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

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Message-ID: <20240630105322.GA17573@altlinux.org>
Date: Sun, 30 Jun 2024 13:53:23 +0300
From: "Dmitry V. Levin" <ldv@...ace.io>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: rostedt@...dmis.org, mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com, 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
Subject: Re: [PATCH v23 3/5] tracing: Allow user-space mapping of the
 ring-buffer

On Fri, May 10, 2024 at 03:04:32PM +0100, Vincent Donnefort wrote:
[...]
> diff --git a/include/uapi/linux/trace_mmap.h b/include/uapi/linux/trace_mmap.h
> index b682e9925539..bd1066754220 100644
> --- a/include/uapi/linux/trace_mmap.h
> +++ b/include/uapi/linux/trace_mmap.h
> @@ -43,4 +43,6 @@ struct trace_buffer_meta {
>  	__u64	Reserved2;
>  };
>  
> +#define TRACE_MMAP_IOCTL_GET_READER		_IO('T', 0x1)
> +

I'm sorry but among all the numbers this one was probably the least
fortunate choice because it collides with TCGETS on most of architectures.

For example, this is how strace output would look like when
TRACE_MMAP_IOCTL_GET_READER support is added:

$ strace -e ioctl stty
ioctl(0, TCGETS or TRACE_MMAP_IOCTL_GET_READER, {c_iflag=ICRNL|IXON, c_oflag=NL0|CR0|TAB0|BS0|VT0|FF0|OPOST|ONLCR, c_cflag=B38400|CS8|CREAD, c_lflag=ISIG|ICANON|ECHO|ECHOE|ECHOK|IEXTEN|ECHOCTL|ECHOKE, ...}) = 0

Even though ioctl numbers are inherently not unique, TCGETS is
a very traditional one, so it would be great if you could change
TRACE_MMAP_IOCTL_GET_READER to avoid this collision.

Given that _IO('T', 0x1) is _IOC(_IOC_NONE, 'T', 0x1, 0),
something like _IOC(_IOC_NONE, 'T', 0x1, 0x1) should be OK.


-- 
ldv