linux-kernel - Re: [PATCH v6 1/4] cramfs: direct memory access support

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Message-ID: <nycvar.YSQ.7.76.1710131356360.1652@knanqh.ubzr>
Date:   Fri, 13 Oct 2017 14:18:55 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Al Viro <viro@...IV.linux.org.uk>
cc:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-embedded@...r.kernel.org, linux-kernel@...r.kernel.org,
        Chris Brandt <Chris.Brandt@...esas.com>
Subject: Re: [PATCH v6 1/4] cramfs: direct memory access support

On Fri, 13 Oct 2017, Al Viro wrote:

> On Fri, Oct 13, 2017 at 01:39:13PM -0400, Nicolas Pitre wrote:
> > On Fri, 13 Oct 2017, Al Viro wrote:
> > 
> > > On Thu, Oct 12, 2017 at 02:16:10AM -0400, Nicolas Pitre wrote:
> > > 
> > > >  static void cramfs_kill_sb(struct super_block *sb)
> > > >  {
> > > >  	struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
> > > >  
> > > > -	kill_block_super(sb);
> > > > +	if (IS_ENABLED(CCONFIG_CRAMFS_MTD)) {
> > > > +		if (sbi->mtd_point_size)
> > > > +			mtd_unpoint(sb->s_mtd, 0, sbi->mtd_point_size);
> > > > +		if (sb->s_mtd)
> > > > +			kill_mtd_super(sb);
> > > 
> > > ...
> > > 
> > > > +	mtd_unpoint(sb->s_mtd, 0, PAGE_SIZE);
> > > > +	err = mtd_point(sb->s_mtd, 0, sbi->size, &sbi->mtd_point_size,
> > > > +			&sbi->linear_virt_addr, &sbi->linear_phys_addr);
> > > > +	if (err || sbi->mtd_point_size != sbi->size) {
> > > 
> > > What happens if that mtd_point() fails?  Note that ->kill_sb() will be
> > > called anyway and ->mtd_point_size is going to be non-zero here...
> > 
> > mtd_point() always clears sbi->mtd_point_size first thing upon entry 
> > even before it has a chance to fail. So it it fails then 
> > sbi->mtd_point_size will be zero and ->kill_sb() will skip the unpoint 
> > call.
> 
> OK...  I wonder if it should simply define stubs for kill_mtd_super(),
> mtd_unpoint() and kill_block_super() in !CONFIG_MTD and !CONFIG_BLOCK
> cases.  mount_mtd() and mount_bdev() as well - e.g.  mount_bdev()
> returning ERR_PTR(-ENODEV) and kill_block_super() being simply BUG()
> in !CONFIG_BLOCK case.  Then cramfs_kill_sb() would be
> 	if (sb->s_mtd) {
> 		if (sbi->mtd_point_size)
> 			mtd_unpoint(sb->s_mtd, 0, sbi->mtd_point_size);
> 		kill_mtd_super(sb);
> 	} else {
> 		kill_block_super(sb);
> 	}
> 	kfree(sbi);

What I really like about IS_ENABLED() usage is the immediate build 
coverage without having to run all config combinations. The compiler 
will discard unneeded code and avoid pesky unused variable warnings that 
require ugly #ifdefs otherwise.

> Wait.  Looking at that code... what happens if you hit this failure
> exit:
>         sbi = kzalloc(sizeof(struct cramfs_sb_info), GFP_KERNEL);
>         if (!sbi)
>                 return -ENOMEM;
> 
> Current cramfs_kill_sb() will do kill_block_super() and kfree(NULL), which
> works nicely, but you are dereferencing that sucker, not just passing it
> to kfree().  IOW, that if (sbi->....) ought to be if (sbi && sbi->...)

Right, good catch.
Fixed in my tree now.


Nicolas