Merge branch 'work.d_path' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

Pull vfs d_path() updates from Al Viro:
 "d_path.c refactoring"

* 'work.d_path' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  getcwd(2): clean up error handling
  d_path: prepend_path() is unlikely to return non-zero
  d_path: prepend_path(): lift the inner loop into a new helper
  d_path: prepend_path(): lift resetting b in case when we'd return 3 out of loop
  d_path: prepend_path(): get rid of vfsmnt
  d_path: introduce struct prepend_buffer
  d_path: make prepend_name() boolean
  d_path: lift -ENAMETOOLONG handling into callers of prepend_path()
  d_path: don't bother with return value of prepend()
  getcwd(2): saner logics around prepend_path() call
  d_path: get rid of path_with_deleted()
  d_path: regularize handling of root dentry in __dentry_path()
  d_path: saner calling conventions for __dentry_path()
  d_path: "\0" is {0,0}, not {0}

fs/d_path.c

@@ -8,14 +8,27 @@
 #include <linux/prefetch.h>
 #include "mount.h"
 
-static int prepend(char **buffer, int *buflen, const char *str, int namelen)
+struct prepend_buffer {
+	char *buf;
+	int len;
+};
+#define DECLARE_BUFFER(__name, __buf, __len) \
+	struct prepend_buffer __name = {.buf = __buf + __len, .len = __len}
+
+static char *extract_string(struct prepend_buffer *p)
 {
-	*buflen -= namelen;
-	if (*buflen < 0)
-		return -ENAMETOOLONG;
-	*buffer -= namelen;
-	memcpy(*buffer, str, namelen);
-	return 0;
+	if (likely(p->len >= 0))
+		return p->buf;
+	return ERR_PTR(-ENAMETOOLONG);
+}
+
+static void prepend(struct prepend_buffer *p, const char *str, int namelen)
+{
+	p->len -= namelen;
+	if (likely(p->len >= 0)) {
+		p->buf -= namelen;
+		memcpy(p->buf, str, namelen);
+	}
 }
 
 /**
@@ -35,22 +48,58 @@ static int prepend(char **buffer, int *buflen, const char *str, int namelen)
  *
  * Load acquire is needed to make sure that we see that terminating NUL.
  */
-static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
+static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
 {
 	const char *dname = smp_load_acquire(&name->name); /* ^^^ */
 	u32 dlen = READ_ONCE(name->len);
-	char *p;
+	char *s;
 
-	*buflen -= dlen + 1;
-	if (*buflen < 0)
-		return -ENAMETOOLONG;
-	p = *buffer -= dlen + 1;
-	*p++ = '/';
+	p->len -= dlen + 1;
+	if (unlikely(p->len < 0))
+		return false;
+	s = p->buf -= dlen + 1;
+	*s++ = '/';
 	while (dlen--) {
 		char c = *dname++;
 		if (!c)
 			break;
-		*p++ = c;
+		*s++ = c;
+	}
+	return true;
+}
+
+static int __prepend_path(const struct dentry *dentry, const struct mount *mnt,
+			  const struct path *root, struct prepend_buffer *p)
+{
+	while (dentry != root->dentry || &mnt->mnt != root->mnt) {
+		const struct dentry *parent = READ_ONCE(dentry->d_parent);
+
+		if (dentry == mnt->mnt.mnt_root) {
+			struct mount *m = READ_ONCE(mnt->mnt_parent);
+			struct mnt_namespace *mnt_ns;
+
+			if (likely(mnt != m)) {
+				dentry = READ_ONCE(mnt->mnt_mountpoint);
+				mnt = m;
+				continue;
+			}
+			/* Global root */
+			mnt_ns = READ_ONCE(mnt->mnt_ns);
+			/* open-coded is_mounted() to use local mnt_ns */
+			if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
+				return 1;	// absolute root
+			else
+				return 2;	// detached or not attached yet
+		}
+
+		if (unlikely(dentry == parent))
+			/* Escaped? */
+			return 3;
+
+		prefetch(parent);
+		if (!prepend_name(p, &dentry->d_name))
+			break;
+		dentry = parent;
 	}
 	return 0;
 }
@@ -74,15 +123,11 @@ static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
  */
 static int prepend_path(const struct path *path,
 			const struct path *root,
-			char **buffer, int *buflen)
+			struct prepend_buffer *p)
 {
-	struct dentry *dentry;
-	struct vfsmount *vfsmnt;
-	struct mount *mnt;
-	int error = 0;
 	unsigned seq, m_seq = 0;
-	char *bptr;
-	int blen;
+	struct prepend_buffer b;
+	int error;
 
 	rcu_read_lock();
 restart_mnt:
@@ -90,50 +135,9 @@ static int prepend_path(const struct path *path,
 	seq = 0;
 	rcu_read_lock();
 restart:
-	bptr = *buffer;
-	blen = *buflen;
-	error = 0;
-	dentry = path->dentry;
-	vfsmnt = path->mnt;
-	mnt = real_mount(vfsmnt);
+	b = *p;
 	read_seqbegin_or_lock(&rename_lock, &seq);
-	while (dentry != root->dentry || vfsmnt != root->mnt) {
-		struct dentry * parent;
-
-		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
-			struct mount *parent = READ_ONCE(mnt->mnt_parent);
-			struct mnt_namespace *mnt_ns;
-
-			/* Escaped? */
-			if (dentry != vfsmnt->mnt_root) {
-				bptr = *buffer;
-				blen = *buflen;
-				error = 3;
-				break;
-			}
-			/* Global root? */
-			if (mnt != parent) {
-				dentry = READ_ONCE(mnt->mnt_mountpoint);
-				mnt = parent;
-				vfsmnt = &mnt->mnt;
-				continue;
-			}
-			mnt_ns = READ_ONCE(mnt->mnt_ns);
-			/* open-coded is_mounted() to use local mnt_ns */
-			if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
-				error = 1;	// absolute root
-			else
-				error = 2;	// detached or not attached yet
-			break;
-		}
-		parent = dentry->d_parent;
-		prefetch(parent);
-		error = prepend_name(&bptr, &blen, &dentry->d_name);
-		if (error)
-			break;
-
-		dentry = parent;
-	}
+	error = __prepend_path(path->dentry, real_mount(path->mnt), root, &b);
 	if (!(seq & 1))
 		rcu_read_unlock();
 	if (need_seqretry(&rename_lock, seq)) {
@@ -150,14 +154,13 @@ static int prepend_path(const struct path *path,
 	}
 	done_seqretry(&mount_lock, m_seq);
 
-	if (error >= 0 && bptr == *buffer) {
-		if (--blen < 0)
-			error = -ENAMETOOLONG;
-		else
-			*--bptr = '/';
-	}
-	*buffer = bptr;
-	*buflen = blen;
+	if (unlikely(error == 3))
+		b = *p;
+
+	if (b.len == p->len)
+		prepend(&b, "/", 1);
+
+	*p = b;
 	return error;
 }
 
@@ -181,56 +184,24 @@ char *__d_path(const struct path *path,
 	       const struct path *root,
 	       char *buf, int buflen)
 {
-	char *res = buf + buflen;
-	int error;
-
-	prepend(&res, &buflen, "\0", 1);
-	error = prepend_path(path, root, &res, &buflen);
+	DECLARE_BUFFER(b, buf, buflen);
 
-	if (error < 0)
-		return ERR_PTR(error);
-	if (error > 0)
+	prepend(&b, "", 1);
+	if (unlikely(prepend_path(path, root, &b) > 0))
 		return NULL;
-	return res;
+	return extract_string(&b);
 }
 
 char *d_absolute_path(const struct path *path,
 	       char *buf, int buflen)
 {
 	struct path root = {};
-	char *res = buf + buflen;
-	int error;
-
-	prepend(&res, &buflen, "\0", 1);
-	error = prepend_path(path, &root, &res, &buflen);
-
-	if (error > 1)
-		error = -EINVAL;
-	if (error < 0)
-		return ERR_PTR(error);
-	return res;
-}
-
-/*
- * same as __d_path but appends "(deleted)" for unlinked files.
- */
-static int path_with_deleted(const struct path *path,
-			     const struct path *root,
-			     char **buf, int *buflen)
-{
-	prepend(buf, buflen, "\0", 1);
-	if (d_unlinked(path->dentry)) {
-		int error = prepend(buf, buflen, " (deleted)", 10);
-		if (error)
-			return error;
-	}
-
-	return prepend_path(path, root, buf, buflen);
-}
+	DECLARE_BUFFER(b, buf, buflen);
 
-static int prepend_unreachable(char **buffer, int *buflen)
-{
-	return prepend(buffer, buflen, "(unreachable)", 13);
+	prepend(&b, "", 1);
+	if (unlikely(prepend_path(path, &root, &b) > 1))
+		return ERR_PTR(-EINVAL);
+	return extract_string(&b);
 }
 
 static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
@@ -261,9 +232,8 @@ static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
  */
 char *d_path(const struct path *path, char *buf, int buflen)
 {
-	char *res = buf + buflen;
+	DECLARE_BUFFER(b, buf, buflen);
 	struct path root;
-	int error;
 
 	/*
 	 * We have various synthetic filesystems that never get mounted.  On
@@ -282,12 +252,14 @@ char *d_path(const struct path *path, char *buf, int buflen)
 
 	rcu_read_lock();
 	get_fs_root_rcu(current->fs, &root);
-	error = path_with_deleted(path, &root, &res, &buflen);
+	if (unlikely(d_unlinked(path->dentry)))
+		prepend(&b, " (deleted)", 11);
+	else
+		prepend(&b, "", 1);
+	prepend_path(path, &root, &b);
 	rcu_read_unlock();
 
-	if (error < 0)
-		res = ERR_PTR(error);
-	return res;
+	return extract_string(&b);
 }
 EXPORT_SYMBOL(d_path);
 
@@ -314,47 +286,34 @@ char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
 
 char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
 {
-	char *end = buffer + buflen;
+	DECLARE_BUFFER(b, buffer, buflen);
 	/* these dentries are never renamed, so d_lock is not needed */
-	if (prepend(&end, &buflen, " (deleted)", 11) ||
-	    prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
-	    prepend(&end, &buflen, "/", 1))  
-		end = ERR_PTR(-ENAMETOOLONG);
-	return end;
+	prepend(&b, " (deleted)", 11);
+	prepend(&b, dentry->d_name.name, dentry->d_name.len);
+	prepend(&b, "/", 1);
+	return extract_string(&b);
 }
 
 /*
  * Write full pathname from the root of the filesystem into the buffer.
  */
-static char *__dentry_path(const struct dentry *d, char *buf, int buflen)
+static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
 {
 	const struct dentry *dentry;
-	char *end, *retval;
-	int len, seq = 0;
-	int error = 0;
-
-	if (buflen < 2)
-		goto Elong;
+	struct prepend_buffer b;
+	int seq = 0;
 
 	rcu_read_lock();
 restart:
 	dentry = d;
-	end = buf + buflen;
-	len = buflen;
-	prepend(&end, &len, "\0", 1);
-	/* Get '/' right */
-	retval = end-1;
-	*retval = '/';
+	b = *p;
 	read_seqbegin_or_lock(&rename_lock, &seq);
 	while (!IS_ROOT(dentry)) {
 		const struct dentry *parent = dentry->d_parent;
 
 		prefetch(parent);
-		error = prepend_name(&end, &len, &dentry->d_name);
-		if (error)
+		if (!prepend_name(&b, &dentry->d_name))
 			break;
-
-		retval = end;
 		dentry = parent;
 	}
 	if (!(seq & 1))
@@ -364,36 +323,29 @@ static char *__dentry_path(const struct dentry *d, char *buf, int buflen)
 		goto restart;
 	}
 	done_seqretry(&rename_lock, seq);
-	if (error)
-		goto Elong;
-	return retval;
-Elong:
-	return ERR_PTR(-ENAMETOOLONG);
+	if (b.len == p->len)
+		prepend(&b, "/", 1);
+	return extract_string(&b);
 }
 
 char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
 {
-	return __dentry_path(dentry, buf, buflen);
+	DECLARE_BUFFER(b, buf, buflen);
+
+	prepend(&b, "", 1);
+	return __dentry_path(dentry, &b);
 }
 EXPORT_SYMBOL(dentry_path_raw);
 
 char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
 {
-	char *p = NULL;
-	char *retval;
-
-	if (d_unlinked(dentry)) {
-		p = buf + buflen;
-		if (prepend(&p, &buflen, "//deleted", 10) != 0)
-			goto Elong;
-		buflen++;
-	}
-	retval = __dentry_path(dentry, buf, buflen);
-	if (!IS_ERR(retval) && p)
-		*p = '/';	/* restore '/' overriden with '\0' */
-	return retval;
-Elong:
-	return ERR_PTR(-ENAMETOOLONG);
+	DECLARE_BUFFER(b, buf, buflen);
+
+	if (unlikely(d_unlinked(dentry)))
+		prepend(&b, "//deleted", 10);
+	else
+		prepend(&b, "", 1);
+	return __dentry_path(dentry, &b);
 }
 
 static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,
@@ -438,38 +390,28 @@ SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 	rcu_read_lock();
 	get_fs_root_and_pwd_rcu(current->fs, &root, &pwd);
 
-	error = -ENOENT;
-	if (!d_unlinked(pwd.dentry)) {
-		unsigned long len;
-		char *cwd = page + PATH_MAX;
-		int buflen = PATH_MAX;
-
-		prepend(&cwd, &buflen, "\0", 1);
-		error = prepend_path(&pwd, &root, &cwd, &buflen);
+	if (unlikely(d_unlinked(pwd.dentry))) {
 		rcu_read_unlock();
+		error = -ENOENT;
+	} else {
+		unsigned len;
+		DECLARE_BUFFER(b, page, PATH_MAX);
 
-		if (error < 0)
-			goto out;
-
-		/* Unreachable from current root */
-		if (error > 0) {
-			error = prepend_unreachable(&cwd, &buflen);
-			if (error)
-				goto out;
-		}
+		prepend(&b, "", 1);
+		if (unlikely(prepend_path(&pwd, &root, &b) > 0))
+			prepend(&b, "(unreachable)", 13);
+		rcu_read_unlock();
 
-		error = -ERANGE;
-		len = PATH_MAX + page - cwd;
-		if (len <= size) {
+		len = PATH_MAX - b.len;
+		if (unlikely(len > PATH_MAX))
+			error = -ENAMETOOLONG;
+		else if (unlikely(len > size))
+			error = -ERANGE;
+		else if (copy_to_user(buf, b.buf, len))
+			error = -EFAULT;
+		else
 			error = len;
-			if (copy_to_user(buf, cwd, len))
-				error = -EFAULT;
-		}
-	} else {
-		rcu_read_unlock();
 	}
-
-out:
 	__putname(page);
 	return error;
 }