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

Mathieu Desnoyers
2024-07-02T15:32:36Z

On 2024-07-02 11:18, Steven Rostedt wrote:
> On Tue, 2 Jul 2024 10:36:03 -0400
> Mathieu Desnoyers <mathieu.desnoyers@efficios.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.