Incorrect error message from erofs "backed by file" in 6.12-rc

[Allison Karlitskaya]

hi,

In context of my work on composefs/bootc I've been testing the new support
for directly mounting files with erofs (ie: without a loopback device) and
it's working well.  Thanks for adding this feature --- it's a huge quality
of life improvement for us.

I've observed a strange behaviour, though: when mounting a file as an
erofs, if you read() the filesystem context fd, you always get the
following error message reported: Can't lookup blockdev.

That's caused by the code in erofs_fc_get_tree() trying to call
get_tree_bdev() and recovering from the error in case it was ENOTBLK and
CONFIG_EROFS_FS_BACKED_BY_FILE.  Unfortunately, get_tree_bdev() logs the
error directly on the fs_context, so you get the error message even on
successful mounts.

It looks something like this at the syscall level:

fsopen("erofs", FSOPEN_CLOEXEC)         = 3
fsconfig(3, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0
fsconfig(3, FSCONFIG_SET_STRING, "source", "/home/lis/src/mountcfs/cfs", 0)
= 0
fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
fsmount(3, FSMOUNT_CLOEXEC, 0)          = 5
move_mount(5, "", AT_FDCWD, "/tmp/composefs.upper.KuT5aV",
MOVE_MOUNT_F_EMPTY_PATH) = 0
read(3, "e /home/lis/src/mountcfs/cfs: Can't lookup blockdev\n", 1024) = 52

This is kernel 6.12.0-0.rc0.20240926git11a299a7933e.13.fc42.x86_64 from
Fedora Rawhide.

It's a pretty minor issue, but it sent me on a wild goose chase for an hour
or two, so probably it should get fixed before the final release.

Thanks again for this awesome feature!

Allison Karlitskaya
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