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

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Date: Sun, 30 Jun 2024 08:40:53 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Dmitry V. Levin" <ldv@...ace.io>
Cc: Vincent Donnefort <vdonnefort@...gle.com>, 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, Linus Torvalds
 <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v23 3/5] tracing: Allow user-space mapping of the
 ring-buffer

On Sun, 30 Jun 2024 13:53:23 +0300
"Dmitry V. Levin" <ldv@...ace.io> wrote:

> 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.

Hmm, that is unfortunate.

> 
> 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.

Well, it may not be too late to update this as it hasn't been
officially released in 6.10 yet. It's still only in the -rc and the
library doesn't rely on this yet (I've been holding off until 6.10 was
officially released before releasing the library that uses it).

I can send a patch this week to update it. Or feel free to send a patch
yourself.

Thanks,

-- Steve