linux-kernel - Re: [PATCH] Silence an uninitialized variable warning

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Message-ID: <20191204184247.GG1765@kadam>
Date:   Wed, 4 Dec 2019 21:42:47 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] Silence an uninitialized variable warning

On Wed, Dec 04, 2019 at 09:26:40AM -0500, Steven Rostedt wrote:
> On Tue, 26 Nov 2019 15:19:34 +0300
> Dan Carpenter <dan.carpenter@...cle.com> wrote:
> 
> > Smatch complains that "ret" could be uninitialized if we don't enter the
> > loop.  I don't know if that's possible, but it's nicer to return a
> > literal zero instead.
> > 
> > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> > ---
> >  kernel/trace/trace_syscalls.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
> > index 73140d80dd46..63528f458826 100644
> > --- a/kernel/trace/trace_syscalls.c
> > +++ b/kernel/trace/trace_syscalls.c
> > @@ -286,7 +286,7 @@ static int __init syscall_enter_define_fields(struct trace_event_call *call)
> >  		offset += sizeof(unsigned long);
> >  	}
> >  
> > -	return ret;
> > +	return 0;
> >  }
> >  
> >  static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
> 
> The current code has this:
> 
> static int __init syscall_enter_define_fields(struct trace_event_call *call)
> {
> 	struct syscall_trace_enter trace;
> 	struct syscall_metadata *meta = call->data;
> 	int ret;
> 	int i;
> 	int offset = offsetof(typeof(trace), args);
> 
> 	ret = trace_define_field(call, SYSCALL_FIELD(int, nr, __syscall_nr),
> 				 FILTER_OTHER);

In linux-next this ret = trace_define_field() assignment is removed.
That was commit 60fdad00827c ("ftrace: Rework event_create_dir()").

> 	if (ret)
> 		return ret;
> 
> 	for (i = 0; i < meta->nb_args; i++) {
> 		ret = trace_define_field(call, meta->types[i],
> 					 meta->args[i], offset,
> 					 sizeof(unsigned long), 0,
> 					 FILTER_OTHER);
> 		offset += sizeof(unsigned long);
> 	}
> 
> 	return ret;
> }
> 
> 
> How can ret possibly be uninitialized?

I should have written this commit more carefully and verified whether
meta->nb_args can actually be zero instead of just assuming it was a
false positive...

regards,
dan carpenter