Move mountpoint checking into bootc_utils

Also:

- Add a new API that hard requires a new enough kernel, for
  cases where we really need to know for sure if it's a mountpoint
  or not.
- Change to use BorrowedFd since we don't need cap-std specifically
  here

Signed-off-by: Colin Walters <walters@verbum.org>

lib/src/install.rs

@@ -1711,7 +1711,7 @@ pub(crate) async fn install_to_filesystem(
 
         tracing::debug!("Root filesystem: {root_path}");
 
-        if let Some(false) = ostree_ext::mountutil::is_mountpoint(&rootfs_fd, ".")? {
+        if let Some(false) = bootc_utils::mount::is_mountpoint_compat(rootfs_fd.as_fd(), ".")? {
             anyhow::bail!("Not a mountpoint: {root_path}");
         }
         rootfs_fd

ostree-ext/src/commit.rs

@@ -2,18 +2,20 @@
 //! procedures as part of building an ostree container image.
 //! <https://github.com/ostreedev/ostree-rs-ext/issues/159>
 
-use crate::container_utils::require_ostree_container;
-use crate::mountutil::is_mountpoint;
 use anyhow::Context;
 use anyhow::Result;
 use cap_std::fs::Dir;
 use cap_std::fs::MetadataExt;
 use cap_std_ext::cap_std;
 use cap_std_ext::dirext::CapStdExtDirExt;
+use std::os::fd::AsFd;
 use std::path::Path;
 use std::path::PathBuf;
 use tokio::task;
 
+use crate::container_utils::require_ostree_container;
+use bootc_utils::mount::is_mountpoint_compat;
+
 /// Directories for which we will always remove all content.
 const FORCE_CLEAN_PATHS: &[&str] = &["run", "tmp", "var/tmp", "var/cache"];
 
@@ -60,7 +62,7 @@ fn clean_subdir(root: &Dir, rootdev: u64) -> Result<()> {
         }
         // Also ignore bind mounts, if we have a new enough kernel with statx()
         // that will tell us.
-        if is_mountpoint(root, &path)?.unwrap_or_default() {
+        if is_mountpoint_compat(root.as_fd(), &path)?.unwrap_or_default() {
             tracing::trace!("Skipping mount point {path:?}");
             continue;
         }

ostree-ext/src/lib.rs

@@ -39,7 +39,6 @@ pub mod diff;
 pub mod ima;
 pub mod keyfileext;
 pub(crate) mod logging;
-pub mod mountutil;
 pub mod ostree_prepareroot;
 pub mod refescape;
 #[doc(hidden)]

utils/Cargo.toml

@@ -8,12 +8,14 @@ repository = "https://github.com/containers/bootc"
 
 [dependencies]
 anyhow = { workspace = true }
+cap-std-ext = { workspace = true, features = ["fs_utf8"] }
 rustix = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 tempfile = { workspace = true }
 tracing = { workspace = true }
 tokio = { workspace = true, features = ["process"] }
+libc = { workspace = true }
 
 [dev-dependencies]
 similar-asserts = { workspace = true }

utils/src/lib.rs

@@ -4,3 +4,4 @@
 //!
 mod command;
 pub use command::*;
+pub mod mount;

ostree-ext/src/mountutil.rs → utils/src/mount.rs

@@ -1,19 +1,17 @@
 //! Helpers for interacting with mounts.
 
-use std::os::fd::AsFd;
+use std::os::fd::{AsFd, BorrowedFd};
 use std::path::Path;
 
 use anyhow::Result;
-use cap_std::fs::Dir;
-use cap_std_ext::cap_std;
 
 // Fix musl support
 #[cfg(target_env = "gnu")]
 use libc::STATX_ATTR_MOUNT_ROOT;
 #[cfg(target_env = "musl")]
 const STATX_ATTR_MOUNT_ROOT: libc::c_int = 0x2000;
 
-fn is_mountpoint_impl_statx(root: &Dir, path: &Path) -> Result<Option<bool>> {
+fn is_mountpoint_impl_statx(root: BorrowedFd, path: &Path) -> Result<Option<bool>> {
     // https://github.com/systemd/systemd/blob/8fbf0a214e2fe474655b17a4b663122943b55db0/src/basic/mountpoint-util.c#L176
     use rustix::fs::{AtFlags, StatxFlags};
 
@@ -34,27 +32,39 @@ fn is_mountpoint_impl_statx(root: &Dir, path: &Path) -> Result<Option<bool>> {
     }
 }
 
-/// Try to (heuristically) determine if the provided path is a mount root.
-pub fn is_mountpoint(root: &Dir, path: impl AsRef<Path>) -> Result<Option<bool>> {
+/// Check if the target path is a mount point. On older systems without
+/// `statx` support or that is missing support for the `STATX_ATTR_MOUNT_ROOT`,
+/// this will return `Ok(None)`.
+pub fn is_mountpoint_compat(root: BorrowedFd, path: impl AsRef<Path>) -> Result<Option<bool>> {
     is_mountpoint_impl_statx(root, path.as_ref())
 }
 
+/// Check if the target path is a mount point.
+pub fn is_mountpoint(root: BorrowedFd, path: impl AsRef<Path>) -> Result<bool> {
+    match is_mountpoint_compat(root, path)? {
+        Some(r) => Ok(r),
+        None => anyhow::bail!("statx missing mountpoint support"),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
-    use cap_std_ext::cap_tempfile;
+    use cap_std_ext::{cap_std, cap_tempfile};
 
     #[test]
     fn test_is_mountpoint() -> Result<()> {
         let root = cap_std::fs::Dir::open_ambient_dir("/", cap_std::ambient_authority())?;
-        let supported = is_mountpoint(&root, Path::new("/")).unwrap();
+        let supported = is_mountpoint_compat(root.as_fd(), Path::new("/")).unwrap();
         match supported {
             Some(r) => assert!(r),
             // If the host doesn't support statx, ignore this for now
             None => return Ok(()),
         }
         let tmpdir = cap_tempfile::TempDir::new(cap_std::ambient_authority())?;
-        assert!(!is_mountpoint(&tmpdir, Path::new(".")).unwrap().unwrap());
+        assert!(!is_mountpoint_compat(tmpdir.as_fd(), Path::new("."))
+            .unwrap()
+            .unwrap());
         Ok(())
     }
 }