Avoid the hard requirement of struct mount_attr/MOUNT_ATTR_IDMAP/struct loop_config/LOOP_CONFIGURE

[rborn-tx]

When building the project on a Linux Mint 20.3 (based on Ubuntu Focal) I get the following errors:

...
  CC       composefs/libcomposefs/libcomposefs_la-lcfs-writer.lo
  CC       composefs/libcomposefs/libcomposefs_la-lcfs-mount.lo
composefs/libcomposefs/lcfs-mount.c:112:13: warning: ‘struct mount_attr’ declared inside parameter list will not be visible outside of this definition or declaration
  112 |      struct mount_attr *attr, size_t usize)
      |             ^~~~~~~~~~
composefs/libcomposefs/lcfs-mount.c: In function ‘setup_loopback’:
composefs/libcomposefs/lcfs-mount.c:273:9: error: variable ‘loopconfig’ has initializer but incomplete type
  273 |  struct loop_config loopconfig = { 0 };
      |         ^~~~~~~~~~~
composefs/libcomposefs/lcfs-mount.c:273:36: warning: excess elements in struct initializer
  273 |  struct loop_config loopconfig = { 0 };
      |                                    ^
composefs/libcomposefs/lcfs-mount.c:273:36: note: (near initialization for ‘loopconfig’)
composefs/libcomposefs/lcfs-mount.c:273:21: error: storage size of ‘loopconfig’ isn’t known
  273 |  struct loop_config loopconfig = { 0 };
      |                     ^~~~~~~~~~
composefs/libcomposefs/lcfs-mount.c:303:20: error: ‘LOOP_CONFIGURE’ undeclared (first use in this function)
  303 |  if (ioctl(loopfd, LOOP_CONFIGURE, &loopconfig) < 0) {
      |                    ^~~~~~~~~~~~~~
composefs/libcomposefs/lcfs-mount.c:303:20: note: each undeclared identifier is reported only once for each function it appears in
composefs/libcomposefs/lcfs-mount.c:273:21: warning: unused variable ‘loopconfig’ [-Wunused-variable]
  273 |  struct loop_config loopconfig = { 0 };
      |                     ^~~~~~~~~~
composefs/libcomposefs/lcfs-mount.c: In function ‘lcfs_mount_erofs’:
composefs/libcomposefs/lcfs-mount.c:384:10: error: variable ‘attr’ has initializer but incomplete type
  384 |   struct mount_attr attr = {
      |          ^~~~~~~~~~
composefs/libcomposefs/lcfs-mount.c:385:5: error: ‘struct mount_attr’ has no member named ‘attr_set’
  385 |    .attr_set = MOUNT_ATTR_IDMAP,
      |     ^~~~~~~~
composefs/libcomposefs/lcfs-mount.c:385:16: error: ‘MOUNT_ATTR_IDMAP’ undeclared (first use in this function); did you mean ‘MOUNT_ATTR_NODEV’?
  385 |    .attr_set = MOUNT_ATTR_IDMAP,
      |                ^~~~~~~~~~~~~~~~
      |                MOUNT_ATTR_NODEV
composefs/libcomposefs/lcfs-mount.c:385:16: warning: excess elements in struct initializer
composefs/libcomposefs/lcfs-mount.c:385:16: note: (near initialization for ‘attr’)
composefs/libcomposefs/lcfs-mount.c:386:5: error: ‘struct mount_attr’ has no member named ‘userns_fd’
  386 |    .userns_fd = state->options->idmap_fd,
      |     ^~~~~~~~~
composefs/libcomposefs/lcfs-mount.c:386:17: warning: excess elements in struct initializer
  386 |    .userns_fd = state->options->idmap_fd,
      |                 ^~~~~
composefs/libcomposefs/lcfs-mount.c:386:17: note: (near initialization for ‘attr’)
composefs/libcomposefs/lcfs-mount.c:384:21: error: storage size of ‘attr’ isn’t known
  384 |   struct mount_attr attr = {
      |                     ^~~~
composefs/libcomposefs/lcfs-mount.c:390:17: error: invalid application of ‘sizeof’ to incomplete type ‘struct mount_attr’
  390 |          sizeof(struct mount_attr));
      |                 ^~~~~~
composefs/libcomposefs/lcfs-mount.c:384:21: warning: unused variable ‘attr’ [-Wunused-variable]
  384 |   struct mount_attr attr = {
      |                     ^~~~
make[2]: *** [Makefile:5304: composefs/libcomposefs/libcomposefs_la-lcfs-mount.lo] Error 1
  CC       src/rofiles-fuse/rofiles_fuse-main.o
  CC       src/libostree/tests_test_rollsum_cli-ostree-rollsum.o
...

Basically, the build fails because of two reasons:

1. The header linux/mount.h misses the struct mount_attr and macro MOUNT_ATTR_IDMAP; this happens despite the fact that "new mount API" has been detected.
2. The header linux/loop.h misses the struct loop_config and macro LOOP_CONFIGURE.

I was wondering if it would make sense to solve these issues in the upstream project to allow the build on machines having older Linux headers. If it does, I have a patch that tries to tackle them and I could create a PR for it. What do you guys think?

[alexlarsson]

I don't mind making this work on older kernels. The idmap is just about adding some checks, but for missing LOOP_CONFIGURE you eill need to have some alternative implementations of some things.