linux-kernel - Re: [PATCH v2 2/2] fat: always use dir_emit

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Date: Wed, 26 Jun 2024 16:46:26 -0300
From: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
To: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Gwendal Grignou <gwendal@...omium.org>, dlunev@...omium.org
Subject: Re: [PATCH v2 2/2] fat: always use dir_emit_dots and ignore . and ..
 entries

On Wed, Jun 26, 2024 at 06:47:15AM +0900, OGAWA Hirofumi wrote:
> Thadeu Lima de Souza Cascardo <cascardo@...lia.com> writes:
> 
> > Instead of only using dir_emit_dots for the root inode and explictily
> > requiring the . and .. entries to emit them, use dir_emit_dots for all
> > directories.
> >
> > That allows filesystems with directories without the . or .. entries to
> > still show them.
> 
> Unacceptable to change the correct behavior to broken format. And
> unlikely break the userspace, however this still has the user visible
> change of seek pos.
> 
> Thanks.
> 

I agree that if this breaks userspace with a good filesystem or regresses
in a way that real applications would break, that this needs to be redone.

However, I spent a few hours doing some extra testing (I had already run
some xfstests that include directory testing) and I failed to find any
issues with this fix.

If this would break, it would have broken the root directory. In the case
of a directory including the . and .. entries, the d_off for the .. entry
will be set for the first non-dot-or-dotdot entry. For ., it will be set as
1, which, if used by telldir (or llseek), will emit the .. entry, as
expected.

For the case where both . and .. are absent, the first real entry will have
d_off as 2, and it will just work.

So everything seems to work as expected. Do you see any user visible change
that would break any applications?

Thanks.
Cascardo.

> > Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
> > ---
> >  fs/fat/dir.c | 24 +++++++++---------------
> >  1 file changed, 9 insertions(+), 15 deletions(-)
> >
> > diff --git a/fs/fat/dir.c b/fs/fat/dir.c
> > index 4e4a359a1ea3..e70781569de5 100644
> > --- a/fs/fat/dir.c
> > +++ b/fs/fat/dir.c
> > @@ -583,15 +583,14 @@ static int __fat_readdir(struct inode *inode, struct file *file,
> >  	mutex_lock(&sbi->s_lock);
> >  
> >  	cpos = ctx->pos;
> > -	/* Fake . and .. for the root directory. */
> > -	if (inode->i_ino == MSDOS_ROOT_INO) {
> > -		if (!dir_emit_dots(file, ctx))
> > -			goto out;
> > -		if (ctx->pos == 2) {
> > -			fake_offset = 1;
> > -			cpos = 0;
> > -		}
> > +
> > +	if (!dir_emit_dots(file, ctx))
> > +		goto out;
> > +	if (ctx->pos == 2) {
> > +		fake_offset = 1;
> > +		cpos = 0;
> >  	}
> > +
> >  	if (cpos & (sizeof(struct msdos_dir_entry) - 1)) {
> >  		ret = -ENOENT;
> >  		goto out;
> > @@ -671,13 +670,8 @@ static int __fat_readdir(struct inode *inode, struct file *file,
> >  	if (fake_offset && ctx->pos < 2)
> >  		ctx->pos = 2;
> >  
> > -	if (!memcmp(de->name, MSDOS_DOT, MSDOS_NAME)) {
> > -		if (!dir_emit_dot(file, ctx))
> > -			goto fill_failed;
> > -	} else if (!memcmp(de->name, MSDOS_DOTDOT, MSDOS_NAME)) {
> > -		if (!dir_emit_dotdot(file, ctx))
> > -			goto fill_failed;
> > -	} else {
> > +	if (memcmp(de->name, MSDOS_DOT, MSDOS_NAME) &&
> > +	    memcmp(de->name, MSDOS_DOTDOT, MSDOS_NAME)) {
> >  		unsigned long inum;
> >  		loff_t i_pos = fat_make_i_pos(sb, bh, de);
> >  		struct inode *tmp = fat_iget(sb, i_pos);
> 
> -- 
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>