfig                     |    4 +-
>  samples/fprobe/fprobe_example.c     |   49 +++++--------------------
>  samples/kprobes/kprobe_example.c    |   69 ++++++++++++++++++-----------------
>  samples/kprobes/kretprobe_example.c |   10 +++--
>  6 files changed, 66 insertions(+), 82 deletions(-)
> 
> --
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>