[PATCH v3] erofs: accept source file descriptor via fsconfig

[PATCH v3] erofs: accept source file descriptor via fsconfig

[Giuseppe Scrivano]

Allow userspace to pass an already-opened file descriptor as the mount
source instead of a path string.  This is useful for tools that already
hold an fd to the image, such as composefs reusing an existing erofs
backing file.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
---

v2: https://lore.kernel.org/linux-fsdevel/20260711071137.4130824-1-gscrivan@redhat.com/
v1: https://lore.kernel.org/linux-fsdevel/ak5GfvVfWLJU1EwK@debian/

 fs/erofs/super.c | 87 ++++++++++++++++++++++++++++++++++++------------
 1 file changed, 65 insertions(+), 22 deletions(-)

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 86fa5c6a0c70..d7baf9f34dc0 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,
 };
 
 static const struct constant_table erofs_param_cache_strategy[] = {
@@ -402,17 +403,18 @@ static const struct constant_table erofs_dax_param_enums[] = {
 };
 
 static const struct fs_parameter_spec erofs_fs_parameters[] = {
-	fsparam_flag_no("user_xattr",	Opt_user_xattr),
-	fsparam_flag_no("acl",		Opt_acl),
-	fsparam_enum("cache_strategy",	Opt_cache_strategy,
+	fsparam_flag_no("user_xattr",		Opt_user_xattr),
+	fsparam_flag_no("acl",			Opt_acl),
+	fsparam_enum("cache_strategy",		Opt_cache_strategy,
 		     erofs_param_cache_strategy),
-	fsparam_flag("dax",             Opt_dax),
-	fsparam_enum("dax",		Opt_dax_enum, erofs_dax_param_enums),
-	fsparam_string("device",	Opt_device),
-	fsparam_string("domain_id",	Opt_domain_id),
-	fsparam_flag_no("directio",	Opt_directio),
-	fsparam_u64("fsoffset",		Opt_fsoffset),
-	fsparam_flag("inode_share",	Opt_inode_share),
+	fsparam_flag("dax",			Opt_dax),
+	fsparam_enum("dax",			Opt_dax_enum, erofs_dax_param_enums),
+	fsparam_string("device",		Opt_device),
+	fsparam_string("domain_id",		Opt_domain_id),
+	fsparam_flag_no("directio",		Opt_directio),
+	fsparam_u64("fsoffset",			Opt_fsoffset),
+	fsparam_flag("inode_share",		Opt_inode_share),
+	fsparam_file_or_string("source",	Opt_source),
 	{}
 };
 
@@ -437,6 +439,40 @@ static bool erofs_fc_set_dax_mode(struct fs_context *fc, unsigned int mode)
 	return false;
 }
 
+static int erofs_fc_parse_source(struct fs_context *fc,
+				 struct fs_parameter *param)
+{
+	struct erofs_sb_info *sbi = fc->s_fs_info;
+
+	if (fc->source || sbi->dif0.file)
+		return invalf(fc, "Multiple sources");
+
+	switch (param->type) {
+	case fs_value_is_string:
+		fc->source = param->string;
+		param->string = NULL;
+		return 0;
+	case fs_value_is_file: {
+		char *buf __free(kfree) = kmalloc(PATH_MAX, GFP_KERNEL);
+		char *p;
+
+		if (!buf)
+			return -ENOMEM;
+		p = file_path(param->file, buf, PATH_MAX);
+		if (IS_ERR(p))
+			return PTR_ERR(p);
+		fc->source = kstrdup(p, GFP_KERNEL);
+		if (!fc->source)
+			return -ENOMEM;
+		sbi->dif0.file = no_free_ptr(param->file);
+		return 0;
+	}
+	default:
+		WARN_ON_ONCE(true);
+		return -EINVAL;
+	}
+}
+
 static int erofs_fc_parse_param(struct fs_context *fc,
 				struct fs_parameter *param)
 {
@@ -524,6 +560,8 @@ static int erofs_fc_parse_param(struct fs_context *fc,
 		else
 			set_opt(&sbi->opt, INODE_SHARE);
 		break;
+	case Opt_source:
+		return erofs_fc_parse_source(fc, param);
 	}
 	return 0;
 }
@@ -752,14 +790,18 @@ 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 = sbi->dif0.file;
 
-	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) {
-		struct erofs_sb_info *sbi = fc->s_fs_info;
-		struct file *file;
+	if (!IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_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 +809,13 @@ 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(file)->i_mode) ||
+	    !file->f_mapping->a_ops->read_folio) {
+		errorfc(fc, "source is unsupported");
+		return -EINVAL;
+	}
+	return get_tree_nodev(fc, erofs_fc_fill_super);
 }
 
 static int erofs_fc_reconfigure(struct fs_context *fc)
-- 
2.55.0

Re: [PATCH v3] erofs: accept source file descriptor via fsconfig

[Gao Xiang]

Hi Giuseppe,

On 2026/7/14 23:48, Giuseppe Scrivano wrote:
> Allow userspace to pass an already-opened file descriptor as the mount
> source instead of a path string.  This is useful for tools that already
> hold an fd to the image, such as composefs reusing an existing erofs
> backing file.
> 
> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Use the current erofs-utils testsuite, the following tests seems skipped incorrectly.

[Error 22] Invalid argument
SKIP: erofs/008
[Error 22] Invalid argument
SKIP: erofs/009
[Error 22] Invalid argument
SKIP: erofs/010
[Error 22] Invalid argument
SKIP: erofs/011

[Error 22] Invalid argument
SKIP: erofs/017
[Error 22] Invalid argument
SKIP: erofs/018

And the kernel message shows:
[  116.477816] erofs: source is unsupported
...

git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git -b experimental-tests
$ sudo make check

Thanks,
Gao Xiang

Re: [PATCH v3] erofs: accept source file descriptor via fsconfig

[Giuseppe Scrivano]

Gao Xiang <hsiangkao@linux.alibaba.com> writes:

> Hi Giuseppe,
>
> On 2026/7/14 23:48, Giuseppe Scrivano wrote:
>> Allow userspace to pass an already-opened file descriptor as the mount
>> source instead of a path string.  This is useful for tools that already
>> hold an fd to the image, such as composefs reusing an existing erofs
>> backing file.
>> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
>
> Use the current erofs-utils testsuite, the following tests seems skipped incorrectly.
>
> [Error 22] Invalid argument
> SKIP: erofs/008
> [Error 22] Invalid argument
> SKIP: erofs/009
> [Error 22] Invalid argument
> SKIP: erofs/010
> [Error 22] Invalid argument
> SKIP: erofs/011
>
> [Error 22] Invalid argument
> SKIP: erofs/017
> [Error 22] Invalid argument
> SKIP: erofs/018
>
> And the kernel message shows:
> [  116.477816] erofs: source is unsupported
> ...
>
> git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git -b experimental-tests
> $ sudo make check
>
> Thanks,
> Gao Xiang

it seems to be caused by the different error returned by erofs_fc_get_tree.

Would you be fine if I amend this on top of v3?

Thanks,
Giuseppe



diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index d7baf9f34dc0..829ae530d7ca 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -791,17 +791,22 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 static int erofs_fc_get_tree(struct fs_context *fc)
 {
 	struct erofs_sb_info *sbi = fc->s_fs_info;
-	struct file *file = sbi->dif0.file;
+	int ret;
 
-	if (!IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE) || !file) {
-		int ret;
+	if (IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE) && sbi->dif0.file) {
+		if (!S_ISREG(file_inode(sbi->dif0.file)->i_mode) ||
+		    !sbi->dif0.file->f_mapping->a_ops->read_folio) {
+			errorfc(fc, "source is unsupported");
+			return -EINVAL;
+		}
+		return get_tree_nodev(fc, erofs_fc_fill_super);
+	}
 
-		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;
+	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) {
+		struct file *file;
 
 		if (!fc->source)
 			return invalf(fc, "No source specified");
@@ -809,13 +814,12 @@ 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);
 	}
-	if (!S_ISREG(file_inode(file)->i_mode) ||
-	    !file->f_mapping->a_ops->read_folio) {
-		errorfc(fc, "source is unsupported");
-		return -EINVAL;
-	}
-	return get_tree_nodev(fc, erofs_fc_fill_super);
+	return ret;
 }
 
 static int erofs_fc_reconfigure(struct fs_context *fc)

Re: [PATCH v3] erofs: accept source file descriptor via fsconfig

[Gao Xiang]

On 2026/7/17 16:28, Giuseppe Scrivano wrote:
> Gao Xiang <hsiangkao@linux.alibaba.com> writes:
> 
>> Hi Giuseppe,
>>
>> On 2026/7/14 23:48, Giuseppe Scrivano wrote:
>>> Allow userspace to pass an already-opened file descriptor as the mount
>>> source instead of a path string.  This is useful for tools that already
>>> hold an fd to the image, such as composefs reusing an existing erofs
>>> backing file.
>>> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
>>
>> Use the current erofs-utils testsuite, the following tests seems skipped incorrectly.
>>
>> [Error 22] Invalid argument
>> SKIP: erofs/008
>> [Error 22] Invalid argument
>> SKIP: erofs/009
>> [Error 22] Invalid argument
>> SKIP: erofs/010
>> [Error 22] Invalid argument
>> SKIP: erofs/011
>>
>> [Error 22] Invalid argument
>> SKIP: erofs/017
>> [Error 22] Invalid argument
>> SKIP: erofs/018
>>
>> And the kernel message shows:
>> [  116.477816] erofs: source is unsupported
>> ...
>>
>> git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git -b experimental-tests
>> $ sudo make check
>>
>> Thanks,
>> Gao Xiang
> 
> it seems to be caused by the different error returned by erofs_fc_get_tree.
> 
> Would you be fine if I amend this on top of v3?

I think you could just send out the diff from the baseline
since it might be harder to follow an incremental diff here.

But anyway, new versions are always welcome, I will play
with them when I am availble.

Thanks,
Gao Xiang

> 
> Thanks,
> Giuseppe