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Date:   Wed, 7 Sep 2022 02:45:13 +0100
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From:   Daniel Pinto <danielpinto52@gmail.com>
To:     Konstantin Komarov <almaz.alexandrovich@paragon-software.com>,
        ntfs3@lists.linux.dev, linux-kernel@vger.kernel.org
Cc:     "Artem S. Tashkinov" <aros@gmx.com>
Subject: [PATCH] ntfs3: fix junction point resolution
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The ntfs3 file system driver does not convert the target path of
junction points to a proper Linux path. As junction points targets
are always absolute paths (they start with a drive letter), all
junctions will result in broken links.

Translate the targets of junction points to relative paths so they
point to directories inside the mounted volume. Note that Windows
allows junction points to reference directories in another drive.
However, as there is no way to know which drive the junctions refer
to, we assume they always target the same file system they are in.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=214833
Signed-off-by: Daniel Pinto <danielpinto52@gmail.com>
---
 fs/ntfs3/inode.c | 103 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 101 insertions(+), 2 deletions(-)

diff --git a/fs/ntfs3/inode.c b/fs/ntfs3/inode.c
index 51363d4e8636..f7e8876f56d3 100644
--- a/fs/ntfs3/inode.c
+++ b/fs/ntfs3/inode.c
@@ -1749,7 +1749,101 @@ void ntfs_evict_inode(struct inode *inode)
 	ni_clear(ntfs_i(inode));
 }
 
-static noinline int ntfs_readlink_hlp(struct inode *inode, char *buffer,
+/*
+ * ntfs_translate_junction
+ *
+ * Translate a Windows junction target to the Linux equivalent.
+ * On junctions, targets are always absolute (they include the drive
+ * letter). We have no way of knowing if the target is for the current
+ * mounted device or not so we just assume it is.
+ */
+static int ntfs_translate_junction(const struct super_block *sb,
+				   const struct dentry *link_de, char *target,
+				   int target_len, int target_max)
+{
+	int tl_len, err = target_len;
+	char *link_path_buffer = NULL, *link_path;
+	char *translated = NULL;
+	char *target_start;
+	int copy_len;
+
+	link_path_buffer = kmalloc(PATH_MAX, GFP_NOFS);
+	if (!link_path_buffer) {
+		err = -ENOMEM;
+		goto out;
+	}
+	/* Get link path, relative to mount point */
+	link_path = dentry_path_raw(link_de, link_path_buffer, PATH_MAX);
+	if (IS_ERR(link_path)) {
+		ntfs_err(sb, "Error getting link path");
+		err = -EINVAL;
+		goto out;
+	}
+
+	translated = kmalloc(PATH_MAX, GFP_NOFS);
+	if (!translated) {
+		err = -ENOMEM;
+		goto out;
+	}
+
+	/* Make translated path a relative path to mount point */
+	strcpy(translated, "./");
+	++link_path;	/* Skip leading / */
+	for (tl_len = sizeof("./") - 1; *link_path; ++link_path) {
+		if (*link_path == '/') {
+			if (PATH_MAX - tl_len < sizeof("../")) {
+				ntfs_err(sb, "Link path %s has too many components",
+					 link_path);
+				err = -EINVAL;
+				goto out;
+			}
+			strcpy(translated + tl_len, "../");
+			tl_len += sizeof("../") - 1;
+		}
+	}
+
+	/* Skip drive letter */
+	target_start = target;
+	while (*target_start && *target_start != ':')
+		++target_start;
+
+	if (!*target_start) {
+		ntfs_err(sb, "Link target (%s) missing drive separator", target);
+		err = -EINVAL;
+		goto out;
+	}
+
+	/* Skip drive separator and leading /, if exists */
+	target_start += 1 + (target_start[1] == '/');
+	copy_len = target_len - (target_start - target);
+
+	if (PATH_MAX - tl_len <= copy_len) {
+		ntfs_err(sb, "Link target %s too large for buffer (%d <= %d)",
+			 target_start, PATH_MAX - tl_len, copy_len);
+		err = -EINVAL;
+		goto out;
+	}
+
+	/* translated path has a trailing / and target_start does not */
+	strcpy(translated + tl_len, target_start);
+	tl_len += copy_len;
+	if (target_max <= tl_len) {
+		ntfs_err(sb, "Target path %s too large for buffer (%d <= %d)",
+			 translated, target_max, tl_len);
+		err = -EINVAL;
+		goto out;
+	}
+	strcpy(target, translated);
+	err = tl_len;
+
+out:
+	kfree(link_path_buffer);
+	kfree(translated);
+	return err;
+}
+
+static noinline int ntfs_readlink_hlp(const struct dentry *link_de,
+				      struct inode *inode, char *buffer,
 				      int buflen)
 {
 	int i, err = -EINVAL;
@@ -1892,6 +1986,11 @@ static noinline int ntfs_readlink_hlp(struct inode *inode, char *buffer,
 
 	/* Always set last zero. */
 	buffer[err] = 0;
+
+	/* If this is a junction, translate the link target. */
+	if (rp->ReparseTag == IO_REPARSE_TAG_MOUNT_POINT)
+		err = ntfs_translate_junction(sb, link_de, buffer, err, buflen);
+
 out:
 	kfree(to_free);
 	return err;
@@ -1910,7 +2009,7 @@ static const char *ntfs_get_link(struct dentry *de, struct inode *inode,
 	if (!ret)
 		return ERR_PTR(-ENOMEM);
 
-	err = ntfs_readlink_hlp(inode, ret, PAGE_SIZE);
+	err = ntfs_readlink_hlp(de, inode, ret, PAGE_SIZE);
 	if (err < 0) {
 		kfree(ret);
 		return ERR_PTR(err);