erofs_fc_get_tree() falls back to filp_open(fc->source, ...) itself when get_tree_bdev_flags() reports the source isn't a block device. That is the only way to reach erofs's file-backed mount support today, which means an in-kernel caller that already opened and validated (e.g. fsverity-checked) that exact file has no way to make erofs mount it: it can only hand over the path again and trust erofs's independent lookup to land on the same file, which it has no way to guarantee.
Check fc->source_file first and use it directly when set, taking a reference and skipping the independent filp_open(fc->source) entirely (fc->source is unset in this case). This has no effect on the existing path-based fsconfig()/mount(2) flow, since userspace has no way to set fc->source_file (see the previous patch); it only enables in-kernel callers such as the following init/do_mounts.c change. Signed-off-by: Eric Curtin <[email protected]> --- fs/erofs/super.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/fs/erofs/super.c b/fs/erofs/super.c index 802add6652fd..b6cd4fde3ca2 100644 --- a/fs/erofs/super.c +++ b/fs/erofs/super.c @@ -799,6 +799,25 @@ static int erofs_fc_get_tree(struct fs_context *fc) if (IS_ENABLED(CONFIG_EROFS_FS_ONDEMAND) && sbi->fsid) return get_tree_nodev(fc, erofs_fc_fill_super); + /* + * An in-kernel caller (see path_mount_file()) has handed us the + * exact, already-open file to mount, rather than a path for us to + * resolve ourselves: use it directly instead of independently + * re-opening fc->source (which isn't set in this case), so that + * whatever the caller validated about this file - e.g. an fsverity + * digest - is guaranteed to still apply to what actually gets + * mounted. + */ + if (fc->source_file) { + if (!IS_ENABLED(CONFIG_EROFS_FS_BACKED_BY_FILE)) + return invalf(fc, "File-backed mounts not supported"); + if (!S_ISREG(file_inode(fc->source_file)->i_mode) || + !fc->source_file->f_mapping->a_ops->read_folio) + return invalf(fc, "Source file is not a plain, read-foliable file"); + sbi->dif0.file = get_file(fc->source_file); + 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); -- 2.43.0
