passthrough_ll: remove symlink fallbacks (#514)

* passthrough_ll/hp: remove symlink fallbacks

Path lookup in the kernel has special rules for looking up magic symlinks
under /proc.  If a filesystem operation is instructed to follow symlinks
(e.g. via AT_SYMLINK_FOLLOW or lack of AT_SYMLINK_NOFOLLOW), and the final
component is such a proc symlink, then the target of the magic symlink is
used for the operation, even if the target itself is a symlink.  I.e. path
lookup is always terminated after following a final magic symlink.

I was erronously assuming that in the above case the target symlink would
also be followed, and so workarounds were added for a couple of operations
to handle the symlink case.  Since the symlink can be handled simply by
following the proc symlink, these workardouds are not needed.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Co-authored-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 8 insertions, 70 deletions

diff --git a/example/passthrough_ll.c b/example/passthrough_ll.c
index 5667223..06bb50f 100644
--- a/example/passthrough_ll.c
+++ b/example/passthrough_ll.c
@@ -74,7 +74,6 @@ struct lo_inode {
 	struct lo_inode *next; /* protected by lo->mutex */
 	struct lo_inode *prev; /* protected by lo->mutex */
 	int fd;
-	bool is_symlink;
 	ino_t ino;
 	dev_t dev;
 	uint64_t refcount; /* protected by lo->mutex */
@@ -188,26 +187,6 @@ static void lo_getattr(fuse_req_t req, fuse_ino_t ino,
 	fuse_reply_attr(req, &buf, lo->timeout);
 }
 
-static int utimensat_empty_nofollow(struct lo_inode *inode,
-				    const struct timespec *tv)
-{
-	int res;
-	char procname[64];
-
-	if (inode->is_symlink) {
-		res = utimensat(inode->fd, "", tv,
-				AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
-		if (res == -1 && errno == EINVAL) {
-			/* Sorry, no race free way to set times on symlink. */
-			errno = EPERM;
-		}
-		return res;
-	}
-	sprintf(procname, "/proc/self/fd/%i", inode->fd);
-
-	return utimensat(AT_FDCWD, procname, tv, 0);
-}
-
 static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
 		       int valid, struct fuse_file_info *fi)
 {
@@ -268,8 +247,10 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
 
 		if (fi)
 			res = futimens(fi->fh, tv);
-		else
-			res = utimensat_empty_nofollow(inode, tv);
+		else {
+			sprintf(procname, "/proc/self/fd/%i", ifd);
+			res = utimensat(AT_FDCWD, procname, tv, 0);
+		}
 		if (res == -1)
 			goto out_err;
 	}
@@ -332,7 +313,6 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
 		if (!inode)
 			goto out_err;
 
-		inode->is_symlink = S_ISLNK(e->attr.st_mode);
 		inode->refcount = 1;
 		inode->fd = newfd;
 		inode->ino = e->attr.st_ino;
@@ -426,26 +406,6 @@ static void lo_symlink(fuse_req_t req, const char *link,
 	lo_mknod_symlink(req, parent, name, S_IFLNK, 0, link);
 }
 
-static int linkat_empty_nofollow(struct lo_inode *inode, int dfd,
-				 const char *name)
-{
-	int res;
-	char procname[64];
-
-	if (inode->is_symlink) {
-		res = linkat(inode->fd, "", dfd, name, AT_EMPTY_PATH);
-		if (res == -1 && (errno == ENOENT || errno == EINVAL)) {
-			/* Sorry, no race free way to hard-link a symlink. */
-			errno = EPERM;
-		}
-		return res;
-	}
-
-	sprintf(procname, "/proc/self/fd/%i", inode->fd);
-
-	return linkat(AT_FDCWD, procname, dfd, name, AT_SYMLINK_FOLLOW);
-}
-
 static void lo_link(fuse_req_t req, fuse_ino_t ino, fuse_ino_t parent,
 		    const char *name)
 {
@@ -453,13 +413,16 @@ static void lo_link(fuse_req_t req, fuse_ino_t ino, fuse_ino_t parent,
 	struct lo_data *lo = lo_data(req);
 	struct lo_inode *inode = lo_inode(req, ino);
 	struct fuse_entry_param e;
+	char procname[64];
 	int saverr;
 
 	memset(&e, 0, sizeof(struct fuse_entry_param));
 	e.attr_timeout = lo->timeout;
 	e.entry_timeout = lo->timeout;
 
-	res = linkat_empty_nofollow(inode, lo_fd(req, parent), name);
+	sprintf(procname, "/proc/self/fd/%i", inode->fd);
+	res = linkat(AT_FDCWD, procname, lo_fd(req, parent), name,
+		     AT_SYMLINK_FOLLOW);
 	if (res == -1)
 		goto out_err;
 
@@ -976,12 +939,6 @@ static void lo_getxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
 			ino, name, size);
 	}
 
-	if (inode->is_symlink) {
-		/* Sorry, no race free way to getxattr on symlink. */
-		saverr = EPERM;
-		goto out;
-	}
-
 	sprintf(procname, "/proc/self/fd/%i", inode->fd);
 
 	if (size) {
@@ -1032,12 +989,6 @@ static void lo_listxattr(fuse_req_t req, fuse_ino_t ino, size_t size)
 			ino, size);
 	}
 
-	if (inode->is_symlink) {
-		/* Sorry, no race free way to listxattr on symlink. */
-		saverr = EPERM;
-		goto out;
-	}
-
 	sprintf(procname, "/proc/self/fd/%i", inode->fd);
 
 	if (size) {
@@ -1088,12 +1039,6 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *name,
 			ino, name, value, size);
 	}
 
-	if (inode->is_symlink) {
-		/* Sorry, no race free way to setxattr on symlink. */
-		saverr = EPERM;
-		goto out;
-	}
-
 	sprintf(procname, "/proc/self/fd/%i", inode->fd);
 
 	ret = setxattr(procname, name, value, size, flags);
@@ -1119,12 +1064,6 @@ static void lo_removexattr(fuse_req_t req, fuse_ino_t ino, const char *name)
 			ino, name);
 	}
 
-	if (inode->is_symlink) {
-		/* Sorry, no race free way to setxattr on symlink. */
-		saverr = EPERM;
-		goto out;
-	}
-
 	sprintf(procname, "/proc/self/fd/%i", inode->fd);
 
 	ret = removexattr(procname, name);
@@ -1274,7 +1213,6 @@ int main(int argc, char *argv[])
 	} else {
 		lo.source = "/";
 	}
-	lo.root.is_symlink = false;
 	if (!lo.timeout_set) {
 		switch (lo.cache) {
 		case CACHE_NEVER: