linux-kernel - Re: [PATCH] tracing: Fix tracing_marker may trigger page fault during preempt

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Message-ID: <aLVt30-Lc9lesqS6@J2N7QTR9R3>
Date: Mon, 1 Sep 2025 10:56:47 +0100
From: Mark Rutland <mark.rutland@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Luo Gengkun <luogengkun@...weicloud.com>, mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] tracing: Fix tracing_marker may trigger page fault
 during preempt_disable

On Sat, Aug 30, 2025 at 11:22:51AM +0100, Catalin Marinas wrote:
> On Fri, Aug 29, 2025 at 06:13:11PM -0400, Steven Rostedt wrote:
> > On Fri, 29 Aug 2025 20:53:40 +0100
> > Catalin Marinas <catalin.marinas@....com> wrote:
> > valid user address.
> > > BTW, arm64 also bails out early in do_page_fault() if in_atomic() but I
> > > suspect that's not the case here.
> > > 
> > > Adding Al Viro since since he wrote a large part of uaccess.h.
> > 
> > So, __copy_from_user_inatomic() is supposed to be called if
> > pagefault_disable() has already been called? If this is the case, can we
> > add more comments to this code? I've been using the inatomic() version this
> > way in preempt disabled locations since 2016.
> 
> This should work as long as in_atomic() returns true as it's checked in
> the arm64 fault code. With PREEMPT_NONE, however, I don't think this
> works.

Sorry, what exactly breaks for the PREEMPT_NONE case?

> __copy_from_user_inatomic() could be changed to call
> pagefault_disable() if !in_atomic() but you might as well call
> copy_from_user_nofault() in the trace code directly as per Luo's patch.

That makes sense to me. I'll go check the arm64 users of
__copy_from_user_inatomic(), in case they're doing something dodgy.

> > I just wanted to figure out why __copy_from_user_inatomic() wasn't atomic.
> > If anything, it needs to be better documented.
> 
> Yeah, I had no idea until I looked at the code. I guess it means it can
> be safely used if in_atomic() == true (well, making it up, not sure what
> the intention was).

I think that was the intention -- it's the caller asserting that they
know the access won't fault (and hence won't sleep), and that's why
__copy_to_user_inatomic() and __copy_to_user() only differ by the latter
calling might_sleep().

It looks like other inconsistencies have crept in by accident. AFAICT
the should_fail_usercopy() check in __copy_from_user() was accidentally
missed from __copy_from_user_inatomic() when it was introduced in
commit:

  4d0e9df5e43dba52 ("lib, uaccess: add failure injection to usercopy functions")

... and the instrumentation is ordered inconsistently w.r.t.
should_fail_usercopy() since commit:

  33b75c1d884e81ec ("instrumented.h: allow instrumenting both sides of copy_from_user()")

... so there's a bunch of scope for cleanup, and we could probably have:

	/*
	 * TODO: comment saying to only call this directly when you know
	 * that the fault handler won't sleep.
	 */
	static __always_inline __must_check unsigned long
	__copy_from_user_inatomic(void *to, const void __user *from, unsigned long n)
	{
		...
	}

	static __always_inline __must_check unsigned long
	__copy_from_user(void *to, const void __user *from, unsigned long n)
	{
		might_fault();
		return __copy_from_user_inatomic();
	}

... to avoid the inconsistency.

Mark.