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From: Beau Belgrave <beaub@linux.microsoft.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-trace-kernel@vger.kernel.org,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	David Vernet <void@manifault.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	dthaler@microsoft.com, brauner@kernel.org, hch@infradead.org
Subject: Re: [PATCH] tracing/user_events: Run BPF program if attached
Date: Mon, 15 May 2023 12:35:32 -0700	[thread overview]
Message-ID: <20230515193532.GA153@W11-BEAU-MD.localdomain> (raw)
In-Reply-To: <20230515143305.4f731fa9@gandalf.local.home>

On Mon, May 15, 2023 at 02:33:05PM -0400, Steven Rostedt wrote:
> On Mon, 15 May 2023 09:57:07 -0700
> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> 
> > Thank you for these details. Answer below...
> 
> Thanks for this well thought out reply!
> 
> 

[...]

> > 
> > > 	if (unlikely(ret <= 0)) {
> > > 		if (!fixup_fault)
> > > 			return -EFAULT;
> > > 
> > > 		if (!user_event_enabler_queue_fault(mm, enabler, *attempt))
> > > 			pr_warn("user_events: Unable to queue fault handler\n");  
> > 
> > This part looks questionable.
> > 
> > The only users of fixup_user_fault() were futex and KVM.
> > Now user_events are calling it too from user_event_mm_fault_in() where
> > "bool unlocked;" is uninitialized and state of this flag is not checked
> > after fixup_user_fault() call.
> > Not an MM expert, but this is suspicious.
> 
> Hmm, yeah, this should be:
> 
> static int user_event_mm_fault_in()
> {
> 	bool unlocked = false;
> 
> 	[..]
> 
> out:
> 	if (!unlocked)
> 		mmap_read_unlock(mm->mm);
> }
> 
> Good catch!
> 

I don't believe that's correct. fixup_user_fault() re-acquires the
mmap lock, and when it does, it lets you know via unlocked getting set
to true. IE: Something COULD have changed in the mmap during this call,
but the lock is still held.

See comments here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/gup.c#n1287

Thanks,
-Beau

> 
> Thank you Alexei for asking these. The above are all valid concerns.
> 
> -- Steve
> 

  reply	other threads:[~2023-05-15 19:35 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-05-08 16:37 [PATCH] tracing/user_events: Run BPF program if attached Beau Belgrave
2023-05-09 15:24 ` Alexei Starovoitov
2023-05-09 17:01   ` Steven Rostedt
2023-05-09 20:30     ` Steven Rostedt
2023-05-09 20:42       ` Steven Rostedt
2023-05-15 16:57       ` Alexei Starovoitov
2023-05-15 18:33         ` Steven Rostedt
2023-05-15 19:35           ` Beau Belgrave [this message]
2023-05-15 21:38             ` Steven Rostedt
2023-05-15 19:24         ` Beau Belgrave
2023-05-15 21:57           ` Steven Rostedt
2023-05-17  0:36           ` Alexei Starovoitov
2023-05-17  0:56             ` Linus Torvalds
2023-05-17  1:46               ` Linus Torvalds
2023-05-17  2:29                 ` Steven Rostedt
2023-05-17  3:03                   ` Linus Torvalds
2023-05-17 17:22                     ` Beau Belgrave
2023-05-17 18:15                       ` Linus Torvalds
2023-05-17 19:07                         ` Beau Belgrave
2023-05-17 19:26                           ` Linus Torvalds
2023-05-17 19:36                             ` Beau Belgrave
2023-05-17 19:36                             ` Linus Torvalds
2023-05-17 19:37                               ` Linus Torvalds
2023-05-17 23:00                                 ` Beau Belgrave
2023-05-17 23:14                                   ` Linus Torvalds
2023-05-17 23:25                                     ` Steven Rostedt
2023-05-18  0:14                                       ` Beau Belgrave
2023-05-18  0:23                                         ` Linus Torvalds
2023-05-17 20:08                               ` Linus Torvalds
2023-05-17  1:26             ` Steven Rostedt
2023-05-17 16:50               ` Beau Belgrave
2023-05-18  0:10                 ` Alexei Starovoitov
2023-05-18  0:19                   ` Beau Belgrave
2023-05-18  0:56                     ` Alexei Starovoitov
2023-05-18  1:18                       ` Beau Belgrave
2023-05-18  2:08                         ` Steven Rostedt
2023-05-18  3:14                           ` Alexei Starovoitov
2023-05-18 13:36                             ` Steven Rostedt
2023-05-18 17:28                               ` Beau Belgrave
2023-06-01  9:46                   ` Christian Brauner
2023-06-01 15:24                     ` Beau Belgrave
2023-06-01 15:57                       ` Christian Brauner
2023-06-01 16:29                         ` Beau Belgrave
2023-06-06 13:37                           ` Masami Hiramatsu
2023-06-06 17:05                             ` Beau Belgrave
2023-06-07 14:07                               ` Masami Hiramatsu
2023-06-07 19:26                                 ` Beau Belgrave
2023-06-08  0:25                                   ` Masami Hiramatsu
2023-05-17 17:51             ` Beau Belgrave
2023-06-06 13:57             ` Masami Hiramatsu
2023-06-06 16:57               ` Andrii Nakryiko
2023-06-06 20:57                 ` Beau Belgrave

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