Date:   Thu, 20 Jan 2022 13:05:16 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Miaoqian Lin <linmq006@...il.com>, linux-kernel@...r.kernel.org,
        linux-remoteproc@...r.kernel.org, ohad@...ery.com
Subject: Re: [PATCH v3] remoteproc: Fix NULL vs IS_ERR() checking in
 rproc_create_trace_file

On Tue, Jan 18, 2022 at 01:17:50PM -0600, Bjorn Andersson wrote:
> On Tue 18 Jan 10:56 CST 2022, Mathieu Poirier wrote:
> 
> > On Mon, Jan 17, 2022 at 04:31:23PM -0600, Bjorn Andersson wrote:
> > > On Mon 17 Jan 11:06 CST 2022, Mathieu Poirier wrote:
> > > 
> > > > On Wed, Jan 05, 2022 at 01:10:22PM +0000, Miaoqian Lin wrote:
> > > > > The debugfs_create_file() function doesn't return NULL.
> > > > > It returns error pointers. Fix check in rproc_create_trace_file
> > > > > and make it returns return error pointers.
> > > > 
> > > > s/"returns return"/return
> > > > 
> > > > > Fix check in rproc_handle_trace to propagate the error code.
> > > > > 
> > > > > Signed-off-by: Miaoqian Lin <linmq006@...il.com>
> > > > > ---
> > > > > Changes in v2:
> > > > > - return PTR_ERR(tfile) in rproc_create_trace_file
> > > > > - fix check in rproc_handle_trace()
> > > > > Changes in v3:
> > > > > - return tfile to fix incorrect return type in v2
> > > > > ---
> > > > >  drivers/remoteproc/remoteproc_core.c    | 6 ++++--
> > > > >  drivers/remoteproc/remoteproc_debugfs.c | 4 +---
> > > > >  2 files changed, 5 insertions(+), 5 deletions(-)
> > > > >
> > > > 
> > > > I will fix the above, add a proper "Fixes" tag and apply this patch to
> > > > rproc-next when v5.17-rc1 comes out next week.
> > > > 
> > > 
> > > We're actually not supposed to check debugfs_create_*() for errors.
> > 
> > I'm interested in knowing more about this - can you expand on the specifics or
> > perharps provide a link?
> > 
> 
> I'm not able to find anything going into the reasoning behind it, but
> you can find lots of examples where Greg says that we shouldn't do this:
> 
> $ git log --grep "no need to check return value of debugfs_create functions"
> 
> E.g.:
> https://lore.kernel.org/r/20200818133701.462958-1-gregkh@linuxfoundation.org

Greg's changelog leaves little doubt, thanks for pointing that out.

> 
> > > 
> > > > Thanks,
> > > > Mathieu
> > > > 
> > > > > diff --git a/drivers/remoteproc/remoteproc_core.c b/drivers/remoteproc/remoteproc_core.c
> > > > > index 775df165eb45..5608408f8eac 100644
> > > > > --- a/drivers/remoteproc/remoteproc_core.c
> > > > > +++ b/drivers/remoteproc/remoteproc_core.c
> > > > > @@ -656,6 +656,7 @@ static int rproc_handle_trace(struct rproc *rproc, void *ptr,
> > > > >  	struct rproc_debug_trace *trace;
> > > > >  	struct device *dev = &rproc->dev;
> > > > >  	char name[15];
> > > > > +	int ret;
> > > > >  
> > > > >  	if (sizeof(*rsc) > avail) {
> > > > >  		dev_err(dev, "trace rsc is truncated\n");
> > > > > @@ -684,9 +685,10 @@ static int rproc_handle_trace(struct rproc *rproc, void *ptr,
> > > > >  
> > > > >  	/* create the debugfs entry */
> > > > >  	trace->tfile = rproc_create_trace_file(name, rproc, trace);
> > > > > -	if (!trace->tfile) {
> > > > > +	if (IS_ERR(trace->tfile)) {
> > > > > +		ret = PTR_ERR(trace->tfile);
> > > > >  		kfree(trace);
> > > > > -		return -EINVAL;
> > > > > +		return ret;
> > > 
> > > 
> > > And actually catching and propagating the error here means that we will
> > > start failing rproc_boot() for firmware including a RSC_TRACE when
> > > debugfs is disabled...
> > > 
> > > So if we really want to save the heap space we should at least cleanly
> > > ignore the error, by cleaning up and returning 0 here.
> > 
> > Humm... To me the _intent_ of the upstream code has always been to propagate
> > errors reported by rproc_create_trace_file().  The fact that is hasn't happen
> > because of inappropriate error handling is something that should be corrected.  
> > 
> 
> I share that view, in general. I suspect that the idea with debugfs is
> that it's for debugging purposes and you don't want your remoteproc to
> stop working just because there might be an issue debugging it.
> 
> > That being said disabling debugfs is a common practice for production systems
> > and I agree that handling such a condition by returning 0 when
> > rproc_create_trace_file() returns -ENODEV is the right thing to do.   
> > 
> 
> Right, but even with debugfs enabled, do you want to prevent your
> remoteproc from booting just because the debugfs, for some reason,
> wasn't able to add the trace file?

Greg's comments brings a different angle to the conversation - I agree there is
no need to dwell on debugfs error conditions.

> 
> For me the question is if we should clean up the "trace" object or not,
> as this only relates to the debugfs file. Ignoring the error would imply
> that we just keep this memory allocated - which I'm fine with for the
> sake of avoiding the error handling.

I would also be fine with not cleaning up the trace object, it will be free'd
as part of the resource cleanup process anyway.

> 
> > Thanks,
> > Mathieu
> > 
> > > 
> > > > >  	}
> > > > >  
> > > > >  	list_add_tail(&trace->node, &rproc->traces);
> > > > > diff --git a/drivers/remoteproc/remoteproc_debugfs.c b/drivers/remoteproc/remoteproc_debugfs.c
> > > > > index b5a1e3b697d9..2ae59a365b7e 100644
> > > > > --- a/drivers/remoteproc/remoteproc_debugfs.c
> > > > > +++ b/drivers/remoteproc/remoteproc_debugfs.c
> > > > > @@ -390,10 +390,8 @@ struct dentry *rproc_create_trace_file(const char *name, struct rproc *rproc,
> > > > >  
> > > > >  	tfile = debugfs_create_file(name, 0400, rproc->dbg_dir, trace,
> > > > >  				    &trace_rproc_ops);
> > > > > -	if (!tfile) {
> > > > > +	if (IS_ERR(tfile))
> > > > >  		dev_err(&rproc->dev, "failed to create debugfs trace entry\n");
> > > 
> > > And I therefor think this function would be better reduced to:
> > > 
> > > 	return debugfs_create_file(...);
> > > 
> 
> Taking another look at the implementation of debugfs_create_file() this
> dev_err() should be removed, because there will already be a more useful
> error printed by debugfs_create_file().
> 
> Regards,
> Bjorn
> 
> > > Regards,
> > > Bjorn
> > > 
> > > > > -		return NULL;
> > > > > -	}
> > > > >  
> > > > >  	return tfile;
> > > > >  }
> > > > > -- 
> > > > > 2.17.1
> > > > > 

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