[PATCH] proc/sysctl: don't return ENOMEM on lookup when a table is unregistering

[PATCH] proc/sysctl: don't return ENOMEM on lookup when a table is unregistering

[Ivan Delalande]

proc_sys_lookup can fail with ENOMEM instead of ENOENT when the
corresponding sysctl table is being unregistered. In our case we see
this upon opening /proc/sys/net/*/conf files while network interfaces
are being removed, which confuses our configuration daemon.

The problem was successfully reproduced and this fix tested on v4.9.122
and v4.20-rc6.

Fixes: ace0c791e6c3 ("proc/sysctl: Don't grab i_lock under sysctl_lock.")
Cc: stable@vger.kernel.org
Signed-off-by: Ivan Delalande <colona@arista.com>
---
 fs/proc/proc_sysctl.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 89921a0d2ebb..834be5bc3d07 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -474,7 +474,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 	if (unlikely(head->unregistering)) {
 		spin_unlock(&sysctl_lock);
 		iput(inode);
-		inode = NULL;
+		inode = ERR_PTR(-ENOENT);
 		goto out;
 	}
 	ei->sysctl = head;
@@ -549,10 +549,11 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
 			goto out;
 	}
 
-	err = ERR_PTR(-ENOMEM);
 	inode = proc_sys_make_inode(dir->i_sb, h ? h : head, p);
-	if (!inode)
+	if (IS_ERR_OR_NULL(inode)) {
+		err = inode ? ERR_CAST(inode) : ERR_PTR(-ENOMEM);
 		goto out;
+	}
 
 	d_set_d_op(dentry, &proc_sys_dentry_operations);
 	err = d_splice_alias(inode, dentry);
@@ -685,7 +686,7 @@ static bool proc_sys_fill_cache(struct file *file,
 		if (d_in_lookup(child)) {
 			struct dentry *res;
 			inode = proc_sys_make_inode(dir->d_sb, head, table);
-			if (!inode) {
+			if (IS_ERR_OR_NULL(inode)) {
 				d_lookup_done(child);
 				dput(child);
 				return false;
-- 
2.20.0

Re: [PATCH] proc/sysctl: don't return ENOMEM on lookup when a table is unregistering

[Al Viro]

On Wed, Dec 12, 2018 at 05:57:43PM -0800, Ivan Delalande wrote:
> proc_sys_lookup can fail with ENOMEM instead of ENOENT when the
> corresponding sysctl table is being unregistered. In our case we see
> this upon opening /proc/sys/net/*/conf files while network interfaces
> are being removed, which confuses our configuration daemon.
> 
> The problem was successfully reproduced and this fix tested on v4.9.122
> and v4.20-rc6.
> 
> Fixes: ace0c791e6c3 ("proc/sysctl: Don't grab i_lock under sysctl_lock.")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ivan Delalande <colona@arista.com>
> ---
>  fs/proc/proc_sysctl.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 89921a0d2ebb..834be5bc3d07 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -474,7 +474,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>  	if (unlikely(head->unregistering)) {
>  		spin_unlock(&sysctl_lock);
>  		iput(inode);
> -		inode = NULL;
> +		inode = ERR_PTR(-ENOENT);
>  		goto out;
>  	}
>  	ei->sysctl = head;
> @@ -549,10 +549,11 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
>  			goto out;
>  	}
>  
> -	err = ERR_PTR(-ENOMEM);
>  	inode = proc_sys_make_inode(dir->i_sb, h ? h : head, p);
> -	if (!inode)
> +	if (IS_ERR_OR_NULL(inode)) {
> +		err = inode ? ERR_CAST(inode) : ERR_PTR(-ENOMEM);

*gags*

If you want to return specific errors, do just that and for pity sake,
do *NOT* invent such hybrids.  "Pointer to object on success,
ERR_PTR(-E...) on failure, NULL means ERR_PTR(-ENOMEM)" is a bitch to
reason about and prone to breakage.

"Return NULL on error" and "return ERR_PTR() on error" do not mix.
Just make proc_sys_make_inode() return ERR_PTR(-ENOMEM) on allocation
failures and update the callers (as you have to, anyway).

IS_ERR_OR_NULL() is usually a sign of bad calling conventions and it
certainly is just that in this case.

Just do
        inode = new_inode(sb);
        if (!inode)
                return ERR_PTR(-ENOMEM);
in there, in addition to your return of ERR_PTR(-ENOENT), and lose those
IS_ERR_OR_NULL() things.  Make those IS_ERR() and turn the assignment
to err into straight ERR_CAST().  All there is to it...

[PATCH v2] proc/sysctl: don't return ENOMEM on lookup when a table is unregistering

[Ivan Delalande]

proc_sys_lookup can fail with ENOMEM instead of ENOENT when the
corresponding sysctl table is being unregistered. In our case we see
this upon opening /proc/sys/net/*/conf files while network interfaces
are being deleted, which confuses our configuration daemon.

The problem was successfully reproduced and this fix tested on v4.9.122
and v4.20-rc6.

v2: return ERR_PTRs in all cases when proc_sys_make_inode fails instead
of mixing them with NULL. Thanks Al Viro for the feedback.

Fixes: ace0c791e6c3 ("proc/sysctl: Don't grab i_lock under sysctl_lock.")
Cc: stable@vger.kernel.org
Signed-off-by: Ivan Delalande <colona@arista.com>
---
 fs/proc/proc_sysctl.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 89921a0d2ebb..e9d4fc40d832 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -464,7 +464,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 
 	inode = new_inode(sb);
 	if (!inode)
-		goto out;
+		return ERR_PTR(-ENOMEM);
 
 	inode->i_ino = get_next_ino();
 
@@ -474,7 +474,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
 	if (unlikely(head->unregistering)) {
 		spin_unlock(&sysctl_lock);
 		iput(inode);
-		inode = NULL;
+		inode = ERR_PTR(-ENOENT);
 		goto out;
 	}
 	ei->sysctl = head;
@@ -549,10 +549,11 @@ static struct dentry *proc_sys_lookup(struct inode *dir, struct dentry *dentry,
 			goto out;
 	}
 
-	err = ERR_PTR(-ENOMEM);
 	inode = proc_sys_make_inode(dir->i_sb, h ? h : head, p);
-	if (!inode)
+	if (IS_ERR(inode)) {
+		err = ERR_CAST(inode);
 		goto out;
+	}
 
 	d_set_d_op(dentry, &proc_sys_dentry_operations);
 	err = d_splice_alias(inode, dentry);
@@ -685,7 +686,7 @@ static bool proc_sys_fill_cache(struct file *file,
 		if (d_in_lookup(child)) {
 			struct dentry *res;
 			inode = proc_sys_make_inode(dir->d_sb, head, table);
-			if (!inode) {
+			if (IS_ERR(inode)) {
 				d_lookup_done(child);
 				dput(child);
 				return false;
-- 
2.20.0

Re: [PATCH v2] proc/sysctl: don't return ENOMEM on lookup when a table is unregistering

[Al Viro]

On Thu, Dec 13, 2018 at 03:20:52PM -0800, Ivan Delalande wrote:

> @@ -474,7 +474,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>  	if (unlikely(head->unregistering)) {
>  		spin_unlock(&sysctl_lock);
>  		iput(inode);
> -		inode = NULL;
> +		inode = ERR_PTR(-ENOENT);
>  		goto out;
>  	}

Applied, with one modification: if you look at the target of that goto,
you'll see
out:
        return inode;
so this place should be simply
 		spin_unlock(&sysctl_lock);
 		iput(inode);
		return ERR_PTR(-ENOENT);
	}

That way the label becomes unused and goes away.