Re: [PATCH 1/2] erofs: accept source file descriptor via fsconfig

Add fsparam_fd("source") so that userspace can pass an already-opened
file descriptor instead of a path string.  When the fd is provided via
fsconfig(FSCONFIG_SET_FD, "source", NULL, fd), it is stored directly
in sbi->dif0.file and erofs_fc_get_tree() skips the filp_open() call.

This is useful for mount namespaces where the backing file may not be
reachable by path, and for tools that already hold an fd to the image
(e.g. composefs reusing an erofs mount's backing file).

Signed-off-by: Giuseppe Scrivano <redacted>
---
 fs/erofs/super.c | 36 +++++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 86fa5c6a0c70..8ad1689f74b2 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c

@@ -386,6 +386,7 @@ static void erofs_default_options(struct erofs_sb_info *sbi)
 enum {
 	Opt_user_xattr, Opt_acl, Opt_cache_strategy, Opt_dax, Opt_dax_enum,
 	Opt_device, Opt_domain_id, Opt_directio, Opt_fsoffset, Opt_inode_share,
+	Opt_source_fd,
 };
 
 static const struct constant_table erofs_param_cache_strategy[] = {

@@ -413,6 +414,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[] = {
 	fsparam_flag_no("directio",	Opt_directio),
 	fsparam_u64("fsoffset",		Opt_fsoffset),
 	fsparam_flag("inode_share",	Opt_inode_share),
+	fsparam_fd("source",		Opt_source_fd),
 	{}
 };
 

@@ -524,6 +526,15 @@ static int erofs_fc_parse_param(struct fs_context *fc,
 		else
 			set_opt(&sbi->opt, INODE_SHARE);
 		break;
+	case Opt_source_fd:
+		if (!IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE)) {
+			errorfc(fc, "source fd option not supported");

+			fput(sbi->dif0.file);
+		sbi->dif0.file = get_file(param->file);
+		break;
 	}
 	return 0;
 }

@@ -752,14 +763,18 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 
 static int erofs_fc_get_tree(struct fs_context *fc)
 {
-	int ret;
+	struct erofs_sb_info *sbi = fc->s_fs_info;

 		struct file *file;
+		int ret;
+
+		ret = get_tree_bdev_flags(fc, erofs_fc_fill_super,
+			IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE) ?
+				GET_TREE_BDEV_QUIET_LOOKUP : 0);
+		if (!IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE) ||
+		    ret != -ENOTBLK)
+			return ret;
 
 		if (!fc->source)
 			return invalf(fc, "No source specified");

@@ -767,12 +782,11 @@ static int erofs_fc_get_tree(struct fs_context *fc)
 		if (IS_ERR(file))
 			return PTR_ERR(file);
 		sbi->dif0.file = file;
-
-		if (S_ISREG(file_inode(sbi->dif0.file)->i_mode) &&
-		    sbi->dif0.file->f_mapping->a_ops->read_folio)
-			return get_tree_nodev(fc, erofs_fc_fill_super);
 	}
-	return ret;
+	if (S_ISREG(file_inode(sbi->dif0.file)->i_mode) &&
+	    sbi->dif0.file->f_mapping->a_ops->read_folio)
+		return get_tree_nodev(fc, erofs_fc_fill_super);
+	return -EINVAL;