[PATCH 6.18.y 1/2] erofs: separate plain and compressed filesystems formally

[Sasha Levin <sashal@kernel.org>]

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

[ Upstream commit 7cef3c8341940febf75db6c25199cd83fb74d52f ]

The EROFS on-disk format uses a tiny, plain metadata design that
prioritizes performance and minimizes complex inconsistencies against
common writable disk filesystems (almost all serious metadata
inconsistency cannot happen in well-designed immutable filesystems like
EROFS). EROFS deliberately avoids artificial design flaws to eliminate
serious security risks from untrusted remote sources by design,
although human-made implementation bugs can still happen sometimes.

Currently, there is no strict check to prevent compressed inodes,
especially LZ4-compressed inodes, from being read in plain filesystems.

Starting with erofs-utils 1.0 and Linux 5.3, LZ4_0PADDING sb feature
is automatically enabled for LZ4-compressed EROFS images to support
in-place decompression. Furthermore, since Linux 5.4 LTS is no longer
supported, we no longer need to handle ancient LZ4-compressed EROFS
images generated by erofs-utils prior to 1.0.

To formally distinguish different filesystem types for improved
security:

 - Use the presence of LZ4_0PADDING or a non-zero
   `dsb->u1.lz4_max_distance` as a marker for compressed filesystems
   containing LZ4-compressed inodes only;

 - For other algorithms, use `dsb->u1.available_compr_algs` bitmap.

Note: LZ4_0PADDING has been supported since Linux 5.4 (the first formal
kernel version), so exposing it via sysfs is no longer necessary and is
now deprecated (but remain it for five more years until 2031):

  `dsb->u1` has been strictly non-zero for all EROFS images containing
  compressed inodes starting with erofs-utils v1.3 and it is actually
  a much better marker for compressed filesystems.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Backport dependency note for Linux 6.18:

Retain only the zero_padding sysfs feature removal and the corresponding
feature-list documentation update.  The documentation layout is required
for target 839f075aabdf5c21048f9801b87c0b841dd3d064 ("erofs: add sysfs
feature entry for xattr prefixes") to apply without modification, and the
sysfs removal keeps that list consistent with the attributes exposed.

Drop the decompressor, inode, superblock, and internal-header changes.
Those changes are unrelated to the target, and the decompressor hunks
assume upstream's newer string-based error reporting instead of this
stable branch's integer-return implementation.  Leave the existing
compression behavior intact; no functions are added by this dependency.

[ sashal: Reduced backport -- upstream 7cef3c8341940 touches 7 file(s), this
  backport carries 2. Not backported here:
  fs/erofs/decompressor.c
  fs/erofs/erofs_fs.h
  fs/erofs/inode.c
  fs/erofs/internal.h
  fs/erofs/super.c
  This note is generated from the file lists only; see the resolution record
  for the reasoning. ]

Stable-dep-of: 839f075aabdf ("erofs: add sysfs feature entry for xattr prefixes")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 Documentation/ABI/testing/sysfs-fs-erofs | 6 +++---
 fs/erofs/sysfs.c                         | 2 --
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-fs-erofs b/Documentation/ABI/testing/sysfs-fs-erofs
index b9243c7f28d71..e4cf6fc6a1066 100644
--- a/Documentation/ABI/testing/sysfs-fs-erofs
+++ b/Documentation/ABI/testing/sysfs-fs-erofs
@@ -3,9 +3,9 @@ Date:		November 2021
 Contact:	"Huang Jianan" <huangjianan@oppo.com>
 Description:	Shows all enabled kernel features.
 		Supported features:
-		zero_padding, compr_cfgs, big_pcluster, chunked_file,
-		device_table, compr_head2, sb_chksum, ztailpacking,
-		dedupe, fragments, 48bit, metabox.
+		compr_cfgs, big_pcluster, chunked_file, device_table,
+		compr_head2, sb_chksum, ztailpacking, dedupe, fragments,
+		48bit, metabox.
 
 What:		/sys/fs/erofs/<disk>/sync_decompress
 Date:		November 2021
diff --git a/fs/erofs/sysfs.c b/fs/erofs/sysfs.c
index 86b22b9f0c191..3a9a5fa000ae9 100644
--- a/fs/erofs/sysfs.c
+++ b/fs/erofs/sysfs.c
@@ -86,7 +86,6 @@ static struct attribute *erofs_attrs[] = {
 ATTRIBUTE_GROUPS(erofs);
 
 /* Features this copy of erofs supports */
-EROFS_ATTR_FEATURE(zero_padding);
 EROFS_ATTR_FEATURE(compr_cfgs);
 EROFS_ATTR_FEATURE(big_pcluster);
 EROFS_ATTR_FEATURE(chunked_file);
@@ -100,7 +99,6 @@ EROFS_ATTR_FEATURE(48bit);
 EROFS_ATTR_FEATURE(metabox);
 
 static struct attribute *erofs_feat_attrs[] = {
-	ATTR_LIST(zero_padding),
 	ATTR_LIST(compr_cfgs),
 	ATTR_LIST(big_pcluster),
 	ATTR_LIST(chunked_file),
-- 
2.53.0