[PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Icenowy Zheng]

The function ovl_dir_real_file() currently uses the semaphore of the
inode to synchronize write to the upperfile cache field.

However, this function will get called by ovl_ioctl_set_flags(), which
utilizes the inode semaphore too. In this case ovl_dir_real_file() will
try to claim a lock that is owned by a function in its call stack, which
won't get released before ovl_dir_real_file() returns.

Define a dedicated semaphore for the upperfile cache, so that the
deadlock won't happen.

Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
Cc: stable@vger.kernel.org # v5.10
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
---
Changes in v2:
- Fixed missing replacement in error handling path.
Changes in v3:
- Use mutex instead of semaphore.

 fs/overlayfs/readdir.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 01620ebae1bd..3980f9982f34 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -56,6 +56,7 @@ struct ovl_dir_file {
 	struct list_head *cursor;
 	struct file *realfile;
 	struct file *upperfile;
+	struct mutex upperfile_mutex;
 };
 
 static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
@@ -874,8 +875,6 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 	 * Need to check if we started out being a lower dir, but got copied up
 	 */
 	if (!od->is_upper) {
-		struct inode *inode = file_inode(file);
-
 		realfile = READ_ONCE(od->upperfile);
 		if (!realfile) {
 			struct path upperpath;
@@ -883,10 +882,10 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 			ovl_path_upper(dentry, &upperpath);
 			realfile = ovl_dir_open_realfile(file, &upperpath);
 
-			inode_lock(inode);
+			mutex_lock(&od->upperfile_mutex);
 			if (!od->upperfile) {
 				if (IS_ERR(realfile)) {
-					inode_unlock(inode);
+					mutex_unlock(&od->upperfile_mutex);
 					return realfile;
 				}
 				smp_store_release(&od->upperfile, realfile);
@@ -896,7 +895,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 					fput(realfile);
 				realfile = od->upperfile;
 			}
-			inode_unlock(inode);
+			mutex_unlock(&od->upperfile_mutex);
 		}
 	}
 
@@ -959,6 +958,7 @@ static int ovl_dir_open(struct inode *inode, struct file *file)
 	od->realfile = realfile;
 	od->is_real = ovl_dir_is_real(file->f_path.dentry);
 	od->is_upper = OVL_TYPE_UPPER(type);
+	mutex_init(&od->upperfile_mutex);
 	file->private_data = od;
 
 	return 0;
-- 
2.28.0

Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Amir Goldstein]

On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io> wrote:
>
> The function ovl_dir_real_file() currently uses the semaphore of the
> inode to synchronize write to the upperfile cache field.

Although the inode lock is a rw_sem it is referred to as the "inode lock"
and you also left semaphore in the commit subject.
No need to re-post. This can be fixed on commit.

>
> However, this function will get called by ovl_ioctl_set_flags(), which
> utilizes the inode semaphore too. In this case ovl_dir_real_file() will
> try to claim a lock that is owned by a function in its call stack, which
> won't get released before ovl_dir_real_file() returns.
>
> Define a dedicated semaphore for the upperfile cache, so that the
> deadlock won't happen.
>
> Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
> Cc: stable@vger.kernel.org # v5.10
> Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> ---
> Changes in v2:
> - Fixed missing replacement in error handling path.
> Changes in v3:
> - Use mutex instead of semaphore.
>
>  fs/overlayfs/readdir.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 01620ebae1bd..3980f9982f34 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -56,6 +56,7 @@ struct ovl_dir_file {
>         struct list_head *cursor;
>         struct file *realfile;
>         struct file *upperfile;
> +       struct mutex upperfile_mutex;

That's a very specific name.
This mutex protects members of struct ovl_dir_file, which could evolve
into struct ovl_file one day (because no reason to cache only dir upper file),
so I would go with a more generic name, but let's leave it to Miklos to decide.

He could have a different idea altogether for fixing this bug.

Thanks,
Amir.

Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Amir Goldstein]

On Tue, Jan 5, 2021 at 8:47 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io> wrote:
> >
> > The function ovl_dir_real_file() currently uses the semaphore of the
> > inode to synchronize write to the upperfile cache field.
>
> Although the inode lock is a rw_sem it is referred to as the "inode lock"
> and you also left semaphore in the commit subject.
> No need to re-post. This can be fixed on commit.
>
> >
> > However, this function will get called by ovl_ioctl_set_flags(), which
> > utilizes the inode semaphore too. In this case ovl_dir_real_file() will
> > try to claim a lock that is owned by a function in its call stack, which
> > won't get released before ovl_dir_real_file() returns.
> >
> > Define a dedicated semaphore for the upperfile cache, so that the
> > deadlock won't happen.
> >
> > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
> > Cc: stable@vger.kernel.org # v5.10
> > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > ---
> > Changes in v2:
> > - Fixed missing replacement in error handling path.
> > Changes in v3:
> > - Use mutex instead of semaphore.
> >
> >  fs/overlayfs/readdir.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > index 01620ebae1bd..3980f9982f34 100644
> > --- a/fs/overlayfs/readdir.c
> > +++ b/fs/overlayfs/readdir.c
> > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> >         struct list_head *cursor;
> >         struct file *realfile;
> >         struct file *upperfile;
> > +       struct mutex upperfile_mutex;
>
> That's a very specific name.
> This mutex protects members of struct ovl_dir_file, which could evolve
> into struct ovl_file one day (because no reason to cache only dir upper file),
> so I would go with a more generic name, but let's leave it to Miklos to decide.
>
> He could have a different idea altogether for fixing this bug.
>

Miklos,

Please fast track this or an alternative fix.
It fixes an easy to reproduce deadlock introduced in 5.10.
Icenowy Zheng has written a simple xfstest reproducer, but it wasn't
posted - best to avoid hanging tester's machines until a fix is merged...

Thanks,
Amir.

Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Miklos Szeredi]

On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:
> On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io> wrote:
> >
> > The function ovl_dir_real_file() currently uses the semaphore of the
> > inode to synchronize write to the upperfile cache field.
> 
> Although the inode lock is a rw_sem it is referred to as the "inode lock"
> and you also left semaphore in the commit subject.
> No need to re-post. This can be fixed on commit.
> 
> >
> > However, this function will get called by ovl_ioctl_set_flags(), which
> > utilizes the inode semaphore too. In this case ovl_dir_real_file() will
> > try to claim a lock that is owned by a function in its call stack, which
> > won't get released before ovl_dir_real_file() returns.
> >
> > Define a dedicated semaphore for the upperfile cache, so that the
> > deadlock won't happen.
> >
> > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
> > Cc: stable@vger.kernel.org # v5.10
> > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > ---
> > Changes in v2:
> > - Fixed missing replacement in error handling path.
> > Changes in v3:
> > - Use mutex instead of semaphore.
> >
> >  fs/overlayfs/readdir.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > index 01620ebae1bd..3980f9982f34 100644
> > --- a/fs/overlayfs/readdir.c
> > +++ b/fs/overlayfs/readdir.c
> > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> >         struct list_head *cursor;
> >         struct file *realfile;
> >         struct file *upperfile;
> > +       struct mutex upperfile_mutex;
> 
> That's a very specific name.
> This mutex protects members of struct ovl_dir_file, which could evolve
> into struct ovl_file one day (because no reason to cache only dir upper file),
> so I would go with a more generic name, but let's leave it to Miklos to decide.
> 
> He could have a different idea altogether for fixing this bug.

How about this (untested) patch?

It's a cleanup as well as a fix, but maybe we should separate the cleanup from
the fix...

Thanks,
Miklos
---

 fs/overlayfs/readdir.c |   23 +++++++----------------
 1 file changed, 7 insertions(+), 16 deletions(-)

--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const str
 
 	struct ovl_dir_file *od = file->private_data;
 	struct dentry *dentry = file->f_path.dentry;
-	struct file *realfile = od->realfile;
+	struct file *old, *realfile = od->realfile;
 
 	if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
 		return want_upper ? NULL : realfile;
@@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const str
 	 * Need to check if we started out being a lower dir, but got copied up
 	 */
 	if (!od->is_upper) {
-		struct inode *inode = file_inode(file);
-
 		realfile = READ_ONCE(od->upperfile);
 		if (!realfile) {
 			struct path upperpath;
 
 			ovl_path_upper(dentry, &upperpath);
 			realfile = ovl_dir_open_realfile(file, &upperpath);
+			if (IS_ERR(realfile))
+				return realfile;
 
-			inode_lock(inode);
-			if (!od->upperfile) {
-				if (IS_ERR(realfile)) {
-					inode_unlock(inode);
-					return realfile;
-				}
-				smp_store_release(&od->upperfile, realfile);
-			} else {
-				/* somebody has beaten us to it */
-				if (!IS_ERR(realfile))
-					fput(realfile);
-				realfile = od->upperfile;
+			old = cmpxchg_release(&od->upperfile, NULL, realfile);
+			if (old) {
+				fput(realfile);
+				realfile = old;
 			}
-			inode_unlock(inode);
 		}
 	}
 

Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Amir Goldstein]

On Wed, Jan 20, 2021 at 12:20 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:
> > On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io> wrote:
> > >
> > > The function ovl_dir_real_file() currently uses the semaphore of the
> > > inode to synchronize write to the upperfile cache field.
> >
> > Although the inode lock is a rw_sem it is referred to as the "inode lock"
> > and you also left semaphore in the commit subject.
> > No need to re-post. This can be fixed on commit.
> >
> > >
> > > However, this function will get called by ovl_ioctl_set_flags(), which
> > > utilizes the inode semaphore too. In this case ovl_dir_real_file() will
> > > try to claim a lock that is owned by a function in its call stack, which
> > > won't get released before ovl_dir_real_file() returns.
> > >
> > > Define a dedicated semaphore for the upperfile cache, so that the
> > > deadlock won't happen.
> > >
> > > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
> > > Cc: stable@vger.kernel.org # v5.10
> > > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > > ---
> > > Changes in v2:
> > > - Fixed missing replacement in error handling path.
> > > Changes in v3:
> > > - Use mutex instead of semaphore.
> > >
> > >  fs/overlayfs/readdir.c | 10 +++++-----
> > >  1 file changed, 5 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > > index 01620ebae1bd..3980f9982f34 100644
> > > --- a/fs/overlayfs/readdir.c
> > > +++ b/fs/overlayfs/readdir.c
> > > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> > >         struct list_head *cursor;
> > >         struct file *realfile;
> > >         struct file *upperfile;
> > > +       struct mutex upperfile_mutex;
> >
> > That's a very specific name.
> > This mutex protects members of struct ovl_dir_file, which could evolve
> > into struct ovl_file one day (because no reason to cache only dir upper file),
> > so I would go with a more generic name, but let's leave it to Miklos to decide.
> >
> > He could have a different idea altogether for fixing this bug.
>
> How about this (untested) patch?
>

Much better :)

> It's a cleanup as well as a fix, but maybe we should separate the cleanup from
> the fix...
>
> Thanks,
> Miklos
> ---
>
>  fs/overlayfs/readdir.c |   23 +++++++----------------
>  1 file changed, 7 insertions(+), 16 deletions(-)
>
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const str
>
>         struct ovl_dir_file *od = file->private_data;
>         struct dentry *dentry = file->f_path.dentry;
> -       struct file *realfile = od->realfile;
> +       struct file *old, *realfile = od->realfile;
>
>         if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
>                 return want_upper ? NULL : realfile;
> @@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const str
>          * Need to check if we started out being a lower dir, but got copied up
>          */
>         if (!od->is_upper) {
> -               struct inode *inode = file_inode(file);
> -
>                 realfile = READ_ONCE(od->upperfile);
>                 if (!realfile) {
>                         struct path upperpath;
>
>                         ovl_path_upper(dentry, &upperpath);
>                         realfile = ovl_dir_open_realfile(file, &upperpath);
> +                       if (IS_ERR(realfile))
> +                               return realfile;
>
> -                       inode_lock(inode);
> -                       if (!od->upperfile) {
> -                               if (IS_ERR(realfile)) {
> -                                       inode_unlock(inode);
> -                                       return realfile;
> -                               }
> -                               smp_store_release(&od->upperfile, realfile);
> -                       } else {
> -                               /* somebody has beaten us to it */
> -                               if (!IS_ERR(realfile))
> -                                       fput(realfile);
> -                               realfile = od->upperfile;
> +                       old = cmpxchg_release(&od->upperfile, NULL, realfile);
> +                       if (old) {
> +                               fput(realfile);
> +                               realfile = old;
>                         }
> -                       inode_unlock(inode);
>                 }
>         }
>

Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Icenowy Zheng]

在 2021-01-20星期三的 11:20 +0100，Miklos Szeredi写道：
> On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:
> > On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io>
> > wrote:
> > > 
> > > The function ovl_dir_real_file() currently uses the semaphore of
> > > the
> > > inode to synchronize write to the upperfile cache field.
> > 
> > Although the inode lock is a rw_sem it is referred to as the "inode
> > lock"
> > and you also left semaphore in the commit subject.
> > No need to re-post. This can be fixed on commit.
> > 
> > > 
> > > However, this function will get called by ovl_ioctl_set_flags(),
> > > which
> > > utilizes the inode semaphore too. In this case
> > > ovl_dir_real_file() will
> > > try to claim a lock that is owned by a function in its call
> > > stack, which
> > > won't get released before ovl_dir_real_file() returns.
> > > 
> > > Define a dedicated semaphore for the upperfile cache, so that the
> > > deadlock won't happen.
> > > 
> > > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and
> > > FS[S|G]ETXATTR ioctls for directories")
> > > Cc: stable@vger.kernel.org # v5.10
> > > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > > ---
> > > Changes in v2:
> > > - Fixed missing replacement in error handling path.
> > > Changes in v3:
> > > - Use mutex instead of semaphore.
> > > 
> > >  fs/overlayfs/readdir.c | 10 +++++-----
> > >  1 file changed, 5 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > > index 01620ebae1bd..3980f9982f34 100644
> > > --- a/fs/overlayfs/readdir.c
> > > +++ b/fs/overlayfs/readdir.c
> > > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> > >         struct list_head *cursor;
> > >         struct file *realfile;
> > >         struct file *upperfile;
> > > +       struct mutex upperfile_mutex;
> > 
> > That's a very specific name.
> > This mutex protects members of struct ovl_dir_file, which could
> > evolve
> > into struct ovl_file one day (because no reason to cache only dir
> > upper file),
> > so I would go with a more generic name, but let's leave it to
> > Miklos to decide.
> > 
> > He could have a different idea altogether for fixing this bug.
> 
> How about this (untested) patch?
> 
> It's a cleanup as well as a fix, but maybe we should separate the
> cleanup from
> the fix...

If you are going to post this, feel free to add

Tested-by: Icenowy Zheng <icenowy@aosc.io>

(And if you remove the IS_ERR(realfile) part, the tested-by tag still
applies.)

> 
> Thanks,
> Miklos
> ---
> 
>  fs/overlayfs/readdir.c |   23 +++++++----------------
>  1 file changed, 7 insertions(+), 16 deletions(-)
> 
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const str
>  
>         struct ovl_dir_file *od = file->private_data;
>         struct dentry *dentry = file->f_path.dentry;
> -       struct file *realfile = od->realfile;
> +       struct file *old, *realfile = od->realfile;
>  
>         if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
>                 return want_upper ? NULL : realfile;
> @@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const str
>          * Need to check if we started out being a lower dir, but got
> copied up
>          */
>         if (!od->is_upper) {
> -               struct inode *inode = file_inode(file);
> -
>                 realfile = READ_ONCE(od->upperfile);
>                 if (!realfile) {
>                         struct path upperpath;
>  
>                         ovl_path_upper(dentry, &upperpath);
>                         realfile = ovl_dir_open_realfile(file,
> &upperpath);
> +                       if (IS_ERR(realfile))
> +                               return realfile;
>  
> -                       inode_lock(inode);
> -                       if (!od->upperfile) {
> -                               if (IS_ERR(realfile)) {
> -                                       inode_unlock(inode);
> -                                       return realfile;
> -                               }
> -                               smp_store_release(&od->upperfile,
> realfile);
> -                       } else {
> -                               /* somebody has beaten us to it */
> -                               if (!IS_ERR(realfile))
> -                                       fput(realfile);
> -                               realfile = od->upperfile;
> +                       old = cmpxchg_release(&od->upperfile, NULL,
> realfile);
> +                       if (old) {
> +                               fput(realfile);
> +                               realfile = old;
>                         }
> -                       inode_unlock(inode);
>                 }
>         }
>  

Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Miklos Szeredi]

On Thu, Jan 21, 2021 at 4:43 AM Icenowy Zheng <icenowy@aosc.io> wrote:
>
> 在 2021-01-20星期三的 11:20 +0100，Miklos Szeredi写道：
> > On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:
> > > On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io>
> > > wrote:
> > > >
> > > > The function ovl_dir_real_file() currently uses the semaphore of
> > > > the
> > > > inode to synchronize write to the upperfile cache field.
> > >
> > > Although the inode lock is a rw_sem it is referred to as the "inode
> > > lock"
> > > and you also left semaphore in the commit subject.
> > > No need to re-post. This can be fixed on commit.
> > >
> > > >
> > > > However, this function will get called by ovl_ioctl_set_flags(),
> > > > which
> > > > utilizes the inode semaphore too. In this case
> > > > ovl_dir_real_file() will
> > > > try to claim a lock that is owned by a function in its call
> > > > stack, which
> > > > won't get released before ovl_dir_real_file() returns.
> > > >
> > > > Define a dedicated semaphore for the upperfile cache, so that the
> > > > deadlock won't happen.
> > > >
> > > > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and
> > > > FS[S|G]ETXATTR ioctls for directories")
> > > > Cc: stable@vger.kernel.org # v5.10
> > > > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > > > ---
> > > > Changes in v2:
> > > > - Fixed missing replacement in error handling path.
> > > > Changes in v3:
> > > > - Use mutex instead of semaphore.
> > > >
> > > >  fs/overlayfs/readdir.c | 10 +++++-----
> > > >  1 file changed, 5 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > > > index 01620ebae1bd..3980f9982f34 100644
> > > > --- a/fs/overlayfs/readdir.c
> > > > +++ b/fs/overlayfs/readdir.c
> > > > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> > > >         struct list_head *cursor;
> > > >         struct file *realfile;
> > > >         struct file *upperfile;
> > > > +       struct mutex upperfile_mutex;
> > >
> > > That's a very specific name.
> > > This mutex protects members of struct ovl_dir_file, which could
> > > evolve
> > > into struct ovl_file one day (because no reason to cache only dir
> > > upper file),
> > > so I would go with a more generic name, but let's leave it to
> > > Miklos to decide.
> > >
> > > He could have a different idea altogether for fixing this bug.
> >
> > How about this (untested) patch?
> >
> > It's a cleanup as well as a fix, but maybe we should separate the
> > cleanup from
> > the fix...
>
> If you are going to post this, feel free to add
>
> Tested-by: Icenowy Zheng <icenowy@aosc.io>

Okay, thanks.

> (And if you remove the IS_ERR(realfile) part, the tested-by tag still
> applies.)

Dropping the IS_ERR(realfile) here would mean having to add the same
check before relevant fput() calls, which would make it more complex
not less.

Or did you mean something else?

Thanks,
Miklos

Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Icenowy Zheng]

在 2021-01-21星期四的 09:07 +0100，Miklos Szeredi写道：
> On Thu, Jan 21, 2021 at 4:43 AM Icenowy Zheng <icenowy@aosc.io>
> wrote:
> > 
> > 在 2021-01-20星期三的 11:20 +0100，Miklos Szeredi写道：
> > > On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:
> > > > On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io>
> > > > wrote:
> > > > > 
> > > > > The function ovl_dir_real_file() currently uses the semaphore
> > > > > of
> > > > > the
> > > > > inode to synchronize write to the upperfile cache field.
> > > > 
> > > > Although the inode lock is a rw_sem it is referred to as the
> > > > "inode
> > > > lock"
> > > > and you also left semaphore in the commit subject.
> > > > No need to re-post. This can be fixed on commit.
> > > > 
> > > > > 
> > > > > However, this function will get called by
> > > > > ovl_ioctl_set_flags(),
> > > > > which
> > > > > utilizes the inode semaphore too. In this case
> > > > > ovl_dir_real_file() will
> > > > > try to claim a lock that is owned by a function in its call
> > > > > stack, which
> > > > > won't get released before ovl_dir_real_file() returns.
> > > > > 
> > > > > Define a dedicated semaphore for the upperfile cache, so that
> > > > > the
> > > > > deadlock won't happen.
> > > > > 
> > > > > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and
> > > > > FS[S|G]ETXATTR ioctls for directories")
> > > > > Cc: stable@vger.kernel.org # v5.10
> > > > > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > > > > ---
> > > > > Changes in v2:
> > > > > - Fixed missing replacement in error handling path.
> > > > > Changes in v3:
> > > > > - Use mutex instead of semaphore.
> > > > > 
> > > > >  fs/overlayfs/readdir.c | 10 +++++-----
> > > > >  1 file changed, 5 insertions(+), 5 deletions(-)
> > > > > 
> > > > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > > > > index 01620ebae1bd..3980f9982f34 100644
> > > > > --- a/fs/overlayfs/readdir.c
> > > > > +++ b/fs/overlayfs/readdir.c
> > > > > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> > > > >         struct list_head *cursor;
> > > > >         struct file *realfile;
> > > > >         struct file *upperfile;
> > > > > +       struct mutex upperfile_mutex;
> > > > 
> > > > That's a very specific name.
> > > > This mutex protects members of struct ovl_dir_file, which could
> > > > evolve
> > > > into struct ovl_file one day (because no reason to cache only
> > > > dir
> > > > upper file),
> > > > so I would go with a more generic name, but let's leave it to
> > > > Miklos to decide.
> > > > 
> > > > He could have a different idea altogether for fixing this bug.
> > > 
> > > How about this (untested) patch?
> > > 
> > > It's a cleanup as well as a fix, but maybe we should separate the
> > > cleanup from
> > > the fix...
> > 
> > If you are going to post this, feel free to add
> > 
> > Tested-by: Icenowy Zheng <icenowy@aosc.io>
> 
> Okay, thanks.
> 
> > (And if you remove the IS_ERR(realfile) part, the tested-by tag
> > still
> > applies.)
> 
> Dropping the IS_ERR(realfile) here would mean having to add the same
> check before relevant fput() calls, which would make it more complex
> not less.
> 
> Or did you mean something else?

I mean "seperate the cleanup from the fix".

This is only for when you do the seperation.

> 
> Thanks,
> Miklos