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

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Date:   Sun, 5 Jul 2020 09:30:15 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.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

On Fri, Jul 03, 2020 at 09:21:03PM +0200, Pavel Machek wrote:
>
>> > > 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.

Moving to an EOL kernel is very much unsupported.

-- 
Thanks,
Sasha