[RFC,2/2] libfs: Improve behavior when directory offset values wrap

Commit Message

Chuck Lever Nov. 17, 2024, 9:32 p.m. UTC

From: Chuck Lever <[e-mail]>

The fix in commit 64a7ce76fb90 ("libfs: fix infinite directory reads
for offset dir") introduced a fence in offset_iterate_dir() to stop
the loop from returning child entries created after the directory
was opened. This comparison relies on the strong ordering of
DIR_OFFSET_MIN <= largest child offset <= next_offset to terminate
the directory iteration.

However, because simple_offset_add() uses mtree_alloc_cyclic() to
select each next new directory offset, ctx->next_offset is not
always the highest unused offset. Once mtree_alloc_cyclic() allows
a new offset value to wrap, ctx->next_offset will be set to a value
less than the actual largest child offset.

The result is that readdir(3) no longer shows any entries in the
directory because their offsets are above ctx->next_offset, which is
now a small value. This situation is persistent, and the directory
cannot be removed unless all current children are already known and
can be explicitly removed by name first.

In the current Maple tree implementation, there is no practical way
that 63-bit offset values can ever wrap, so this issue is cleverly
avoided. But the ordering dependency is not documented via comments
or code, making the mechanism somewhat brittle. And it makes the
continued use of mtree_alloc_cyclic() somewhat confusing.

Further, if commit 64a7ce76fb90 ("libfs: fix infinite directory
reads for offset dir") were to be backported to a kernel that still
uses xarray to manage simple directory offsets, the directory offset
value range is limited to 32-bits, which is small enough to allow a
wrap after a few weeks of constant creation of entries in one
directory.

Therefore, replace the use of ctx->next_offset for fencing new
children from appearing in readdir results.

A jiffies timestamp marks the end of each opendir epoch. Entries
created after this timestamp will not be visible to the file
descriptor. I chose jiffies so that the dentry->d_time field can be
re-used for storing the entry creation time.

The new mechanism has its own corner cases. For instance, I think
if jiffies wraps twice while a directory is open, some children
might become invisible. On 32-bit systems, the jiffies value wraps
every 49 days. Double-wrapping is not a risk on systems with 64-bit
jiffies. Unlike with the next_offset-based mechanism, re-opening the
directory will make invisible children re-appear.

Reported-by: Yu Kuai <[e-mail]>
Closes: https://lore.kernel.org/stable/[e-mail]/T/#m1c448e5bd4aae3632a09468affcfe1d1594c6a59
Fixes: 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir")
Signed-off-by: Chuck Lever <[e-mail]>
---
 fs/libfs.c | 36 +++++++++++++++++-------------------
 1 file changed, 17 insertions(+), 19 deletions(-)

Comments

Jeff Layton Nov. 18, 2024, 8 p.m. UTC | #1

On Sun, 2024-11-17 at 16:32 -0500, [e-mail] wrote:
> From: Chuck Lever <[e-mail]>
> 
> The fix in commit 64a7ce76fb90 ("libfs: fix infinite directory reads
> for offset dir") introduced a fence in offset_iterate_dir() to stop
> the loop from returning child entries created after the directory
> was opened. This comparison relies on the strong ordering of
> DIR_OFFSET_MIN <= largest child offset <= next_offset to terminate
> the directory iteration.
> 
> However, because simple_offset_add() uses mtree_alloc_cyclic() to
> select each next new directory offset, ctx->next_offset is not
> always the highest unused offset. Once mtree_alloc_cyclic() allows
> a new offset value to wrap, ctx->next_offset will be set to a value
> less than the actual largest child offset.
> 
> The result is that readdir(3) no longer shows any entries in the
> directory because their offsets are above ctx->next_offset, which is
> now a small value. This situation is persistent, and the directory
> cannot be removed unless all current children are already known and
> can be explicitly removed by name first.
> 
> In the current Maple tree implementation, there is no practical way
> that 63-bit offset values can ever wrap, so this issue is cleverly
> avoided. But the ordering dependency is not documented via comments
> or code, making the mechanism somewhat brittle. And it makes the
> continued use of mtree_alloc_cyclic() somewhat confusing.
> 
> Further, if commit 64a7ce76fb90 ("libfs: fix infinite directory
> reads for offset dir") were to be backported to a kernel that still
> uses xarray to manage simple directory offsets, the directory offset
> value range is limited to 32-bits, which is small enough to allow a
> wrap after a few weeks of constant creation of entries in one
> directory.
> 
> Therefore, replace the use of ctx->next_offset for fencing new
> children from appearing in readdir results.
> 
> A jiffies timestamp marks the end of each opendir epoch. Entries
> created after this timestamp will not be visible to the file
> descriptor. I chose jiffies so that the dentry->d_time field can be
> re-used for storing the entry creation time.
> 
> The new mechanism has its own corner cases. For instance, I think
> if jiffies wraps twice while a directory is open, some children
> might become invisible. On 32-bit systems, the jiffies value wraps
> every 49 days. Double-wrapping is not a risk on systems with 64-bit
> jiffies. Unlike with the next_offset-based mechanism, re-opening the
> directory will make invisible children re-appear.
> 
> Reported-by: Yu Kuai <[e-mail]>
> Closes: https://lore.kernel.org/stable/[e-mail]/T/#m1c448e5bd4aae3632a09468affcfe1d1594c6a59
> Fixes: 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir")
> Signed-off-by: Chuck Lever <[e-mail]>
> ---
>  fs/libfs.c | 36 +++++++++++++++++-------------------
>  1 file changed, 17 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/libfs.c b/fs/libfs.c
> index bf67954b525b..862a603fd454 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -294,6 +294,7 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
>  		return ret;
>  
>  	offset_set(dentry, offset);
> +	WRITE_ONCE(dentry->d_time, jiffies);
>  	return 0;
>  }
>  
> @@ -454,9 +455,7 @@ void simple_offset_destroy(struct offset_ctx *octx)
>  
>  static int offset_dir_open(struct inode *inode, struct file *file)
>  {
> -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> -
> -	file->private_data = (void *)ctx->next_offset;
> +	file->private_data = (void *)jiffies;
>  	return 0;
>  }
>  
> @@ -473,9 +472,6 @@ static int offset_dir_open(struct inode *inode, struct file *file)
>   */
>  static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>  {
> -	struct inode *inode = file->f_inode;
> -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> -
>  	switch (whence) {
>  	case SEEK_CUR:
>  		offset += file->f_pos;
> @@ -490,7 +486,8 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
>  
>  	/* In this case, ->private_data is protected by f_pos_lock */
>  	if (!offset)
> -		file->private_data = (void *)ctx->next_offset;
> +		/* Make newer child entries visible */
> +		file->private_data = (void *)jiffies;
>  	return vfs_setpos(file, offset, LONG_MAX);
>  }
>  
> @@ -521,7 +518,8 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
>  			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
>  }
>  
> -static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
> +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx,
> +			       unsigned long fence)
>  {
>  	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
>  	struct dentry *dentry;
> @@ -531,14 +529,15 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
>  		if (!dentry)
>  			return;
>  
> -		if (dentry2offset(dentry) >= last_index) {
> -			dput(dentry);
> -			return;
> -		}
> -
> -		if (!offset_dir_emit(ctx, dentry)) {
> -			dput(dentry);
> -			return;
> +		/*
> +		 * Output only child entries created during or before
> +		 * the current opendir epoch.
> +		 */
> +		if (time_before_eq(dentry->d_time, fence)) {
> +			if (!offset_dir_emit(ctx, dentry)) {
> +				dput(dentry);
> +				return;
> +			}
>  		}
>  
>  		ctx->pos = dentry2offset(dentry) + 1;
> @@ -569,15 +568,14 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
>   */
>  static int offset_readdir(struct file *file, struct dir_context *ctx)
>  {
> +	unsigned long fence = (unsigned long)file->private_data;
>  	struct dentry *dir = file->f_path.dentry;
> -	long last_index = (long)file->private_data;
>  
>  	lockdep_assert_held(&d_inode(dir)->i_rwsem);
>  
>  	if (!dir_emit_dots(file, ctx))
>  		return 0;
> -
> -	offset_iterate_dir(d_inode(dir), ctx, last_index);
> +	offset_iterate_dir(d_inode(dir), ctx, fence);
>  	return 0;
>  }
>  

Using timestamps instead of directory ordering does seem less brittle,
and the choice to use jiffies makes sense given that d_time is also an
unsigned long.

Reviewed-by: Jeff Layton <[e-mail]>

Chuck Lever Nov. 18, 2024, 8:58 p.m. UTC | #2

On Mon, Nov 18, 2024 at 03:00:56PM -0500, Jeff Layton wrote:
> On Sun, 2024-11-17 at 16:32 -0500, [e-mail] wrote:
> > From: Chuck Lever <[e-mail]>
> > 
> > The fix in commit 64a7ce76fb90 ("libfs: fix infinite directory reads
> > for offset dir") introduced a fence in offset_iterate_dir() to stop
> > the loop from returning child entries created after the directory
> > was opened. This comparison relies on the strong ordering of
> > DIR_OFFSET_MIN <= largest child offset <= next_offset to terminate
> > the directory iteration.
> > 
> > However, because simple_offset_add() uses mtree_alloc_cyclic() to
> > select each next new directory offset, ctx->next_offset is not
> > always the highest unused offset. Once mtree_alloc_cyclic() allows
> > a new offset value to wrap, ctx->next_offset will be set to a value
> > less than the actual largest child offset.
> > 
> > The result is that readdir(3) no longer shows any entries in the
> > directory because their offsets are above ctx->next_offset, which is
> > now a small value. This situation is persistent, and the directory
> > cannot be removed unless all current children are already known and
> > can be explicitly removed by name first.
> > 
> > In the current Maple tree implementation, there is no practical way
> > that 63-bit offset values can ever wrap, so this issue is cleverly
> > avoided. But the ordering dependency is not documented via comments
> > or code, making the mechanism somewhat brittle. And it makes the
> > continued use of mtree_alloc_cyclic() somewhat confusing.
> > 
> > Further, if commit 64a7ce76fb90 ("libfs: fix infinite directory
> > reads for offset dir") were to be backported to a kernel that still
> > uses xarray to manage simple directory offsets, the directory offset
> > value range is limited to 32-bits, which is small enough to allow a
> > wrap after a few weeks of constant creation of entries in one
> > directory.
> > 
> > Therefore, replace the use of ctx->next_offset for fencing new
> > children from appearing in readdir results.
> > 
> > A jiffies timestamp marks the end of each opendir epoch. Entries
> > created after this timestamp will not be visible to the file
> > descriptor. I chose jiffies so that the dentry->d_time field can be
> > re-used for storing the entry creation time.
> > 
> > The new mechanism has its own corner cases. For instance, I think
> > if jiffies wraps twice while a directory is open, some children
> > might become invisible. On 32-bit systems, the jiffies value wraps
> > every 49 days. Double-wrapping is not a risk on systems with 64-bit
> > jiffies. Unlike with the next_offset-based mechanism, re-opening the
> > directory will make invisible children re-appear.
> > 
> > Reported-by: Yu Kuai <[e-mail]>
> > Closes: https://lore.kernel.org/stable/[e-mail]/T/#m1c448e5bd4aae3632a09468affcfe1d1594c6a59
> > Fixes: 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir")
> > Signed-off-by: Chuck Lever <[e-mail]>
> > ---
> >  fs/libfs.c | 36 +++++++++++++++++-------------------
> >  1 file changed, 17 insertions(+), 19 deletions(-)
> > 
> > diff --git a/fs/libfs.c b/fs/libfs.c
> > index bf67954b525b..862a603fd454 100644
> > --- a/fs/libfs.c
> > +++ b/fs/libfs.c
> > @@ -294,6 +294,7 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
> >  		return ret;
> >  
> >  	offset_set(dentry, offset);
> > +	WRITE_ONCE(dentry->d_time, jiffies);
> >  	return 0;
> >  }
> >  
> > @@ -454,9 +455,7 @@ void simple_offset_destroy(struct offset_ctx *octx)
> >  
> >  static int offset_dir_open(struct inode *inode, struct file *file)
> >  {
> > -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > -
> > -	file->private_data = (void *)ctx->next_offset;
> > +	file->private_data = (void *)jiffies;
> >  	return 0;
> >  }
> >  
> > @@ -473,9 +472,6 @@ static int offset_dir_open(struct inode *inode, struct file *file)
> >   */
> >  static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> >  {
> > -	struct inode *inode = file->f_inode;
> > -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > -
> >  	switch (whence) {
> >  	case SEEK_CUR:
> >  		offset += file->f_pos;
> > @@ -490,7 +486,8 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> >  
> >  	/* In this case, ->private_data is protected by f_pos_lock */
> >  	if (!offset)
> > -		file->private_data = (void *)ctx->next_offset;
> > +		/* Make newer child entries visible */
> > +		file->private_data = (void *)jiffies;
> >  	return vfs_setpos(file, offset, LONG_MAX);
> >  }
> >  
> > @@ -521,7 +518,8 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
> >  			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
> >  }
> >  
> > -static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
> > +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx,
> > +			       unsigned long fence)
> >  {
> >  	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
> >  	struct dentry *dentry;
> > @@ -531,14 +529,15 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
> >  		if (!dentry)
> >  			return;
> >  
> > -		if (dentry2offset(dentry) >= last_index) {
> > -			dput(dentry);
> > -			return;
> > -		}
> > -
> > -		if (!offset_dir_emit(ctx, dentry)) {
> > -			dput(dentry);
> > -			return;
> > +		/*
> > +		 * Output only child entries created during or before
> > +		 * the current opendir epoch.
> > +		 */
> > +		if (time_before_eq(dentry->d_time, fence)) {
> > +			if (!offset_dir_emit(ctx, dentry)) {
> > +				dput(dentry);
> > +				return;
> > +			}
> >  		}
> >  
> >  		ctx->pos = dentry2offset(dentry) + 1;
> > @@ -569,15 +568,14 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
> >   */
> >  static int offset_readdir(struct file *file, struct dir_context *ctx)
> >  {
> > +	unsigned long fence = (unsigned long)file->private_data;
> >  	struct dentry *dir = file->f_path.dentry;
> > -	long last_index = (long)file->private_data;
> >  
> >  	lockdep_assert_held(&d_inode(dir)->i_rwsem);
> >  
> >  	if (!dir_emit_dots(file, ctx))
> >  		return 0;
> > -
> > -	offset_iterate_dir(d_inode(dir), ctx, last_index);
> > +	offset_iterate_dir(d_inode(dir), ctx, fence);
> >  	return 0;
> >  }
> >  
> 
> Using timestamps instead of directory ordering does seem less brittle,
> and the choice to use jiffies makes sense given that d_time is also an
> unsigned long.
> 
> Reviewed-by: Jeff Layton <[e-mail]>

Precisely. The goal was to re-use as much code as possible to avoid
perturbing the current size of "struct dentry".

That said, I'm not overjoyed with using jiffies, given it has
similar wrapping issues as ctx->next_offset on 32-bit systems. The
consequences of an offset value wrap are less severe, though, since
that can no longer make children entries disappear permanently.

I've been trying to imagine a solution that does not depend on the
range of an integer value and has solidly deterministic behavior in
the face of multiple child entry creations during one timer tick.

We could partially re-use the legacy cursor/list mechanism.

* When a child entry is created, it is added at the end of the
  parent's d_children list.
* When a child entry is unlinked, it is removed from the parent's
  d_children list.

This includes creation and removal of entries due to a rename.


* When a directory is opened, mark the current end of the d_children
  list with a cursor dentry. New entries would then be added to this
  directory following this cursor dentry in the directory's
  d_children list.
* When a directory is closed, its cursor dentry is removed from the
  d_children list and freed.

Each cursor dentry would need to refer to an opendir instance
(using, say, a pointer to the "struct file" for that open) so that
multiple cursors in the same directory can reside in its d_chilren
list and won't interfere with each other. Re-use the cursor dentry's
d_fsdata field for that.


* offset_readdir gets its starting entry using the mtree/xarray to
  map ctx->pos to a dentry.
* offset_readdir continues iterating by following the .next pointer
  in the current dentry's d_child field.
* offset_readdir returns EOD when it hits the cursor dentry matching
  this opendir instance.


I think all of these operations could be O(1), but it might require
some additional locking.

Christian Brauner Nov. 20, 2024, 8:59 a.m. UTC | #3

On Mon, Nov 18, 2024 at 03:58:09PM -0500, Chuck Lever wrote:
> On Mon, Nov 18, 2024 at 03:00:56PM -0500, Jeff Layton wrote:
> > On Sun, 2024-11-17 at 16:32 -0500, [e-mail] wrote:
> > > From: Chuck Lever <[e-mail]>
> > > 
> > > The fix in commit 64a7ce76fb90 ("libfs: fix infinite directory reads
> > > for offset dir") introduced a fence in offset_iterate_dir() to stop
> > > the loop from returning child entries created after the directory
> > > was opened. This comparison relies on the strong ordering of
> > > DIR_OFFSET_MIN <= largest child offset <= next_offset to terminate
> > > the directory iteration.
> > > 
> > > However, because simple_offset_add() uses mtree_alloc_cyclic() to
> > > select each next new directory offset, ctx->next_offset is not
> > > always the highest unused offset. Once mtree_alloc_cyclic() allows
> > > a new offset value to wrap, ctx->next_offset will be set to a value
> > > less than the actual largest child offset.
> > > 
> > > The result is that readdir(3) no longer shows any entries in the
> > > directory because their offsets are above ctx->next_offset, which is
> > > now a small value. This situation is persistent, and the directory
> > > cannot be removed unless all current children are already known and
> > > can be explicitly removed by name first.
> > > 
> > > In the current Maple tree implementation, there is no practical way
> > > that 63-bit offset values can ever wrap, so this issue is cleverly
> > > avoided. But the ordering dependency is not documented via comments
> > > or code, making the mechanism somewhat brittle. And it makes the
> > > continued use of mtree_alloc_cyclic() somewhat confusing.
> > > 
> > > Further, if commit 64a7ce76fb90 ("libfs: fix infinite directory
> > > reads for offset dir") were to be backported to a kernel that still
> > > uses xarray to manage simple directory offsets, the directory offset
> > > value range is limited to 32-bits, which is small enough to allow a
> > > wrap after a few weeks of constant creation of entries in one
> > > directory.
> > > 
> > > Therefore, replace the use of ctx->next_offset for fencing new
> > > children from appearing in readdir results.
> > > 
> > > A jiffies timestamp marks the end of each opendir epoch. Entries
> > > created after this timestamp will not be visible to the file
> > > descriptor. I chose jiffies so that the dentry->d_time field can be
> > > re-used for storing the entry creation time.
> > > 
> > > The new mechanism has its own corner cases. For instance, I think
> > > if jiffies wraps twice while a directory is open, some children
> > > might become invisible. On 32-bit systems, the jiffies value wraps
> > > every 49 days. Double-wrapping is not a risk on systems with 64-bit
> > > jiffies. Unlike with the next_offset-based mechanism, re-opening the
> > > directory will make invisible children re-appear.
> > > 
> > > Reported-by: Yu Kuai <[e-mail]>
> > > Closes: https://lore.kernel.org/stable/[e-mail]/T/#m1c448e5bd4aae3632a09468affcfe1d1594c6a59
> > > Fixes: 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir")
> > > Signed-off-by: Chuck Lever <[e-mail]>
> > > ---
> > >  fs/libfs.c | 36 +++++++++++++++++-------------------
> > >  1 file changed, 17 insertions(+), 19 deletions(-)
> > > 
> > > diff --git a/fs/libfs.c b/fs/libfs.c
> > > index bf67954b525b..862a603fd454 100644
> > > --- a/fs/libfs.c
> > > +++ b/fs/libfs.c
> > > @@ -294,6 +294,7 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
> > >  		return ret;
> > >  
> > >  	offset_set(dentry, offset);
> > > +	WRITE_ONCE(dentry->d_time, jiffies);
> > >  	return 0;
> > >  }
> > >  
> > > @@ -454,9 +455,7 @@ void simple_offset_destroy(struct offset_ctx *octx)
> > >  
> > >  static int offset_dir_open(struct inode *inode, struct file *file)
> > >  {
> > > -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > > -
> > > -	file->private_data = (void *)ctx->next_offset;
> > > +	file->private_data = (void *)jiffies;
> > >  	return 0;
> > >  }
> > >  
> > > @@ -473,9 +472,6 @@ static int offset_dir_open(struct inode *inode, struct file *file)
> > >   */
> > >  static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> > >  {
> > > -	struct inode *inode = file->f_inode;
> > > -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > > -
> > >  	switch (whence) {
> > >  	case SEEK_CUR:
> > >  		offset += file->f_pos;
> > > @@ -490,7 +486,8 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> > >  
> > >  	/* In this case, ->private_data is protected by f_pos_lock */
> > >  	if (!offset)
> > > -		file->private_data = (void *)ctx->next_offset;
> > > +		/* Make newer child entries visible */
> > > +		file->private_data = (void *)jiffies;
> > >  	return vfs_setpos(file, offset, LONG_MAX);
> > >  }
> > >  
> > > @@ -521,7 +518,8 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
> > >  			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
> > >  }
> > >  
> > > -static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
> > > +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx,
> > > +			       unsigned long fence)
> > >  {
> > >  	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
> > >  	struct dentry *dentry;
> > > @@ -531,14 +529,15 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
> > >  		if (!dentry)
> > >  			return;
> > >  
> > > -		if (dentry2offset(dentry) >= last_index) {
> > > -			dput(dentry);
> > > -			return;
> > > -		}
> > > -
> > > -		if (!offset_dir_emit(ctx, dentry)) {
> > > -			dput(dentry);
> > > -			return;
> > > +		/*
> > > +		 * Output only child entries created during or before
> > > +		 * the current opendir epoch.
> > > +		 */
> > > +		if (time_before_eq(dentry->d_time, fence)) {
> > > +			if (!offset_dir_emit(ctx, dentry)) {
> > > +				dput(dentry);
> > > +				return;
> > > +			}
> > >  		}
> > >  
> > >  		ctx->pos = dentry2offset(dentry) + 1;
> > > @@ -569,15 +568,14 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
> > >   */
> > >  static int offset_readdir(struct file *file, struct dir_context *ctx)
> > >  {
> > > +	unsigned long fence = (unsigned long)file->private_data;
> > >  	struct dentry *dir = file->f_path.dentry;
> > > -	long last_index = (long)file->private_data;
> > >  
> > >  	lockdep_assert_held(&d_inode(dir)->i_rwsem);
> > >  
> > >  	if (!dir_emit_dots(file, ctx))
> > >  		return 0;
> > > -
> > > -	offset_iterate_dir(d_inode(dir), ctx, last_index);
> > > +	offset_iterate_dir(d_inode(dir), ctx, fence);
> > >  	return 0;
> > >  }
> > >  
> > 
> > Using timestamps instead of directory ordering does seem less brittle,
> > and the choice to use jiffies makes sense given that d_time is also an
> > unsigned long.
> > 
> > Reviewed-by: Jeff Layton <[e-mail]>
> 
> Precisely. The goal was to re-use as much code as possible to avoid
> perturbing the current size of "struct dentry".
> 
> That said, I'm not overjoyed with using jiffies, given it has
> similar wrapping issues as ctx->next_offset on 32-bit systems. The
> consequences of an offset value wrap are less severe, though, since
> that can no longer make children entries disappear permanently.
> 
> I've been trying to imagine a solution that does not depend on the
> range of an integer value and has solidly deterministic behavior in
> the face of multiple child entry creations during one timer tick.
> 
> We could partially re-use the legacy cursor/list mechanism.
> 
> * When a child entry is created, it is added at the end of the
>   parent's d_children list.
> * When a child entry is unlinked, it is removed from the parent's
>   d_children list.
> 
> This includes creation and removal of entries due to a rename.
> 
> 
> * When a directory is opened, mark the current end of the d_children
>   list with a cursor dentry. New entries would then be added to this
>   directory following this cursor dentry in the directory's
>   d_children list.
> * When a directory is closed, its cursor dentry is removed from the
>   d_children list and freed.
> 
> Each cursor dentry would need to refer to an opendir instance
> (using, say, a pointer to the "struct file" for that open) so that
> multiple cursors in the same directory can reside in its d_chilren
> list and won't interfere with each other. Re-use the cursor dentry's
> d_fsdata field for that.
> 
> 
> * offset_readdir gets its starting entry using the mtree/xarray to
>   map ctx->pos to a dentry.
> * offset_readdir continues iterating by following the .next pointer
>   in the current dentry's d_child field.
> * offset_readdir returns EOD when it hits the cursor dentry matching
>   this opendir instance.
> 
> 
> I think all of these operations could be O(1), but it might require
> some additional locking.

This would be a bigger refactor of the whole stable offset logic. So
even if we end up doing that I think we should use the jiffies solution
for now.

Chuck Lever Nov. 20, 2024, 3:05 p.m. UTC | #4

On Wed, Nov 20, 2024 at 09:59:54AM +0100, Christian Brauner wrote:
> On Mon, Nov 18, 2024 at 03:58:09PM -0500, Chuck Lever wrote:
> > On Mon, Nov 18, 2024 at 03:00:56PM -0500, Jeff Layton wrote:
> > > On Sun, 2024-11-17 at 16:32 -0500, [e-mail] wrote:
> > > > From: Chuck Lever <[e-mail]>
> > > > 
> > > > The fix in commit 64a7ce76fb90 ("libfs: fix infinite directory reads
> > > > for offset dir") introduced a fence in offset_iterate_dir() to stop
> > > > the loop from returning child entries created after the directory
> > > > was opened. This comparison relies on the strong ordering of
> > > > DIR_OFFSET_MIN <= largest child offset <= next_offset to terminate
> > > > the directory iteration.
> > > > 
> > > > However, because simple_offset_add() uses mtree_alloc_cyclic() to
> > > > select each next new directory offset, ctx->next_offset is not
> > > > always the highest unused offset. Once mtree_alloc_cyclic() allows
> > > > a new offset value to wrap, ctx->next_offset will be set to a value
> > > > less than the actual largest child offset.
> > > > 
> > > > The result is that readdir(3) no longer shows any entries in the
> > > > directory because their offsets are above ctx->next_offset, which is
> > > > now a small value. This situation is persistent, and the directory
> > > > cannot be removed unless all current children are already known and
> > > > can be explicitly removed by name first.
> > > > 
> > > > In the current Maple tree implementation, there is no practical way
> > > > that 63-bit offset values can ever wrap, so this issue is cleverly
> > > > avoided. But the ordering dependency is not documented via comments
> > > > or code, making the mechanism somewhat brittle. And it makes the
> > > > continued use of mtree_alloc_cyclic() somewhat confusing.
> > > > 
> > > > Further, if commit 64a7ce76fb90 ("libfs: fix infinite directory
> > > > reads for offset dir") were to be backported to a kernel that still
> > > > uses xarray to manage simple directory offsets, the directory offset
> > > > value range is limited to 32-bits, which is small enough to allow a
> > > > wrap after a few weeks of constant creation of entries in one
> > > > directory.
> > > > 
> > > > Therefore, replace the use of ctx->next_offset for fencing new
> > > > children from appearing in readdir results.
> > > > 
> > > > A jiffies timestamp marks the end of each opendir epoch. Entries
> > > > created after this timestamp will not be visible to the file
> > > > descriptor. I chose jiffies so that the dentry->d_time field can be
> > > > re-used for storing the entry creation time.
> > > > 
> > > > The new mechanism has its own corner cases. For instance, I think
> > > > if jiffies wraps twice while a directory is open, some children
> > > > might become invisible. On 32-bit systems, the jiffies value wraps
> > > > every 49 days. Double-wrapping is not a risk on systems with 64-bit
> > > > jiffies. Unlike with the next_offset-based mechanism, re-opening the
> > > > directory will make invisible children re-appear.
> > > > 
> > > > Reported-by: Yu Kuai <[e-mail]>
> > > > Closes: https://lore.kernel.org/stable/[e-mail]/T/#m1c448e5bd4aae3632a09468affcfe1d1594c6a59
> > > > Fixes: 64a7ce76fb90 ("libfs: fix infinite directory reads for offset dir")
> > > > Signed-off-by: Chuck Lever <[e-mail]>
> > > > ---
> > > >  fs/libfs.c | 36 +++++++++++++++++-------------------
> > > >  1 file changed, 17 insertions(+), 19 deletions(-)
> > > > 
> > > > diff --git a/fs/libfs.c b/fs/libfs.c
> > > > index bf67954b525b..862a603fd454 100644
> > > > --- a/fs/libfs.c
> > > > +++ b/fs/libfs.c
> > > > @@ -294,6 +294,7 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
> > > >  		return ret;
> > > >  
> > > >  	offset_set(dentry, offset);
> > > > +	WRITE_ONCE(dentry->d_time, jiffies);
> > > >  	return 0;
> > > >  }
> > > >  
> > > > @@ -454,9 +455,7 @@ void simple_offset_destroy(struct offset_ctx *octx)
> > > >  
> > > >  static int offset_dir_open(struct inode *inode, struct file *file)
> > > >  {
> > > > -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > > > -
> > > > -	file->private_data = (void *)ctx->next_offset;
> > > > +	file->private_data = (void *)jiffies;
> > > >  	return 0;
> > > >  }
> > > >  
> > > > @@ -473,9 +472,6 @@ static int offset_dir_open(struct inode *inode, struct file *file)
> > > >   */
> > > >  static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> > > >  {
> > > > -	struct inode *inode = file->f_inode;
> > > > -	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> > > > -
> > > >  	switch (whence) {
> > > >  	case SEEK_CUR:
> > > >  		offset += file->f_pos;
> > > > @@ -490,7 +486,8 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> > > >  
> > > >  	/* In this case, ->private_data is protected by f_pos_lock */
> > > >  	if (!offset)
> > > > -		file->private_data = (void *)ctx->next_offset;
> > > > +		/* Make newer child entries visible */
> > > > +		file->private_data = (void *)jiffies;
> > > >  	return vfs_setpos(file, offset, LONG_MAX);
> > > >  }
> > > >  
> > > > @@ -521,7 +518,8 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
> > > >  			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
> > > >  }
> > > >  
> > > > -static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
> > > > +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx,
> > > > +			       unsigned long fence)
> > > >  {
> > > >  	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
> > > >  	struct dentry *dentry;
> > > > @@ -531,14 +529,15 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
> > > >  		if (!dentry)
> > > >  			return;
> > > >  
> > > > -		if (dentry2offset(dentry) >= last_index) {
> > > > -			dput(dentry);
> > > > -			return;
> > > > -		}
> > > > -
> > > > -		if (!offset_dir_emit(ctx, dentry)) {
> > > > -			dput(dentry);
> > > > -			return;
> > > > +		/*
> > > > +		 * Output only child entries created during or before
> > > > +		 * the current opendir epoch.
> > > > +		 */
> > > > +		if (time_before_eq(dentry->d_time, fence)) {
> > > > +			if (!offset_dir_emit(ctx, dentry)) {
> > > > +				dput(dentry);
> > > > +				return;
> > > > +			}
> > > >  		}
> > > >  
> > > >  		ctx->pos = dentry2offset(dentry) + 1;
> > > > @@ -569,15 +568,14 @@ static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
> > > >   */
> > > >  static int offset_readdir(struct file *file, struct dir_context *ctx)
> > > >  {
> > > > +	unsigned long fence = (unsigned long)file->private_data;
> > > >  	struct dentry *dir = file->f_path.dentry;
> > > > -	long last_index = (long)file->private_data;
> > > >  
> > > >  	lockdep_assert_held(&d_inode(dir)->i_rwsem);
> > > >  
> > > >  	if (!dir_emit_dots(file, ctx))
> > > >  		return 0;
> > > > -
> > > > -	offset_iterate_dir(d_inode(dir), ctx, last_index);
> > > > +	offset_iterate_dir(d_inode(dir), ctx, fence);
> > > >  	return 0;
> > > >  }
> > > >  
> > > 
> > > Using timestamps instead of directory ordering does seem less brittle,
> > > and the choice to use jiffies makes sense given that d_time is also an
> > > unsigned long.
> > > 
> > > Reviewed-by: Jeff Layton <[e-mail]>
> > 
> > Precisely. The goal was to re-use as much code as possible to avoid
> > perturbing the current size of "struct dentry".
> > 
> > That said, I'm not overjoyed with using jiffies, given it has
> > similar wrapping issues as ctx->next_offset on 32-bit systems. The
> > consequences of an offset value wrap are less severe, though, since
> > that can no longer make children entries disappear permanently.
> > 
> > I've been trying to imagine a solution that does not depend on the
> > range of an integer value and has solidly deterministic behavior in
> > the face of multiple child entry creations during one timer tick.
> > 
> > We could partially re-use the legacy cursor/list mechanism.
> > 
> > * When a child entry is created, it is added at the end of the
> >   parent's d_children list.
> > * When a child entry is unlinked, it is removed from the parent's
> >   d_children list.
> > 
> > This includes creation and removal of entries due to a rename.
> > 
> > 
> > * When a directory is opened, mark the current end of the d_children
> >   list with a cursor dentry. New entries would then be added to this
> >   directory following this cursor dentry in the directory's
> >   d_children list.
> > * When a directory is closed, its cursor dentry is removed from the
> >   d_children list and freed.
> > 
> > Each cursor dentry would need to refer to an opendir instance
> > (using, say, a pointer to the "struct file" for that open) so that
> > multiple cursors in the same directory can reside in its d_chilren
> > list and won't interfere with each other. Re-use the cursor dentry's
> > d_fsdata field for that.
> > 
> > 
> > * offset_readdir gets its starting entry using the mtree/xarray to
> >   map ctx->pos to a dentry.
> > * offset_readdir continues iterating by following the .next pointer
> >   in the current dentry's d_child field.
> > * offset_readdir returns EOD when it hits the cursor dentry matching
> >   this opendir instance.
> > 
> > 
> > I think all of these operations could be O(1), but it might require
> > some additional locking.
> 
> This would be a bigger refactor of the whole stable offset logic. So
> even if we end up doing that I think we should use the jiffies solution
> for now.

How should I mark patches so they can be posted for discussion and
never applied? This series is marked RFC.

I am actually half-way through implementing the approach described
here. It is not as big a re-write as you might think, and addresses
some fundamental misunderstandings in the offset_iterate_dir() code.

Patch

diff --git a/fs/libfs.c b/fs/libfs.c
index bf67954b525b..862a603fd454 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -294,6 +294,7 @@  int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 		return ret;
 
 	offset_set(dentry, offset);
+	WRITE_ONCE(dentry->d_time, jiffies);
 	return 0;
 }
 
@@ -454,9 +455,7 @@  void simple_offset_destroy(struct offset_ctx *octx)
 
 static int offset_dir_open(struct inode *inode, struct file *file)
 {
-	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
-
-	file->private_data = (void *)ctx->next_offset;
+	file->private_data = (void *)jiffies;
 	return 0;
 }
 
@@ -473,9 +472,6 @@  static int offset_dir_open(struct inode *inode, struct file *file)
  */
 static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 {
-	struct inode *inode = file->f_inode;
-	struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
-
 	switch (whence) {
 	case SEEK_CUR:
 		offset += file->f_pos;
@@ -490,7 +486,8 @@  static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
 
 	/* In this case, ->private_data is protected by f_pos_lock */
 	if (!offset)
-		file->private_data = (void *)ctx->next_offset;
+		/* Make newer child entries visible */
+		file->private_data = (void *)jiffies;
 	return vfs_setpos(file, offset, LONG_MAX);
 }
 
@@ -521,7 +518,8 @@  static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
+static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx,
+			       unsigned long fence)
 {
 	struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
 	struct dentry *dentry;
@@ -531,14 +529,15 @@  static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
 		if (!dentry)
 			return;
 
-		if (dentry2offset(dentry) >= last_index) {
-			dput(dentry);
-			return;
-		}
-
-		if (!offset_dir_emit(ctx, dentry)) {
-			dput(dentry);
-			return;
+		/*
+		 * Output only child entries created during or before
+		 * the current opendir epoch.
+		 */
+		if (time_before_eq(dentry->d_time, fence)) {
+			if (!offset_dir_emit(ctx, dentry)) {
+				dput(dentry);
+				return;
+			}
 		}
 
 		ctx->pos = dentry2offset(dentry) + 1;
@@ -569,15 +568,14 @@  static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, lon
  */
 static int offset_readdir(struct file *file, struct dir_context *ctx)
 {
+	unsigned long fence = (unsigned long)file->private_data;
 	struct dentry *dir = file->f_path.dentry;
-	long last_index = (long)file->private_data;
 
 	lockdep_assert_held(&d_inode(dir)->i_rwsem);
 
 	if (!dir_emit_dots(file, ctx))
 		return 0;
-
-	offset_iterate_dir(d_inode(dir), ctx, last_index);
+	offset_iterate_dir(d_inode(dir), ctx, fence);
 	return 0;
 }