Missing support for mount --bind of nondirectory files

[lyphyser]

Description

mount --bind fails unless the source and destination are directories.

In Linux, any non-directory filesystem node can be bind mounted on any non-directory target (in addition to bind mounting directories on directories).

Steps to reproduce

echo foo > foo
echo bar > bar
mount --bind foo bar

runsc version

docker version (if using docker)

uname

No response

kubectl (if using Kubernetes)

repo state (if built from source)

No response

runsc debug logs (if available)

[tarunbtw]

Tried to reproduce this on current main and it looks like it's already working correctly.

Build: bazel build //runsc on latest main (commit: 0ebccb7857cc4b4adffe2d0dad5ba903cfa7d011)

Ran inside a runsc sandbox (docker run --runtime=runsc --cap-add=SYS_ADMIN ubuntu bash):

echo foo > foo
echo bar > bar
mount --bind foo bar
echo $?      # 0

cat bar      # foo
echo hello >> bar
cat foo      # foo / hello  -> writes through bar affect foo, confirming a real bind, not a no-op

umount bar
cat bar      # bar  -> original content restored after unmount

Bind mount of two regular (non-directory) files works as expected: mount succeeds, content is shared through the bind, and unmounting restores the original file. Looking at vfs.BindAt in pkg/sentry/vfs/mount.go, the type-check there only rejects a mismatch between directory and non-directory (sourceStat.Mode.IsDir() != targetStat.Mode.IsDir()), so file-to-file bind mounts pass through fine.

Might have been fixed as a side effect of other VFS work since this was filed. Can someone confirm whether this can be closed, or point me to a case that still fails?