linux-kernel - Re: [PATCH v23 3/5] tracing: Allow user-space mapping of the ring-buffer

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Message-ID: <cb02f5a0-d6a3-4228-9cbb-473fd392ee48@efficios.com>
Date: Tue, 2 Jul 2024 11:32:53 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>,
 Beau Belgrave <beaub@...ux.microsoft.com>
Cc: "Dmitry V. Levin" <ldv@...ace.io>,
 Vincent Donnefort <vdonnefort@...gle.com>, mhiramat@...nel.org,
 kernel-team@...roid.com, rdunlap@...radead.org, rppt@...nel.org,
 david@...hat.com, linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v23 3/5] tracing: Allow user-space mapping of the
 ring-buffer

On 2024-07-02 11:18, Steven Rostedt wrote:
> On Tue, 2 Jul 2024 10:36:03 -0400
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>>> I can send a patch this week to update it. Or feel free to send a patch
>>> yourself.
>>
>> You need to reserve an unused ioctl Code and Seq# range within:
>>
>> Documentation/userspace-api/ioctl/ioctl-number.rst
> 
> Ug, it's been so long, I completely forgot about that file.
> 
> Thanks for catching this.
> 
>>
>> Otherwise this duplicate will confuse all system call instrumentation
>> tooling.
> 
> Agreed, what if we did this then:
> 
> -- Steve
> 
> diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> index a141e8e65c5d..9a97030c6c8d 100644
> --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> @@ -186,6 +186,7 @@ Code  Seq#    Include File                                           Comments
>   'Q'   all    linux/soundcard.h
>   'R'   00-1F  linux/random.h                                          conflict!
>   'R'   01     linux/rfkill.h                                          conflict!
> +'R'   20-2F  linux/trace_mmap.h
>   'R'   C0-DF  net/bluetooth/rfcomm.h
>   'R'   E0     uapi/linux/fsl_mc.h
>   'S'   all    linux/cdrom.h                                           conflict!
> diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> index bd1066754220..c102ef35d11e 100644
> --- a/include/uapi/linux/trace_mmap.h
> +++ b/include/uapi/linux/trace_mmap.h
> @@ -43,6 +43,6 @@ struct trace_buffer_meta {
>   	__u64	Reserved2;
>   };
>   
> -#define TRACE_MMAP_IOCTL_GET_READER		_IO('T', 0x1)
> +#define TRACE_MMAP_IOCTL_GET_READER		_IO('R', 0x20)

Note that user events also has this issue: the ioctl is not reserved in
the ioctl-number.rst list. See include/uapi/linux/user_events.h:

#define DIAG_IOC_MAGIC '*'

/* Request to register a user_event */
#define DIAG_IOCSREG _IOWR(DIAG_IOC_MAGIC, 0, struct user_reg *)

/* Request to delete a user_event */
#define DIAG_IOCSDEL _IOW(DIAG_IOC_MAGIC, 1, char *)

/* Requests to unregister a user_event */
#define DIAG_IOCSUNREG _IOW(DIAG_IOC_MAGIC, 2, struct user_unreg*)

Where '*' maps to Code 0x2A. Looking at the list I don't see any
conflicts there, but we should definitely add it.

If we use '*' for user events already, perhaps we'd want to consider
using the same range for the ring buffer ioctls ? Arguably one is
about instrumentation and the other is about ring buffer interaction
(data transport), but those are both related to tracing.

Thanks,

Mathieu

>   
>   #endif /* _TRACE_MMAP_H_ */

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com