linux-kernel - Re: [PATCH 4.19 119/131] tracing: Fix event trigger to accept redundant spaces

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Message-ID: <20200703192102.GA31738@amd>
Date:   Fri, 3 Jul 2020 21:21:03 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Pavel Machek <pavel@...x.de>, Sasha Levin <sashal@...nel.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <zanussi@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 4.19 119/131] tracing: Fix event trigger to accept
 redundant spaces


> > > commit 6784beada631800f2c5afd567e5628c843362cee upstream.
> > > 
> > > Fix the event trigger to accept redundant spaces in
> > > the trigger input.
> > > 
> > > For example, these return -EINVAL
> > > 
> > > echo " traceon" > events/ftrace/print/trigger
> > > echo "traceon  if common_pid == 0" > events/ftrace/print/trigger
> > > echo "disable_event:kmem:kmalloc " > events/ftrace/print/trigger
> > > 
> > > But these are hard to find what is wrong.
> > > 
> > > To fix this issue, use skip_spaces() to remove spaces
> > > in front of actual tokens, and set NULL if there is no
> > > token.
> > 
> > For the record, I'm not fan of this one. It is ABI change, not a
> > bugfix.
> > 
> > Yes, it makes kernel interface "easier to use". It also changes
> > interface in the middle of stable series, and if people start relying
> > on new interface and start putting extra spaces, they'll get nasty
> > surprise when they move code to the older kernel.
> 
> If an interface changes anywhere that breaks userspace, it needs to be
> not done, stable kernels are not an issue here or not.

I'm not saying it is a regression; I'd scream way more if that was the
case. I'm saying it is nowhere near a fix.

We really don't want userspace doing:

> > > echo " traceon" > events/ftrace/print/trigger

Because it does not work on older kernels. It will work on 4.19.131
and break on 5.6.19.

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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