From: Beau Belgrave <beaub@linux.microsoft.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: mhiramat@kernel.org, linux-trace-devel@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 02/13] user_events: Add minimal support for trace_event into ftrace
Date: Thu, 9 Dec 2021 11:42:35 -0800 [thread overview]
Message-ID: <20211209194235.GA21676@kbox> (raw)
In-Reply-To: <20211209124735.3d1a9707@gandalf.local.home>
On Thu, Dec 09, 2021 at 12:47:35PM -0500, Steven Rostedt wrote:
> On Thu, 9 Dec 2021 09:40:50 -0800
> Beau Belgrave <beaub@linux.microsoft.com> wrote:
>
> > No, this is not a fast path, and I don't have a problem moving to a
> > mutex if you feel that is better. I've likely become too close to this
> > code to know when things are confusing for others.
>
> Yeah. I really dislike the "protection by algorithms" then protection by
> locking unless it is a fast path.
>
> If this was a fast path then I'd be more concerned. I dislike global locks
> as well, but unless contention becomes a concern, I don't think we should
> worry about it.
Sure thing.
>
> Also, for this comment:
>
> +static int user_events_release(struct inode *node, struct file *file)
> +{
> + struct user_event_refs *refs;
> + struct user_event *user;
> + int i;
> +
> + /*
> + * refs is protected by RCU and could in theory change immediately
> + * before this call on another core. To ensure we read the latest
> + * version of refs we acquire the RCU read lock again.
> + */
> + rcu_read_lock_sched();
> + refs = rcu_dereference_sched(file->private_data);
> + rcu_read_unlock_sched();
>
> How do you see refs changing on another core if this can only be called
> when nothing has a reference to it?
>
> I think this comment and grabbing the rcu locks is what is causing me
> concern.
>
> -- Steve
User program task:
CPU0: ioctl(fd, REG)
CPU1: close(fd)
IE: Some program registers and then immediately calls close on the file.
If the CPU migrates right between the 2 and the close swaps, it is
possible this could occur.
This could be attempted in tight loops maliciously as well.
I assume with a mutex there that some barrier is imposed to ensure
correct reads in this condition? (By virtue of the mutex acquire/check)
Thanks,
-Beau
next prev parent reply other threads:[~2021-12-09 19:42 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-01 18:25 [PATCH v6 00/13] user_events: Enable user processes to create and write to trace events Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 01/13] user_events: Add UABI header for user access to user_events Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 02/13] user_events: Add minimal support for trace_event into ftrace Beau Belgrave
2021-12-08 23:19 ` Steven Rostedt
2021-12-09 0:58 ` Beau Belgrave
2021-12-09 2:03 ` Steven Rostedt
2021-12-09 17:40 ` Beau Belgrave
2021-12-09 17:47 ` Steven Rostedt
2021-12-09 19:42 ` Beau Belgrave [this message]
2021-12-09 19:57 ` Steven Rostedt
2021-12-09 20:11 ` Beau Belgrave
2021-12-09 20:19 ` Steven Rostedt
2021-12-01 18:25 ` [PATCH v6 03/13] user_events: Add print_fmt generation support for basic types Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 04/13] user_events: Handle matching arguments from dyn_events Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 05/13] user_events: Add basic perf and eBPF support Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 06/13] user_events: Add self-test for ftrace integration Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 07/13] user_events: Add self-test for dynamic_events integration Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 08/13] user_events: Add self-test for perf_event integration Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 09/13] user_events: Optimize writing events by only copying data once Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 10/13] user_events: Add documentation file Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 11/13] user_events: Add sample code for typical usage Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 12/13] user_events: Validate user payloads for size and null termination Beau Belgrave
2021-12-01 18:25 ` [PATCH v6 13/13] user_events: Use __get_rel_str for relative string fields Beau Belgrave
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