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Gao Xiang 7388ba6e5c erofs: fix file-backed mounts no longer working on EROFS partitions 
[ Upstream commit 7893cc1225 ]

Sheng Yong reported [1] that Android APEX images didn't work with commit
072a7c7cdb ("erofs: don't bother with s_stack_depth increasing for
now") because "EROFS-formatted APEX file images can be stored within an
EROFS-formatted Android system partition."

In response, I sent a quick fat-fingered [PATCH v3] to address the
report.  Unfortunately, the updated condition was incorrect:

         if (erofs_is_fileio_mode(sbi)) {
-            sb->s_stack_depth =
-                file_inode(sbi->dif0.file)->i_sb->s_stack_depth + 1;
-            if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
-                erofs_err(sb, "maximum fs stacking depth exceeded");
+            inode = file_inode(sbi->dif0.file);
+            if ((inode->i_sb->s_op == &erofs_sops && !sb->s_bdev) ||
+                inode->i_sb->s_stack_depth) {

The condition `!sb->s_bdev` is always true for all file-backed EROFS
mounts, making the check effectively a no-op.

The real fix tested and confirmed by Sheng Yong [2] at that time was
[PATCH v3 RESEND], which correctly ensures the following EROFS^2 setup
works:
    EROFS (on a block device) + EROFS (file-backed mount)

But sadly I screwed it up again by upstreaming the outdated [PATCH v3].

This patch applies the same logic as the delta between the upstream
[PATCH v3] and the real fix [PATCH v3 RESEND].

Reported-by: Sheng Yong <shengyong1@xiaomi.com>
Closes: https://lore.kernel.org/r/3acec686-4020-4609-aee4-5dae7b9b0093@gmail.com [1]
Fixes: 072a7c7cdb ("erofs: don't bother with s_stack_depth increasing for now")
Link: https://lore.kernel.org/r/243f57b8-246f-47e7-9fb1-27a771e8e9e8@gmail.com [2]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2026-01-17 16:31:25 +01:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.