Re: [RFC PATCH v2 0/4] Mark Kprobe debug exception paths noinstr

[Will Deacon]

[+Ada]

On Thu, Aug 13, 2026 at 06:47:31AM +0000, Hongyan Xia wrote:
> From: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
> 
> Commit 879a6754d3d1 ("arm64: kprobes: Only handle faults originating
> from XOL slot") and 23f851ac0078 ("arm64: kprobes: Allow reentering
> kprobes while single-stepping") band-aided two corner cases in the arm64
> kprobe path: Nested page faults inside Kprobe and nested Kprobe SS
> handling. Although the two patches were taken, Will asked whether we
> should keep patching things like this each time we find a corner case,
> and whether making Kprobe noinstr is the correct answer [1].
> 
> This series attempts to do exactly that: Making the Kprobe path noinstr.
> The kprobe handlers become noinstr end to end. Apart from marking
> functions noinstr, we also force-inline some static inline helper
> functions because certain compilers still outline them, breaking
> noinstr.
> 
> Note that arm64 has no objtool so at the moment noinstr is more of a
> contract that must be followed, instead of actually enforcing control
> flow. Instrumentation can still happen if someone with root privileges
> does something stupid, but now it is regarded as a bug rather than a
> legitimate use case that the kernel needs to cover.
> 
> [1] https://lore.kernel.org/all/alpuL10h7-OK2hFb@willie-the-truck/
> 
> Changed in v2:
> - Remove instrumentation_{begin/end}() markers as they do nothing
> - Fold __always_inline into the patches that actually need them
> - Remove the debug exception path changes unrelated to Kprobe. This
>   series is only about Kprobe for now 
> - Drop the kprobe_page_fault() patch for now. It's a question whether we
>   even need it at all. Will send a patch for it separately.
> 
> Hongyan Xia (4):
>   arm64/entry: Make debug_exception_enter/exit() noinstr
>   arm64/debug: Make the Kprobe functions noinstr
>   arm64/kprobes: Make the entire Kprobe noinstr
>   Revert "arm64: kprobes: Allow reentering kprobes while
>     single-stepping"
> 
>  arch/arm64/include/asm/esr.h       |  2 +-
>  arch/arm64/include/asm/kprobes.h   | 15 ++------
>  arch/arm64/include/asm/percpu.h    |  2 +-
>  arch/arm64/include/asm/preempt.h   |  4 +-
>  arch/arm64/include/asm/ptrace.h    |  4 +-
>  arch/arm64/kernel/debug-monitors.c |  6 +--
>  arch/arm64/kernel/entry-common.c   | 13 ++-----
>  arch/arm64/kernel/probes/kprobes.c | 59 ++++++++++--------------------
>  include/linux/kprobes.h            |  8 ++--

Given the lack of objtool enforcement (as you note above), I wonder
whether it would be more robust to do this on a per-compilation unit
basis? i.e. disable instrumentation for debug-monitors.c, entry-common.c
and kprobes.c?

Unfortunately, I couldn't find a way to do that in the Makefile :/ In
which case, perhaps this series is the best we can do, but I thought I'd
ask the question in case somebody has any bright ideas.

Will