If the SMB symlink is stored on NT server in absolute form then it points
to the NT object hierarchy, which is different from POSIX one and needs
some conversion / mapping.

To make interoperability with Windows SMB server and WSL subsystem, reuse
its logic of mapping between NT paths and POSIX paths into Linux SMB
client.

WSL subsystem on Windows uses for -t drvfs mount option -o symlinkroot=
which specifies the POSIX path where are expected to be mounted lowercase
Windows drive letters (without colon).

Do same for Linux SMB client and add a new mount option -o symlinkroot=
which mimics the drvfs mount option of the same name. It specifies where in
the Linux VFS hierarchy is the root of the DOS / Windows drive letters, and
translates between absolute NT-style symlinks and absolute Linux VFS
symlinks. Default value of symlinkroot is "/mnt", same what is using WSL.

Note that DOS / Windows drive letter symlinks are just subset of all
possible NT-style symlinks. Drive letters live in NT subtree \??\ and
important details about NT paths and object hierarchy are in the comments
in this change.

When symlink target location from non-POSIX SMB server is in absolute form
(indicated by absence of SYMLINK_FLAG_RELATIVE) then it is converted to
Linux absolute symlink according to symlinkroot configuration.

And when creating a new symlink on non-POSIX SMB server in absolute form
then Linux absolute target is converted to NT-style according to
symlinkroot configuration.

When SMB server is POSIX, then this change does not affect neither reading
target location of symlink, nor creating a new symlink. It is expected that
POSIX SMB server works with POSIX paths where the absolute root is /.

This change improves interoperability of absolute SMB symlinks with Windows
SMB servers.

Signed-off-by: Pali Rohár <pali@kernel.org>
---
 fs/smb/client/fs_context.c |  22 +++
 fs/smb/client/fs_context.h |   2 +
 fs/smb/client/reparse.c    | 276 ++++++++++++++++++++++++++++++++++---
 3 files changed, 281 insertions(+), 19 deletions(-)

diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
index 2f0c3894b0f7..22b550860cc8 100644
--- a/fs/smb/client/fs_context.c
+++ b/fs/smb/client/fs_context.c
@@ -178,6 +178,7 @@ const struct fs_parameter_spec smb3_fs_parameters[] = {
 	fsparam_string("sec", Opt_sec),
 	fsparam_string("cache", Opt_cache),
 	fsparam_string("reparse", Opt_reparse),
+	fsparam_string("symlinkroot", Opt_symlinkroot),
 
 	/* Arguments that should be ignored */
 	fsparam_flag("guest", Opt_ignore),
@@ -355,6 +356,7 @@ smb3_fs_context_dup(struct smb3_fs_context *new_ctx, struct smb3_fs_context *ctx
 	new_ctx->source = NULL;
 	new_ctx->iocharset = NULL;
 	new_ctx->leaf_fullpath = NULL;
+	new_ctx->symlinkroot = NULL;
 	/*
 	 * Make sure to stay in sync with smb3_cleanup_fs_context_contents()
 	 */
@@ -369,6 +371,7 @@ smb3_fs_context_dup(struct smb3_fs_context *new_ctx, struct smb3_fs_context *ctx
 	DUP_CTX_STR(nodename);
 	DUP_CTX_STR(iocharset);
 	DUP_CTX_STR(leaf_fullpath);
+	DUP_CTX_STR(symlinkroot);
 
 	return 0;
 }
@@ -1614,9 +1617,26 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
 		if (parse_reparse_flavor(fc, param->string, ctx))
 			goto cifs_parse_mount_err;
 		break;
+	case Opt_symlinkroot:
+		if (param->string[0] != '/') {
+			cifs_errorf(fc, "symlinkroot mount options must be absolute path\n");
+			goto cifs_parse_mount_err;
+		}
+		kfree(ctx->symlinkroot);
+		ctx->symlinkroot = kstrdup(param->string, GFP_KERNEL);
+		if (!ctx->symlinkroot)
+			goto cifs_parse_mount_err;
+		break;
 	}
 	/* case Opt_ignore: - is ignored as expected ... */
 
+	/*
+	 * By default resolve all native absolute symlinks relative to "/mnt/".
+	 * Same default has drvfs driver running in WSL for resolving SMB shares.
+	 */
+	if (!ctx->symlinkroot)
+		ctx->symlinkroot = kstrdup("/mnt/", GFP_KERNEL);
+
 	return 0;
 
  cifs_parse_mount_err:
@@ -1747,6 +1767,8 @@ smb3_cleanup_fs_context_contents(struct smb3_fs_context *ctx)
 	ctx->prepath = NULL;
 	kfree(ctx->leaf_fullpath);
 	ctx->leaf_fullpath = NULL;
+	kfree(ctx->symlinkroot);
+	ctx->symlinkroot = NULL;
 }
 
 void
diff --git a/fs/smb/client/fs_context.h b/fs/smb/client/fs_context.h
index cf577ec0dd0a..8dd12498ffd8 100644
--- a/fs/smb/client/fs_context.h
+++ b/fs/smb/client/fs_context.h
@@ -157,6 +157,7 @@ enum cifs_param {
 	Opt_sec,
 	Opt_cache,
 	Opt_reparse,
+	Opt_symlinkroot,
 
 	/* Mount options to be ignored */
 	Opt_ignore,
@@ -284,6 +285,7 @@ struct smb3_fs_context {
 	struct cifs_ses *dfs_root_ses;
 	bool dfs_automount:1; /* set for dfs automount only */
 	enum cifs_reparse_type reparse_type;
+	char *symlinkroot; /* top level directory for native SMB symlinks in absolute format */
 };
 
 extern const struct fs_parameter_spec smb3_fs_parameters[];
diff --git a/fs/smb/client/reparse.c b/fs/smb/client/reparse.c
index ca4f96c43508..5845dfde950b 100644
--- a/fs/smb/client/reparse.c
+++ b/fs/smb/client/reparse.c
@@ -19,35 +19,137 @@ int smb2_create_reparse_symlink(const unsigned int xid, struct inode *inode,
 				const char *full_path, const char *symname)
 {
 	struct reparse_symlink_data_buffer *buf = NULL;
-	struct cifs_open_info_data data;
+	struct cifs_open_info_data data = {};
 	struct cifs_sb_info *cifs_sb = CIFS_SB(inode->i_sb);
 	struct inode *new;
 	struct kvec iov;
-	__le16 *path;
+	char *sym = NULL;
+	__le16 *path = NULL;
 	bool directory = false;
-	char *sym, sep = CIFS_DIR_SEP(cifs_sb);
-	u16 len, plen;
+	char *symlink_target = NULL;
+	char sep = CIFS_DIR_SEP(cifs_sb);
+	u16 len, plen, poff, slen;
 	int rc = 0;
 
 	len = strlen(symname)+1;
-	sym = kzalloc(len+1, GFP_KERNEL); /* +1 for possible directory slash */
-	if (!sym)
-		return -ENOMEM;
-	memcpy(sym, symname, len);
+	symlink_target = kzalloc(len+1, GFP_KERNEL); /* +1 for possible directory slash */
+	if (!symlink_target) {
+		rc = -ENOMEM;
+		goto out;
+	}
+	memcpy(symlink_target, symname, len);
 
 	data = (struct cifs_open_info_data) {
 		.reparse_point = true,
 		.reparse = { .tag = IO_REPARSE_TAG_SYMLINK, },
-		.symlink_target = sym,
+		.symlink_target = symlink_target,
 	};
 
-	convert_delimiter(sym, sep);
+	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIX_PATHS) && symname[0] == '/') {
+		/*
+		 * This is a request to create an absolute symlink on the server
+		 * which does not support POSIX paths, and expects symlink in
+		 * NT-style path. So convert absolute Linux symlink target path
+		 * to the absolute NT-style path. Root of the NT-style path for
+		 * symlinks is specified in "symlinkroot" mount option. This will
+		 * ensure compatibility of this symlink stored in absolute form
+		 * on the SMB server.
+		 */
+		if (!strstarts(symname, cifs_sb->ctx->symlinkroot)) {
+			/*
+			 * If the absolute Linux symlink target path is not
+			 * inside "symlinkroot" location then there is no way
+			 * to convert such Linux symlink to NT-style path.
+			 */
+			cifs_dbg(VFS,
+				 "absolute symlink '%s' cannot be converted to NT format "
+				 "because it is outside of symlinkroot='%s'\n",
+				 symname, cifs_sb->ctx->symlinkroot);
+			rc = -EINVAL;
+			goto out;
+		}
+		len = strlen(cifs_sb->ctx->symlinkroot);
+		if (cifs_sb->ctx->symlinkroot[len-1] != '/')
+			len++;
+		if (symname[len] >= 'a' && symname[len] <= 'z' &&
+		    (symname[len+1] == '/' || symname[len+1] == '\0')) {
+			/*
+			 * Symlink points to Linux target /symlinkroot/x/path/...
+			 * where 'x' is the lowercase local Windows drive.
+			 * NT-style path for 'x' has common form \??\X:\path\...
+			 * with uppercase local Windows drive.
+			 */
+			int common_path_len = strlen(symname+len+1)+1;
+			sym = kzalloc(6+common_path_len, GFP_KERNEL);
+			if (!sym) {
+				rc = -ENOMEM;
+				goto out;
+			}
+			memcpy(sym, "\\??\\", 4);
+			sym[4] = symname[len] - ('a'-'A');
+			sym[5] = ':';
+			memcpy(sym+6, symname+len+1, common_path_len);
+			if (sym[6] == '\0') {
+				/*
+				 * Symlink which target path is just a Windows
+				 * drive letter is definitely directory symlink.
+				 */
+				directory = true;
+			}
+		} else {
+			/* Unhandled absolute symlink. Report an error. */
+			cifs_dbg(
+				 VFS,
+				 "absolute symlink '%s' cannot be converted to NT format "
+				 "because it points to unknown target\n",
+				 symname);
+			rc = -EINVAL;
+			goto out;
+		}
+	} else {
+		/*
+		 * This is request to either create an absolute symlink on
+		 * server which expects POSIX paths or it is an request to
+		 * create a relative symlink from the current directory.
+		 * These paths have same format as relative SMB symlinks,
+		 * so no conversion is needed. So just take symname as-is.
+		 */
+		sym = kstrdup(symname, GFP_KERNEL);
+		if (!sym) {
+			rc = -ENOMEM;
+			goto out;
+		}
+	}
+
+	if (sep == '\\')
+		convert_delimiter(sym, sep);
+
+	/*
+	 * For absolute NT symlinks it is required to pass also leading
+	 * backslash and to not mangle NT object prefix "\\??\\" and not to
+	 * mangle colon in drive letter. But cifs_convert_path_to_utf16()
+	 * removes leading backslash and replaces '?' and ':'. So temporary
+	 * mask these characters in NT object prefix by '_' and then change
+	 * them back.
+	 */
+	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIX_PATHS) && symname[0] == '/')
+		sym[0] = sym[1] = sym[2] = sym[5] = '_';
+
 	path = cifs_convert_path_to_utf16(sym, cifs_sb);
 	if (!path) {
 		rc = -ENOMEM;
 		goto out;
 	}
 
+	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIX_PATHS) && symname[0] == '/') {
+		sym[0] = '\\';
+		sym[1] = sym[2] = '?';
+		sym[5] = ':';
+		path[0] = '\\';
+		path[1] = path[2] = '?';
+		path[5] = ':';
+	}
+
 	/*
 	 * SMB distinguish between symlink to directory and symlink to file.
 	 * They cannot be exchanged (symlink of file type which points to
@@ -167,8 +269,18 @@ int smb2_create_reparse_symlink(const unsigned int xid, struct inode *inode,
 		data.symlink_target[len+1] = '\0';
 	}
 
-	plen = 2 * UniStrnlen((wchar_t *)path, PATH_MAX);
-	len = sizeof(*buf) + plen * 2;
+	slen = 2 * UniStrnlen((wchar_t *)path, PATH_MAX);
+	poff = 0;
+	plen = slen;
+	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIX_PATHS) && symname[0] == '/') {
+		/*
+		 * For absolute NT symlinks skip leading "\\??\\" in PrintName as
+		 * PrintName is user visible location in DOS/Win32 format (not in NT format).
+		 */
+		poff = 4;
+		plen -= 2 * poff;
+	}
+	len = sizeof(*buf) + plen + slen;
 	buf = kzalloc(len, GFP_KERNEL);
 	if (!buf) {
 		rc = -ENOMEM;
@@ -177,17 +289,17 @@ int smb2_create_reparse_symlink(const unsigned int xid, struct inode *inode,
 
 	buf->ReparseTag = cpu_to_le32(IO_REPARSE_TAG_SYMLINK);
 	buf->ReparseDataLength = cpu_to_le16(len - sizeof(struct reparse_data_buffer));
+
 	buf->SubstituteNameOffset = cpu_to_le16(plen);
-	buf->SubstituteNameLength = cpu_to_le16(plen);
-	memcpy(&buf->PathBuffer[plen], path, plen);
+	buf->SubstituteNameLength = cpu_to_le16(slen);
+	memcpy(&buf->PathBuffer[plen], path, slen);
+
 	buf->PrintNameOffset = 0;
 	buf->PrintNameLength = cpu_to_le16(plen);
-	memcpy(buf->PathBuffer, path, plen);
+	memcpy(buf->PathBuffer, path+poff, plen);
+
 	buf->Flags = cpu_to_le32(*symname != '/' ? SYMLINK_FLAG_RELATIVE : 0);
-	if (*sym != sep)
-		buf->Flags = cpu_to_le32(SYMLINK_FLAG_RELATIVE);
 
-	convert_delimiter(sym, '/');
 	iov.iov_base = buf;
 	iov.iov_len = len;
 	new = smb2_create_reparse_inode(&data, inode->i_sb, xid,
@@ -198,6 +310,7 @@ int smb2_create_reparse_symlink(const unsigned int xid, struct inode *inode,
 	else
 		rc = PTR_ERR(new);
 out:
+	kfree(sym);
 	kfree(path);
 	cifs_free_open_info(&data);
 	kfree(buf);
@@ -505,6 +618,9 @@ int smb2_parse_native_symlink(char **target, const char *buf, unsigned int len,
 	char sep = CIFS_DIR_SEP(cifs_sb);
 	char *linux_target = NULL;
 	char *smb_target = NULL;
+	int symlinkroot_len;
+	int abs_path_len;
+	char *abs_path;
 	int levels;
 	int rc;
 	int i;
@@ -525,7 +641,123 @@ int smb2_parse_native_symlink(char **target, const char *buf, unsigned int len,
 		goto out;
 	}
 
-	if (smb_target[0] == sep && relative) {
+	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIX_PATHS) && !relative) {
+		/*
+		 * This is an absolute symlink from the server which does not
+		 * support POSIX paths, so the symlink is in NT-style path.
+		 * So convert it to absolute Linux symlink target path. Root of
+		 * the NT-style path for symlinks is specified in "symlinkroot"
+		 * mount option.
+		 *
+		 * Root of the DOS and Win32 paths is at NT path \??\
+		 * It means that DOS/Win32 path C:\folder\file.txt is
+		 * NT path \??\C:\folder\file.txt
+		 *
+		 * NT systems have some well-known object symlinks in their NT
+		 * hierarchy, which is needed to take into account when resolving
+		 * other symlinks. Most commonly used symlink paths are:
+		 * \?? -> \GLOBAL??
+		 * \DosDevices -> \??
+		 * \GLOBAL??\GLOBALROOT -> \
+		 * \GLOBAL??\Global -> \GLOBAL??
+		 * \GLOBAL??\NUL -> \Device\Null
+		 * \GLOBAL??\UNC -> \Device\Mup
+		 * \GLOBAL??\PhysicalDrive0 -> \Device\Harddisk0\DR0 (for each harddisk)
+		 * \GLOBAL??\A: -> \Device\Floppy0 (if A: is the first floppy)
+		 * \GLOBAL??\C: -> \Device\HarddiskVolume1 (if C: is the first harddisk)
+		 * \GLOBAL??\D: -> \Device\CdRom0 (if D: is first cdrom)
+		 * \SystemRoot -> \Device\Harddisk0\Partition1\WINDOWS (or where is NT system installed)
+		 * \Volume{...} -> \Device\HarddiskVolume1 (where ... is system generated guid)
+		 *
+		 * In most common cases, absolute NT symlinks points to path on
+		 * DOS/Win32 drive letter, system-specific Volume or on UNC share.
+		 * Here are few examples of commonly used absolute NT symlinks
+		 * created by mklink.exe tool:
+		 * \??\C:\folder\file.txt
+		 * \??\\C:\folder\file.txt
+		 * \??\UNC\server\share\file.txt
+		 * \??\\UNC\server\share\file.txt
+		 * \??\Volume{b75e2c83-0000-0000-0000-602f00000000}\folder\file.txt
+		 *
+		 * It means that the most common path prefix \??\ is also NT path
+		 * symlink (to \GLOBAL??). It is less common that second path
+		 * separator is double backslash, but it is valid.
+		 *
+		 * Volume guid is randomly generated by the target system and so
+		 * only the target system knows the mapping between guid and the
+		 * hardisk number. Over SMB it is not possible to resolve this
+		 * mapping, therefore symlinks pointing to target location of
+		 * volume guids are totally unusable over SMB.
+		 *
+		 * For now parse only symlink paths available for DOS and Win32.
+		 * Those are paths with \??\ prefix or paths which points to \??\
+		 * via other NT symlink (\DosDevices\, \GLOBAL??\, ...).
+		 */
+		abs_path = smb_target;
+globalroot:
+		if (strstarts(abs_path, "\\??\\"))
+			abs_path += sizeof("\\??\\")-1;
+		else if (strstarts(abs_path, "\\DosDevices\\"))
+			abs_path += sizeof("\\DosDevices\\")-1;
+		else if (strstarts(abs_path, "\\GLOBAL??\\"))
+			abs_path += sizeof("\\GLOBAL??\\")-1;
+		else {
+			/* Unhandled absolute symlink, points outside of DOS/Win32 */
+			cifs_dbg(VFS,
+				 "absolute symlink '%s' cannot be converted from NT format "
+				 "because points to unknown target\n",
+				 smb_target);
+			rc = -EIO;
+			goto out;
+		}
+
+		/* Sometimes path separator after \?? is double backslash */
+		if (abs_path[0] == '\\')
+			abs_path++;
+
+		while (strstarts(abs_path, "Global\\"))
+			abs_path += sizeof("Global\\")-1;
+
+		if (strstarts(abs_path, "GLOBALROOT\\")) {
+			/* Label globalroot requires path with leading '\\', so do not trim '\\' */
+			abs_path += sizeof("GLOBALROOT")-1;
+			goto globalroot;
+		}
+
+		/* For now parse only paths to drive letters */
+		if (((abs_path[0] >= 'A' && abs_path[0] <= 'Z') ||
+		     (abs_path[0] >= 'a' && abs_path[0] <= 'z')) &&
+		    abs_path[1] == ':' &&
+		    (abs_path[2] == '\\' || abs_path[2] == '\0')) {
+			/* Convert drive letter to lowercase and drop colon */
+			char drive_letter = abs_path[0];
+			if (drive_letter >= 'A' && drive_letter <= 'Z')
+				drive_letter += 'a'-'A';
+			abs_path++;
+			abs_path[0] = drive_letter;
+		} else {
+			/* Unhandled absolute symlink. Report an error. */
+			cifs_dbg(VFS,
+				 "absolute symlink '%s' cannot be converted from NT format "
+				 "because points to unknown target\n",
+				 smb_target);
+			rc = -EIO;
+			goto out;
+		}
+
+		abs_path_len = strlen(abs_path)+1;
+		symlinkroot_len = strlen(cifs_sb->ctx->symlinkroot);
+		if (cifs_sb->ctx->symlinkroot[symlinkroot_len-1] == '/')
+			symlinkroot_len--;
+		linux_target = kmalloc(symlinkroot_len + 1 + abs_path_len, GFP_KERNEL);
+		if (!linux_target) {
+			rc = -ENOMEM;
+			goto out;
+		}
+		memcpy(linux_target, cifs_sb->ctx->symlinkroot, symlinkroot_len);
+		linux_target[symlinkroot_len] = '/';
+		memcpy(linux_target + symlinkroot_len + 1, abs_path, abs_path_len);
+	} else if (smb_target[0] == sep && relative) {
 		/*
 		 * This is a relative SMB symlink from the top of the share,
 		 * which is the top level directory of the Linux mount point.
@@ -554,6 +786,12 @@ int smb2_parse_native_symlink(char **target, const char *buf, unsigned int len,
 		}
 		memcpy(linux_target + levels*3, smb_target+1, smb_target_len); /* +1 to skip leading sep */
 	} else {
+		/*
+		 * This is either an absolute symlink in POSIX-style format
+		 * or relative SMB symlink from the current directory.
+		 * These paths have same format as Linux symlinks, so no
+		 * conversion is needed.
+		 */
 		linux_target = smb_target;
 		smb_target = NULL;
 	}
-- 
2.20.1