Forwarded: [PATCH] 9p: add printk diagnostics to identify -ENOENT source in v9fs_fid_lookup

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Subject: [PATCH] 9p: add printk diagnostics to identify -ENOENT source in v9fs_fid_lookup
Author: kartikey406@xxxxxxxxx

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master


Add printk(KERN_WARNING) at the three points in
v9fs_fid_lookup_with_uid() where a racing unlink/rename can cause a
symlink fid walk to fail, to determine which one produces the
-ENOENT seen in the "WARNING in v9fs_init_request (2)" reproducer.

Diagnostic only, not a functional fix.

Reported-by: syzbot+344c09c64fcd8d3d2782@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Deepanshu Kartikey <kartikey406@xxxxxxxxx>
---
 fs/9p/fid.c      | 10 ++++++++++
 fs/9p/vfs_addr.c | 37 ++++++++++++++++++++++++-------------
 2 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/fs/9p/fid.c b/fs/9p/fid.c
index 76242d450aa7..d8d79a4c88ef 100644
--- a/fs/9p/fid.c
+++ b/fs/9p/fid.c
@@ -194,6 +194,10 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct dentry *dentry,
 		old_fid = fid;
 
 		fid = p9_client_walk(old_fid, 1, &dentry->d_name.name, 1);
+		if (IS_ERR(fid)) {
+			printk(KERN_WARNING "DIAG: single-component p9_client_walk "
+			       "failed, name=%pd err=%ld\n", dentry, PTR_ERR(fid));
+		}
 		p9_fid_put(old_fid);
 		goto fid_out;
 	}
@@ -251,6 +255,9 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct dentry *dentry,
 			old_fid = fid;
 		}
 		if (IS_ERR(fid)) {
+			printk(KERN_WARNING "DIAG: multipath p9_client_walk failed, "
+			       "name=%pd component_index=%d err=%ld\n",
+	                        dentry, i, PTR_ERR(fid));
 			kfree(wnames);
 			goto err_out;
 		}
@@ -262,6 +269,9 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct dentry *dentry,
 		spin_lock(&dentry->d_lock);
 		if (d_unhashed(dentry)) {
 			spin_unlock(&dentry->d_lock);
+			printk(KERN_WARNING "DIAG: walk succeeded but dentry "
+			         "unhashed, manufacturing -ENOENT, name=%pd\n",
+				 dentry);
 			p9_fid_put(fid);
 			fid = ERR_PTR(-ENOENT);
 		} else {
diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
index 1ac0b3dcc077..fc05bd963506 100644
--- a/fs/9p/vfs_addr.c
+++ b/fs/9p/vfs_addr.c
@@ -136,21 +136,37 @@ static int v9fs_init_request(struct netfs_io_request *rreq, struct file *file)
 
 	if (file) {
 		fid = file->private_data;
-		if (!fid)
-			goto no_fid;
+		if (!fid) {
+			WARN_ONCE(1, "DIAG: v9fs_init_request: file has no fid, "
+			           "inode->i_ino=%llx\n", rreq->inode->i_ino);
+			return -EINVAL;
+		}
 		p9_fid_get(fid);
 	} else if (S_ISLNK(rreq->inode->i_mode)) {
 		dentry = d_find_any_alias(rreq->inode);
-		if (!dentry)
-			goto no_fid;
+		if (!dentry) {
+			WARN_ONCE(1, "DIAG: v9fs_init_request: symlink inode has no "
+				  "dentry alias, inode->i_ino=%llx\n",
+				   rreq->inode->i_ino);
+			return -EINVAL;
+		}
 		fid = v9fs_fid_lookup(dentry);
+		if (IS_ERR(fid)) {
+			WARN_ONCE(1, "DIAG: v9fs_init_request: v9fs_fid_lookup failed "
+				"for symlink, err=%ld dentry=%pd4 inode->i_ino=%llx\n",
+			 	PTR_ERR(fid), dentry, rreq->inode->i_ino);
+			dput(dentry);
+			return PTR_ERR(fid);
+		}
 		dput(dentry);
-		if (IS_ERR(fid))
-			goto no_fid;
 	} else {
 		fid = v9fs_fid_find_inode(rreq->inode, writing, INVALID_UID, true);
-		if (!fid)
-			goto no_fid;
+		if (!fid) {
+			WARN_ONCE(1, "DIAG: v9fs_init_request: non-symlink inode has "
+				   "no open/writeback fid, inode->i_ino=%llx\n",
+				   rreq->inode->i_ino);
+			return -EINVAL;
+		}
 	}
 
 	rreq->wsize = fid->clnt->msize - P9_IOHDRSZ;
@@ -163,11 +179,6 @@ static int v9fs_init_request(struct netfs_io_request *rreq, struct file *file)
 	WARN_ON(rreq->origin == NETFS_READ_FOR_WRITE && !(fid->mode & P9_ORDWR));
 	rreq->netfs_priv = fid;
 	return 0;
-
-no_fid:
-	WARN_ONCE(1, "folio expected an open fid inode->i_ino=%llx\n",
-		  rreq->inode->i_ino);
-	return -EINVAL;
 }
 
 /**
-- 
2.43.0