[PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[SeongJae Park]

From: Badari Pulavarty <badari.pulavarty@intel.com>

When user tries to create a DAMON context via the DAMON debugfs
interface with a name of an already existing context, the context
directory creation silently fails but the context is added in the
internal data structure.  As a result, memory could leak and DAMON
cannot be turned on.  An example test case is as below:

    # cd /sys/kernel/debug/damon/
    # echo "off" >  monitor_on
    # echo paddr > target_ids
    # echo "abc" > mk_context
    # echo "abc" > mk_context
    # echo $$ > abc/target_ids
    # echo "on" > monitor_on  <<< fails

This commit fixes the issue by checking if the name already exist and
immediately returning '-EEXIST' in the case.

Fixes: 75c1c2b53c78 ("mm/damon/dbgfs: support multiple contexts")
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Badari Pulavarty <badari.pulavarty@intel.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
---
Changes from v1
(https://lore.kernel.org/damon/DM6PR11MB3978994F75A4104D714437379C679@DM6PR11MB3978.namprd11.prod.outlook.com/)
- Manually check duplicate entry instead of checking
  'debugfs_create_dir()' return value
- Reword commit message and the test case

Seems Badari have some email client issue, so I (SJ) am making this
second version of the patch based on Badari's final proposal and repost
on behalf of Badari.

 mm/damon/dbgfs.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/damon/dbgfs.c b/mm/damon/dbgfs.c
index 51d67c8050dd..364b44063c2f 100644
--- a/mm/damon/dbgfs.c
+++ b/mm/damon/dbgfs.c
@@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
  */
 static int dbgfs_mk_context(char *name)
 {
-	struct dentry *root, **new_dirs, *new_dir;
+	struct dentry *root, **new_dirs, *new_dir, *dir;
 	struct damon_ctx **new_ctxs, *new_ctx;
 
 	if (damon_nr_running_ctxs())
@@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name)
 	if (!root)
 		return -ENOENT;
 
+	dir = debugfs_lookup(name, root);
+	if (dir) {
+		dput(dir);
+		return -EEXIST;
+	}
+
 	new_dir = debugfs_create_dir(name, root);
 	dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;
 
-- 
2.25.1

Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[Andrew Morton]

On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote:

> From: Badari Pulavarty <badari.pulavarty@intel.com>
> 
> When user tries to create a DAMON context via the DAMON debugfs
> interface with a name of an already existing context, the context
> directory creation silently fails but the context is added in the
> internal data structure.  As a result, memory could leak and DAMON
> cannot be turned on.  An example test case is as below:
> 
>     # cd /sys/kernel/debug/damon/
>     # echo "off" >  monitor_on
>     # echo paddr > target_ids
>     # echo "abc" > mk_context
>     # echo "abc" > mk_context
>     # echo $$ > abc/target_ids
>     # echo "on" > monitor_on  <<< fails
> 
> This commit fixes the issue by checking if the name already exist and
> immediately returning '-EEXIST' in the case.
> 
> ...
>
> --- a/mm/damon/dbgfs.c
> +++ b/mm/damon/dbgfs.c
> @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
>   */
>  static int dbgfs_mk_context(char *name)
>  {
> -	struct dentry *root, **new_dirs, *new_dir;
> +	struct dentry *root, **new_dirs, *new_dir, *dir;
>  	struct damon_ctx **new_ctxs, *new_ctx;
>  
>  	if (damon_nr_running_ctxs())
> @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name)
>  	if (!root)
>  		return -ENOENT;
>  
> +	dir = debugfs_lookup(name, root);
> +	if (dir) {
> +		dput(dir);
> +		return -EEXIST;
> +	}
> +
>  	new_dir = debugfs_create_dir(name, root);
>  	dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;

It would be simpler (and less racy) to check the debugfs_create_dir()
return value for IS_ERR()?

Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[SeongJae Park]

On Fri, 19 Aug 2022 14:08:09 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote:
> 
> > From: Badari Pulavarty <badari.pulavarty@intel.com>
> > 
> > When user tries to create a DAMON context via the DAMON debugfs
> > interface with a name of an already existing context, the context
> > directory creation silently fails but the context is added in the
> > internal data structure.  As a result, memory could leak and DAMON
> > cannot be turned on.  An example test case is as below:
> > 
> >     # cd /sys/kernel/debug/damon/
> >     # echo "off" >  monitor_on
> >     # echo paddr > target_ids
> >     # echo "abc" > mk_context
> >     # echo "abc" > mk_context
> >     # echo $$ > abc/target_ids
> >     # echo "on" > monitor_on  <<< fails
> > 
> > This commit fixes the issue by checking if the name already exist and
> > immediately returning '-EEXIST' in the case.
> > 
> > ...
> >
> > --- a/mm/damon/dbgfs.c
> > +++ b/mm/damon/dbgfs.c
> > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
> >   */
> >  static int dbgfs_mk_context(char *name)
> >  {
> > -	struct dentry *root, **new_dirs, *new_dir;
> > +	struct dentry *root, **new_dirs, *new_dir, *dir;
> >  	struct damon_ctx **new_ctxs, *new_ctx;
> >  
> >  	if (damon_nr_running_ctxs())
> > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name)
> >  	if (!root)
> >  		return -ENOENT;
> >  
> > +	dir = debugfs_lookup(name, root);
> > +	if (dir) {
> > +		dput(dir);
> > +		return -EEXIST;
> > +	}
> > +
> >  	new_dir = debugfs_create_dir(name, root);
> >  	dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;
> 
> It would be simpler (and less racy) to check the debugfs_create_dir()
> return value for IS_ERR()?

I was merely following Greg's previous advice for ignoring the return value[1]
of the function, but I might misunderstanding his intention, so CC-ing Greg.
Greg, may I ask your opinion?

[1] https://lore.kernel.org/linux-mm/YB1kZaD%2F7omxXztF@kroah.com/


Thanks,
SJ

Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[Andrew Morton]

On Fri, 19 Aug 2022 21:16:31 +0000 SeongJae Park <sj@kernel.org> wrote:

> > It would be simpler (and less racy) to check the debugfs_create_dir()
> > return value for IS_ERR()?
> 
> I was merely following Greg's previous advice for ignoring the return value[1]
> of the function, but I might misunderstanding his intention, so CC-ing Greg.
> Greg, may I ask your opinion?
> 
> [1] https://lore.kernel.org/linux-mm/YB1kZaD%2F7omxXztF@kroah.com/

Thing is, the correct functioning of the debugfs interfaces is utterly
critical to damon.  And that's apart from these memory leak and
oops-we-killed-damon issues.

So damon simply cannot ignore the state of its debugfs interfaces and
keep going along - because if something goes wrong at the debugfs
layer, damon is dead and useless and the machine needs a reboot.

Perhaps this means that damon should not be using debugfs for its
interfaces at all.  Or it means that the debugfs interfaces are
misdesigned.  I go with the latter, which, alas, also affirms the
former.

From a quick scan it appears that a significant minority (20%?) of
drivers are checking the debugfs_create_dir() return value.

Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[SeongJae Park]

On Fri, 19 Aug 2022 15:44:51 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 19 Aug 2022 21:16:31 +0000 SeongJae Park <sj@kernel.org> wrote:
> 
> > > It would be simpler (and less racy) to check the debugfs_create_dir()
> > > return value for IS_ERR()?
> > 
> > I was merely following Greg's previous advice for ignoring the return value[1]
> > of the function, but I might misunderstanding his intention, so CC-ing Greg.
> > Greg, may I ask your opinion?
> > 
> > [1] https://lore.kernel.org/linux-mm/YB1kZaD%2F7omxXztF@kroah.com/
> 
> Thing is, the correct functioning of the debugfs interfaces is utterly
> critical to damon.  And that's apart from these memory leak and
> oops-we-killed-damon issues.
> 
> So damon simply cannot ignore the state of its debugfs interfaces and
> keep going along - because if something goes wrong at the debugfs
> layer, damon is dead and useless and the machine needs a reboot.
> 
> Perhaps this means that damon should not be using debugfs for its
> interfaces at all.  Or it means that the debugfs interfaces are
> misdesigned.  I go with the latter, which, alas, also affirms the
> former.

I'd save my word about the latter, but agreed on the former.  Fortunately we
already have an alternative (DAMON sysfs interface), and the debugfs interface
deprecation plan was announced for a while ago.  Not sure if the deprecation
will be well as hoped, though.

> 
> From a quick scan it appears that a significant minority (20%?) of
> drivers are checking the debugfs_create_dir() return value.

Maybe partly owing to Greg's previous efforts for removing the checks[1,2]?
Anyway, based on the previous discussions, I'd expect Greg might prefer not
checking the return code.

Anyway, waiting for his opinion.

[1] https://lore.kernel.org/all/20190122152151.16139-14-gregkh@linuxfoundation.org/
[2] https://lore.kernel.org/lkml/20190122152151.16139-7-gregkh@linuxfoundation.org/


Thanks,
SJ

Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[Bagas Sanjaya]

On 8/20/22 00:19, SeongJae Park wrote:
> From: Badari Pulavarty <badari.pulavarty@intel.com>
> 
> When user tries to create a DAMON context via the DAMON debugfs
> interface with a name of an already existing context, the context
> directory creation silently fails but the context is added in the
> internal data structure.  As a result, memory could leak and DAMON
> cannot be turned on.  An example test case is as below:
> 
>     # cd /sys/kernel/debug/damon/
>     # echo "off" >  monitor_on
>     # echo paddr > target_ids
>     # echo "abc" > mk_context
>     # echo "abc" > mk_context
>     # echo $$ > abc/target_ids
>     # echo "on" > monitor_on  <<< fails
> 
> This commit fixes the issue by checking if the name already exist and
> immediately returning '-EEXIST' in the case.
> 

Meh...

SJ, I have seen most (if not all of) your patches uses descriptive mood
instead of imperative. Better say "Fix the issue by checking ...".

-- 
An old man doll... just what I always wanted! - Clara

Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[Greg KH]

On Fri, Aug 19, 2022 at 02:08:09PM -0700, Andrew Morton wrote:
> On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote:
> 
> > From: Badari Pulavarty <badari.pulavarty@intel.com>
> > 
> > When user tries to create a DAMON context via the DAMON debugfs
> > interface with a name of an already existing context, the context
> > directory creation silently fails but the context is added in the
> > internal data structure.  As a result, memory could leak and DAMON
> > cannot be turned on.  An example test case is as below:
> > 
> >     # cd /sys/kernel/debug/damon/
> >     # echo "off" >  monitor_on
> >     # echo paddr > target_ids
> >     # echo "abc" > mk_context
> >     # echo "abc" > mk_context
> >     # echo $$ > abc/target_ids
> >     # echo "on" > monitor_on  <<< fails
> > 
> > This commit fixes the issue by checking if the name already exist and
> > immediately returning '-EEXIST' in the case.
> > 
> > ...
> >
> > --- a/mm/damon/dbgfs.c
> > +++ b/mm/damon/dbgfs.c
> > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
> >   */
> >  static int dbgfs_mk_context(char *name)
> >  {
> > -	struct dentry *root, **new_dirs, *new_dir;
> > +	struct dentry *root, **new_dirs, *new_dir, *dir;
> >  	struct damon_ctx **new_ctxs, *new_ctx;
> >  
> >  	if (damon_nr_running_ctxs())
> > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name)
> >  	if (!root)
> >  		return -ENOENT;
> >  
> > +	dir = debugfs_lookup(name, root);
> > +	if (dir) {
> > +		dput(dir);
> > +		return -EEXIST;
> > +	}
> > +
> >  	new_dir = debugfs_create_dir(name, root);
> >  	dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;
> 
> It would be simpler (and less racy) to check the debugfs_create_dir()
> return value for IS_ERR()?
> 

Yes, if you _HAVE_ to know if the code works properly (i.e. because your
feature totally depends on debugfs like damon does), or you have a
potential duplicate name like this, then sure, check the return value
and do something based on it.

It's odd enough that you should put a comment above the check just so I
don't go and send a patch to delete it later on :)

thanks,

greg k-h

Re: [PATCH v2] mm/damon/dbgfs: avoid duplicate context directory creation

[SeongJae Park]

Hi Greg,

On Sat, 20 Aug 2022 19:32:37 +0200 Greg KH <gregkh@linuxfoundation.org> wrote:

> On Fri, Aug 19, 2022 at 02:08:09PM -0700, Andrew Morton wrote:
> > On Fri, 19 Aug 2022 17:19:30 +0000 SeongJae Park <sj@kernel.org> wrote:
> > 
> > > From: Badari Pulavarty <badari.pulavarty@intel.com>
> > > 
> > > When user tries to create a DAMON context via the DAMON debugfs
> > > interface with a name of an already existing context, the context
> > > directory creation silently fails but the context is added in the
> > > internal data structure.  As a result, memory could leak and DAMON
> > > cannot be turned on.  An example test case is as below:
> > > 
> > >     # cd /sys/kernel/debug/damon/
> > >     # echo "off" >  monitor_on
> > >     # echo paddr > target_ids
> > >     # echo "abc" > mk_context
> > >     # echo "abc" > mk_context
> > >     # echo $$ > abc/target_ids
> > >     # echo "on" > monitor_on  <<< fails
> > > 
> > > This commit fixes the issue by checking if the name already exist and
> > > immediately returning '-EEXIST' in the case.
> > > 
> > > ...
> > >
> > > --- a/mm/damon/dbgfs.c
> > > +++ b/mm/damon/dbgfs.c
> > > @@ -795,7 +795,7 @@ static void dbgfs_destroy_ctx(struct damon_ctx *ctx)
> > >   */
> > >  static int dbgfs_mk_context(char *name)
> > >  {
> > > -	struct dentry *root, **new_dirs, *new_dir;
> > > +	struct dentry *root, **new_dirs, *new_dir, *dir;
> > >  	struct damon_ctx **new_ctxs, *new_ctx;
> > >  
> > >  	if (damon_nr_running_ctxs())
> > > @@ -817,6 +817,12 @@ static int dbgfs_mk_context(char *name)
> > >  	if (!root)
> > >  		return -ENOENT;
> > >  
> > > +	dir = debugfs_lookup(name, root);
> > > +	if (dir) {
> > > +		dput(dir);
> > > +		return -EEXIST;
> > > +	}
> > > +
> > >  	new_dir = debugfs_create_dir(name, root);
> > >  	dbgfs_dirs[dbgfs_nr_ctxs] = new_dir;
> > 
> > It would be simpler (and less racy) to check the debugfs_create_dir()
> > return value for IS_ERR()?
> > 
> 
> Yes, if you _HAVE_ to know if the code works properly (i.e. because your
> feature totally depends on debugfs like damon does), or you have a
> potential duplicate name like this, then sure, check the return value
> and do something based on it.
> 
> It's odd enough that you should put a comment above the check just so I
> don't go and send a patch to delete it later on :)

Thank you for the kind explanation, Greg.  I will revise this patch to simply
check the return value with a comment noticing it's really needed due to the
potential duplicate names.


Thanks,
SJ


> 
> thanks,
> 
> greg k-h
>