Gao Xiang <[email protected]> 于2026年1月8日周四 11:07写道： > > Previously, commit d53cd891f0e4 ("erofs: limit the level of fs stacking > for file-backed mounts") bumped `s_stack_depth` by one to avoid kernel > stack overflow when stacking an unlimited number of EROFS on top of > each other. > > This fix breaks composefs mounts, which need EROFS+ovl^2 sometimes > (and such setups are already used in production for quite a long time). > > One way to fix this regression is to bump FILESYSTEM_MAX_STACK_DEPTH > from 2 to 3, but proving that this is safe in general is a high bar. > > After a long discussion on GitHub issues [1] about possible solutions, > one conclusion is that there is no need to support nesting file-backed > EROFS mounts on stacked filesystems, because there is always the option > to use loopback devices as a fallback. > > As a quick fix for the composefs regression for this cycle, instead of > bumping `s_stack_depth` for file backed EROFS mounts, we disallow > nesting file-backed EROFS over EROFS and over filesystems with > `s_stack_depth` > 0. > > This works for all known file-backed mount use cases (composefs, > containerd, and Android APEX for some Android vendors), and the fix is > self-contained. > > Essentially, we are allowing one extra unaccounted fs stacking level of > EROFS below stacking filesystems, but EROFS can only be used in the read > path (i.e. overlayfs lower layers), which typically has much lower stack > usage than the write path. > > We can consider increasing FILESYSTEM_MAX_STACK_DEPTH later, after more > stack usage analysis or using alternative approaches, such as splitting > the `s_stack_depth` limitation according to different combinations of > stacking. > > Fixes: d53cd891f0e4 ("erofs: limit the level of fs stacking for file-backed > mounts") > Reported-and-tested-by: Dusty Mabe <[email protected]> > Reported-by: Timothée Ravier <[email protected]> > Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1] > Reported-by: "Alekséi Naidénov" <[email protected]> > Closes: > https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0qxwcpec9s...@mail.gmail.com > Acked-by: Amir Goldstein <[email protected]> > Acked-by: Alexander Larsson <[email protected]> > Cc: Christian Brauner <[email protected]> > Cc: Miklos Szeredi <[email protected]> > Cc: Sheng Yong <[email protected]> > Cc: Zhiguo Niu <[email protected]> > Signed-off-by: Gao Xiang <[email protected]> > --- > v2->v3 RESEND: > - Exclude bdev-backed EROFS mounts since it will be a real terminal fs > as pointed out by Sheng Yong (APEX will rely on this); > > - Preserve previous "Acked-by:" and "Tested-by:" since it's trivial. > > fs/erofs/super.c | 19 +++++++++++++------ > 1 file changed, 13 insertions(+), 6 deletions(-) > > diff --git a/fs/erofs/super.c b/fs/erofs/super.c > index 937a215f626c..5136cda5972a 100644 > --- a/fs/erofs/super.c > +++ b/fs/erofs/super.c > @@ -644,14 +644,21 @@ static int erofs_fc_fill_super(struct super_block *sb, > struct fs_context *fc) > * fs contexts (including its own) due to self-controlled RO > * accesses/contexts and no side-effect changes that need to > * context save & restore so it can reuse the current thread > - * context. However, it still needs to bump `s_stack_depth` > to > - * avoid kernel stack overflow from nested filesystems. > + * context. > + * However, we still need to prevent kernel stack overflow due > + * to filesystem nesting: just ensure that s_stack_depth is 0 > + * to disallow mounting EROFS on stacked filesystems. > + * Note: s_stack_depth is not incremented here for now, since > + * EROFS is the only fs supporting file-backed mounts for now. > + * It MUST change if another fs plans to support them, which > + * may also require adjusting FILESYSTEM_MAX_STACK_DEPTH. > */ > 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 && > + !inode->i_sb->s_bdev) || > + inode->i_sb->s_stack_depth) { > + erofs_err(sb, "file-backed mounts cannot be > applied to stacked fses"); Hi Xiang Do we need to print s_stack_depth here to distinguish which specific problem case it is? Other LGTM based on my basic test. so
Reviewed-by: Zhiguo Niu <[email protected]> Thanks！ > return -ENOTBLK; > } > } > -- > 2.43.5 >
