From 62317fd431b2c49979eb14b9307bfedf533f72c9 Mon Sep 17 00:00:00 2001
From: Colin Walters <walters@verbum.org>
Date: Mon, 18 Sep 2023 15:08:06 -0400
Subject: [PATCH] commit: Skip bind mounts too

This fixes a failure I was seeing using `buildah` in Gitlab CI;
it seems like `/run/secrets` ends up as a bind mount here, and
because it's a bind mount it has the same device node.

With new enough Linux there's a handy attribute from the `statx`
system call that will tell us whether a given path is a mount
point root.  Let's use that to start.

(The other alternatives are much uglier; see the linked systemd
 git source code)

We may need to end up doing some of that if we find we
keep hitting this with systems that have older kernels.
---
 lib/src/commit.rs | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/lib/src/commit.rs b/lib/src/commit.rs
index e0bc0cc8..2ab12341 100644
--- a/lib/src/commit.rs
+++ b/lib/src/commit.rs
@@ -9,6 +9,7 @@ use camino::Utf8Path;
 use cap_std::fs::Dir;
 use cap_std_ext::cap_std;
 use cap_std_ext::dirext::CapStdExtDirExt;
+use io_lifetimes::AsFd;
 use rustix::fs::MetadataExt;
 use std::borrow::Cow;
 use std::convert::TryInto;
@@ -101,16 +102,42 @@ fn remove_all_on_mount_recurse(root: &Dir, rootdev: u64, path: &Path) -> Result<
     Ok(skipped)
 }
 
+/// Try to (heuristically) determine if the provided path is a mount root.
+fn is_mountpoint(root: &Dir, path: &Path) -> bool {
+    // https://github.com/systemd/systemd/blob/8fbf0a214e2fe474655b17a4b663122943b55db0/src/basic/mountpoint-util.c#L176
+    use rustix::fs::{AtFlags, StatxFlags};
+
+    let attr_mount_root: u32 = libc::STATX_ATTR_MOUNT_ROOT.try_into().unwrap();
+    let flags: StatxFlags = StatxFlags::from_bits(attr_mount_root).unwrap();
+    if let Ok(meta) = rustix::fs::statx(
+        root.as_fd(),
+        path,
+        AtFlags::NO_AUTOMOUNT | AtFlags::SYMLINK_NOFOLLOW,
+        flags,
+    ) {
+        (meta.stx_attributes & attr_mount_root as u64) > 0
+    } else {
+        return false;
+    }
+}
+
 fn clean_subdir(root: &Dir, rootdev: u64) -> Result<()> {
     for entry in root.entries()? {
         let entry = entry?;
         let metadata = entry.metadata()?;
         let dev = metadata.dev();
+        let path = PathBuf::from(entry.file_name());
         // Ignore other filesystem mounts, e.g. podman injects /run/.containerenv
         if dev != rootdev {
+            tracing::trace!("Skipping entry in foreign dev {path:?}");
+            continue;
+        }
+        // Also ignore bind mounts, if we have a new enough kernel with statx()
+        // that will tell us.
+        if is_mountpoint(root, &path) {
+            tracing::trace!("Skipping mount point {path:?}");
             continue;
         }
-        let path = PathBuf::from(entry.file_name());
         if metadata.is_dir() {
             remove_all_on_mount_recurse(root, rootdev, &path)?;
         } else {