linux-kernel - Re: [PATCH v6 2/4] tracing: Make the backup instance non-reusable

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Message-Id: <20260210141415.01a6907dcb558866e1abb994@kernel.org>
Date: Tue, 10 Feb 2026 14:14:15 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/4] tracing: Make the backup instance non-reusable

On Mon, 9 Feb 2026 18:42:47 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 9 Feb 2026 18:08:44 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> > > > @@ -5152,6 +5157,9 @@ tracing_cpumask_write(struct file *filp, const char __user *ubuf,
> > > >  	cpumask_var_t tracing_cpumask_new;
> > > >  	int err;
> > > >  
> > > > +	if (trace_array_is_readonly(tr))
> > > > +		return -EPERM;
> > > > +  
> > > 
> > > Shouldn't these checks be done in the open function? Doing it now is
> > > too late, as -EPERM on a write is confusing when the open for write
> > > succeeds.  
> > 
> > I've made a small program and straced. Surprisingly, for the super user,
> > open(2) does not return error on opening a readonly file with O_RDWR.
> 
> *blink*
> 
> So if on open, the trace_array_is_read_only(tr) returns true and you
> return -EPREM, it still succeeds? That sounds like a bug!

Hmm, OK. Now I found how sysfs handles it.

	/*
	 * For regular files, if the opener has CAP_DAC_OVERRIDE, open(2)
	 * succeeds regardless of the RW permissions.  sysfs had an extra
	 * layer of enforcement where open(2) fails with -EACCES regardless
	 * of CAP_DAC_OVERRIDE if the permission doesn't have the
	 * respective read or write access at all (none of S_IRUGO or
	 * S_IWUGO) or the respective operation isn't implemented.  The
	 * following flag enables that behavior.
	 */
	KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK	= 0x0002,

So for the similar reason, I will make tracefs to check the permission
even if CAP_DAC_OVERRIDE is set. (But this check should be done in general,
instead of each open() operation)

Thank you,


> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>