Re: [Qemu-devel] [V9fs-developer] [Bug 1336794] Re: 9pfs does not honor open file handles on unlinked files

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Apr 13, 2015 at 04:05:28PM +0000, Eric Van Hensbergen wrote:
> Well, technically fid 3 isn't 'open', only fid 2 is open - at least
> according to the protocol.  fid 3 and fid 2 are both clones of fid 1.
> However, thanks for the alternative workaround.  If you get a chance, can
> you check that my change to the client to partially fix this for the other
> servers doesn't break nfs-ganesha:
> 
> https://github.com/ericvh/linux/commit/fddc7721d6d19e4e6be4905f37ade5b0521f4ed5

BTW, what the hell is going on in v9fs_vfs_mknod() and v9fs_vfs_link()?
You allocate 4Kb buffer, sprintf into it ("b %u %u", "c %u %u", or "%d\n")
feed it to v9fs_vfs_mkspecial() and immediately free it.  What's wrong with
a local array of char?  In the worst case it needs to be char name[24] -
surely, we are not so tight on stack that extra 16 bytes (that array instead
of a pointer) would drive us over the cliff?

IOW, do you have any problem with this:
diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index 703342e..cda68f7 100644
--- a/fs/9p/vfs_inode.c
+++ b/fs/9p/vfs_inode.c
@@ -1370,6 +1370,8 @@ v9fs_vfs_symlink(struct inode *dir, struct dentry *dentry, const char *symname)
 	return v9fs_vfs_mkspecial(dir, dentry, P9_DMSYMLINK, symname);
 }
 
+#define U32_MAX_DIGITS 10
+
 /**
  * v9fs_vfs_link - create a hardlink
  * @old_dentry: dentry for file to link to
@@ -1383,7 +1385,7 @@ v9fs_vfs_link(struct dentry *old_dentry, struct inode *dir,
 	      struct dentry *dentry)
 {
 	int retval;
-	char *name;
+	char name[1 + U32_MAX_DIGITS + 2]; /* sign + number + \n + \0 */
 	struct p9_fid *oldfid;
 
 	p9_debug(P9_DEBUG_VFS, " %lu,%pd,%pd\n",
@@ -1393,20 +1395,12 @@ v9fs_vfs_link(struct dentry *old_dentry, struct inode *dir,
 	if (IS_ERR(oldfid))
 		return PTR_ERR(oldfid);
 
-	name = __getname();
-	if (unlikely(!name)) {
-		retval = -ENOMEM;
-		goto clunk_fid;
-	}
-
 	sprintf(name, "%d\n", oldfid->fid);
 	retval = v9fs_vfs_mkspecial(dir, dentry, P9_DMLINK, name);
-	__putname(name);
 	if (!retval) {
 		v9fs_refresh_inode(oldfid, d_inode(old_dentry));
 		v9fs_invalidate_inode_attr(dir);
 	}
-clunk_fid:
 	p9_client_clunk(oldfid);
 	return retval;
 }
@@ -1425,7 +1419,7 @@ v9fs_vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t rde
 {
 	struct v9fs_session_info *v9ses = v9fs_inode2v9ses(dir);
 	int retval;
-	char *name;
+	char name[2 + U32_MAX_DIGITS + 1 + U32_MAX_DIGITS + 1];
 	u32 perm;
 
 	p9_debug(P9_DEBUG_VFS, " %lu,%pd mode: %hx MAJOR: %u MINOR: %u\n",
@@ -1435,26 +1429,16 @@ v9fs_vfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t rde
 	if (!new_valid_dev(rdev))
 		return -EINVAL;
 
-	name = __getname();
-	if (!name)
-		return -ENOMEM;
 	/* build extension */
 	if (S_ISBLK(mode))
 		sprintf(name, "b %u %u", MAJOR(rdev), MINOR(rdev));
 	else if (S_ISCHR(mode))
 		sprintf(name, "c %u %u", MAJOR(rdev), MINOR(rdev));
-	else if (S_ISFIFO(mode))
-		*name = 0;
-	else if (S_ISSOCK(mode))
+	else
 		*name = 0;
-	else {
-		__putname(name);
-		return -EINVAL;
-	}
 
 	perm = unixmode2p9mode(v9ses, mode);
 	retval = v9fs_vfs_mkspecial(dir, dentry, perm, name);
-	__putname(name);
 
 	return retval;
 }