test markus pr2

Signed-off-by: Markus Rudy <mr@edgeless.systems>

Cargo.toml

@@ -159,6 +159,7 @@ netns-rs = "0.1.0"
 # Note: nix needs to stay sync'd with libs versions
 nix = "0.26.4"
 oci-spec = { version = "0.8.1", features = ["runtime"] }
+pathrs = "0.2.4"
 opentelemetry = { version = "0.17.0", features = ["rt-tokio"] }
 procfs = "0.12.0"
 prometheus = { version = "0.14.0", features = ["process"] }

src/agent/Cargo.toml

@@ -81,6 +81,7 @@ safe-path.workspace = true
 # to be modified at runtime.
 logging.workspace = true
 vsock-exporter.workspace = true
+pathrs.workspace = true
 
 # Initdata
 base64 = "0.22"

src/agent/rustjail/Cargo.toml

@@ -46,6 +46,7 @@ libseccomp = { version = "0.3.0", optional = true }
 zbus = "3.12.0"
 bit-vec = "0.8.0"
 xattr = "0.2.3"
+pathrs.workspace = true
 
 # Local dependencies
 protocols.workspace = true

src/agent/rustjail/src/mount.rs

@@ -16,8 +16,9 @@ use nix::NixPath;
 use oci::{LinuxDevice, Mount, Process, Spec};
 use oci_spec::runtime as oci;
 use std::collections::{HashMap, HashSet};
-use std::fs::{self, OpenOptions};
+use std::fs;
 use std::mem::MaybeUninit;
+use std::os::fd::AsRawFd;
 use std::os::unix;
 use std::os::unix::fs::PermissionsExt;
 use std::os::unix::io::RawFd;
@@ -47,6 +48,7 @@ pub struct Info {
 const MOUNTINFO_FORMAT: &str = "{d} {d} {d}:{d} {} {} {} {}";
 const MOUNTINFO_PATH: &str = "/proc/self/mountinfo";
 const PROC_PATH: &str = "/proc";
+const SHARED_DIRECTORY: &str = "/run/kata-containers/shared/containers";
 
 const ERR_FAILED_PARSE_MOUNTINFO: &str = "failed to parse mountinfo file";
 const ERR_FAILED_PARSE_MOUNTINFO_FINAL_FIELDS: &str =
@@ -233,10 +235,11 @@ pub fn init_rootfs(
 
         // From https://github.com/opencontainers/runtime-spec/blob/main/config.md#mounts
         // type (string, OPTIONAL) The type of the filesystem to be mounted.
-        // bind may be only specified in the oci spec options -> flags update r#type
+        // For bind mounts, this can be empty or any string whatsoever. For consistency, we set it
+        // to 'bind'.
         let m = &{
             let mut mbind = m.clone();
-            if is_none_mount_type(mbind.typ()) && flags & MsFlags::MS_BIND == MsFlags::MS_BIND {
+            if flags.contains(MsFlags::MS_BIND) {
                 mbind.set_typ(Some("bind".to_string()));
             }
             mbind
@@ -397,13 +400,6 @@ fn mount_cgroups_v2(cfd_log: RawFd, m: &Mount, rootfs: &str, flags: MsFlags) ->
     Ok(())
 }
 
-fn is_none_mount_type(typ: &Option<String>) -> bool {
-    match typ {
-        Some(t) => t == "none",
-        None => true,
-    }
-}
-
 fn mount_cgroups(
     cfd_log: RawFd,
     m: &Mount,
@@ -763,48 +759,20 @@ fn mount_from(
     let mut d = String::from(data);
     let mount_dest = m.destination().display().to_string();
     let mount_typ = m.typ().as_ref().unwrap();
+
+    if mount_typ == "bind" {
+        // Bind mounts need special treatment for security, handle them separately.
+        return bind_mount_from(m, rootfs, flags)
+            .inspect_err(|e| log_child!(cfd_log, "bind_mount_from failed: {:?}", e));
+    }
+
     let dest = scoped_join(rootfs, mount_dest)?
         .to_str()
         .ok_or_else(|| anyhow::anyhow!("Failed to convert path to string"))?
         .to_string();
 
     let mount_source = m.source().as_ref().unwrap().display().to_string();
-    let src = if mount_typ == "bind" {
-        let src = fs::canonicalize(&mount_source)?;
-        let dir = if src.is_dir() {
-            Path::new(&dest)
-        } else {
-            Path::new(&dest).parent().unwrap()
-        };
-
-        fs::create_dir_all(dir).inspect_err(|e| {
-            log_child!(
-                cfd_log,
-                "create dir {}: {}",
-                dir.to_str().unwrap(),
-                e.to_string()
-            )
-        })?;
-
-        // make sure file exists so we can bind over it
-        if !src.is_dir() {
-            let _ = OpenOptions::new()
-                .create(true)
-                .truncate(true)
-                .write(true)
-                .open(&dest)
-                .map_err(|e| {
-                    log_child!(
-                        cfd_log,
-                        "open/create dest error. {}: {:?}",
-                        dest.as_str(),
-                        e
-                    );
-                    e
-                })?;
-        }
-        src.to_str().unwrap().to_string()
-    } else {
+    let src = {
         let _ = fs::create_dir_all(&dest);
         if mount_typ == "cgroup2" {
             "cgroup2".to_string()
@@ -864,25 +832,126 @@ fn mount_from(
     if !label.is_empty() && selinux::is_enabled()? && use_xattr {
         xattr::set(dest.as_str(), "security.selinux", label.as_bytes())?;
     }
+    Ok(())
+}
 
-    if flags.contains(MsFlags::MS_BIND)
-        && flags.intersects(
-            !(MsFlags::MS_REC
-                | MsFlags::MS_REMOUNT
-                | MsFlags::MS_BIND
-                | MsFlags::MS_PRIVATE
-                | MsFlags::MS_SHARED
-                | MsFlags::MS_SLAVE),
+fn bind_mount_from(m: &Mount, rootfs: &str, flags: MsFlags) -> Result<()> {
+    let mount_source_fd = {
+        let shared_dir = PathBuf::from(SHARED_DIRECTORY);
+        let unsafe_mount_source = m
+            .source()
+            .as_ref()
+            .ok_or_else(|| anyhow::anyhow!("Missing source for bind mount"))?;
+        // Policy checks ensured earlier that shared mount sources start with the `sfprefix`.
+        // Therefore, it's safe to derive the root for scoping the mount source from that prefix.
+        let (root_path, inner_path) = match unsafe_mount_source.strip_prefix(&shared_dir) {
+            Ok(inner) => (shared_dir, inner), // needs scoping
+            Err(_) => (PathBuf::from("/"), unsafe_mount_source.as_path()), // does not need scoping, i.e. scoped to root
+        };
+        let root = pathrs::Root::open(&root_path)
+            .context(format!("opening mount_source_root {:?} failed", root_path))?;
+        root.open_subpath(inner_path, pathrs::flags::OpenFlags::O_PATH)
+            .context(format!(
+                "opening {:?} in {:?} failed",
+                inner_path, root_path
+            ))?
+    };
+
+    let container_root = pathrs::Root::open(rootfs)
+        .context(format!("opening mount_source_root {:?} failed", rootfs))?;
+
+    // .metadata queries attrs with statx + AT_EMPTY_PATH, i.e. directly from the opened fd.
+    let meta = mount_source_fd.metadata().context("statx failed")?;
+    let mount_destination_fd = if meta.is_dir() {
+        container_root
+            .mkdir_all(m.destination(), &std::fs::Permissions::from_mode(0o755))
+            .context(format!(
+                "mkdir_all for {:?} in {} failed",
+                m.destination(),
+                rootfs
+            ))?
+            .reopen(pathrs::flags::OpenFlags::O_DIRECTORY)?
+    } else if meta.is_symlink() {
+        anyhow::bail!("won't bind mount from symlink {:?}: {:?}", m.destination(), meta)
+    } else {
+        // All other bind mounts (files, devices, sockets) should target a file.
+        if let Some(parent) = m.destination().parent() {
+            let _ = container_root
+                .mkdir_all(parent, &std::fs::Permissions::from_mode(0o755))
+                .context(format!("mkdir_all for {:?} in {} failed", parent, rootfs))?;
+        }
+        container_root
+            .create_file(
+                m.destination(),
+                pathrs::flags::OpenFlags::O_TRUNC,
+                &std::fs::Permissions::from_mode(0o755),
+            )
+            .context(format!(
+                "create_file for {:?} in {} failed",
+                m.destination(),
+                rootfs
+            ))?
+    };
+
+    let open_tree_flags = if flags.intersects(MsFlags::MS_REC) {
+        libc::AT_RECURSIVE
+    } else {
+        0
+    };
+    let empty_path = std::ffi::CString::new("")?;
+
+    let tree_dfd = unsafe {
+        libc::syscall(
+            libc::SYS_open_tree,
+            mount_source_fd.as_raw_fd(),
+            empty_path.as_ptr(),
+            libc::OPEN_TREE_CLONE
+                | libc::OPEN_TREE_CLOEXEC
+                | libc::AT_EMPTY_PATH as u32
+                | open_tree_flags as u32,
         )
-    {
+    };
+
+    if tree_dfd < 0 {
+        return Err(std::io::Error::last_os_error().into());
+    }
+
+    let ret = unsafe {
+        libc::syscall(
+            libc::SYS_move_mount,
+            tree_dfd,
+            empty_path.as_ptr(),
+            mount_destination_fd.as_raw_fd(),
+            empty_path.as_ptr(),
+            0x01 /* MOVE_MOUNT_F_EMPTY_PATH */ | 0x02, /* MOVE_MOUNT_T_EMPTY_PATH */
+        )
+    };
+
+    if ret < 0 {
+        return Err(std::io::Error::last_os_error().into());
+    }
+
+    if flags.intersects(
+        !(MsFlags::MS_REC
+            | MsFlags::MS_REMOUNT
+            | MsFlags::MS_BIND
+            | MsFlags::MS_PRIVATE
+            | MsFlags::MS_SHARED
+            | MsFlags::MS_SLAVE),
+    ) {
+        // TODO(burgerdev): we really should be using mount_setattr here, but the necessary API is not in nix.
+
+        // We successfully resolved the destination within the rootfs above. If nothing else messed
+        // with the filesystem in between, using the path unchecked should be safe.
+        let dest = scoped_join(rootfs, m.destination())?;
         mount(
-            Some(dest.as_str()),
-            dest.as_str(),
+            Some(&dest),
+            &dest,
             None::<&str>,
             flags | MsFlags::MS_REMOUNT,
             None::<&str>,
         )
-        .inspect_err(|e| log_child!(cfd_log, "remout {}: {:?}", dest.as_str(), e))?;
+        .context("remount failed")?;
     }
     Ok(())
 }

src/agent/src/rpc.rs

@@ -4,6 +4,7 @@
 //
 
 use async_trait::async_trait;
+use pathrs::flags::OpenFlags;
 use rustjail::{pipestream::PipeStream, process::StreamType};
 use tokio::io::{AsyncReadExt, AsyncWriteExt, ReadHalf};
 use tokio::sync::Mutex;
@@ -13,6 +14,7 @@ use std::convert::TryFrom;
 use std::ffi::{CString, OsStr};
 use std::fmt::Debug;
 use std::io;
+use std::os::fd::AsRawFd;
 use std::os::unix::ffi::OsStrExt;
 use std::path::Path;
 #[cfg(target_arch = "s390x")]
@@ -99,7 +101,7 @@ use std::os::unix::prelude::PermissionsExt;
 use std::process::{Command, Stdio};
 
 use nix::unistd::{Gid, Uid};
-use std::fs::{File, OpenOptions};
+use std::fs::File;
 use std::io::{BufRead, BufReader, Write};
 use std::os::unix::fs::FileExt;
 use std::path::PathBuf;
@@ -132,6 +134,18 @@ const ERR_NO_SANDBOX_PIDNS: &str = "Sandbox does not have sandbox_pidns";
 // not available.
 const IPTABLES_RESTORE_WAIT_SEC: u64 = 5;
 
+/// This mask is applied to parent directories implicitly created for CopyFile requests.
+const IMPLICIT_DIRECTORY_PERMISSION_MASK: u32 = 0o777;
+
+/// This mask is applied to directories explicitly created for CopyFile requests.
+/// setgid and sticky bit are valid for such directories, whereas setuid is not.
+const EXPLICIT_DIRECTORY_PERMISSION_MASK: u32 = 0o3777;
+
+/// This mask is applied to files created for CopyFile requests.
+/// This constant is used for consistency with *_DIRECTORY_PERMISSION_MASK.
+const FILE_PERMISSION_MASK: u32 = 0o7777;
+
+
 // Convenience function to obtain the scope logger.
 fn sl() -> slog::Logger {
     slog_scope::logger()
@@ -1521,7 +1535,9 @@ impl agent_ttrpc::AgentService for AgentService {
         trace_rpc_call!(ctx, "copy_file", req);
         is_allowed(&req).await?;
 
-        do_copy_file(&req).map_ttrpc_err(same)?;
+        // Potentially untrustworthy data from the host needs to go into the shared dir.
+        let root_path = PathBuf::from(KATA_GUEST_SHARE_DIR);
+        do_copy_file(&req, &root_path).map_ttrpc_err(same)?;
 
         Ok(Empty::new())
     }
@@ -2035,125 +2051,116 @@ fn do_set_guest_date_time(sec: i64, usec: i64) -> Result<()> {
     Ok(())
 }
 
-fn do_copy_file(req: &CopyFileRequest) -> Result<()> {
-    let path = PathBuf::from(req.path.as_str());
+/// do_copy_file creates a file, directory or symlink beneath the provided directory.
+///
+/// The function guarantees that no content is written outside of the directory. However, a symlink
+/// created by this function might point outside the shared directory. Other users of that
+/// directory need to consider whether they trust the host, or handle the directory with the same
+/// care as do_copy_file.
+///
+/// Parent directories are created, if they don't exist already. For these implicit operations, the
+/// permissions are set with req.dir_mode. The actual target is created with permissions from
+/// req.file_mode, even if it's a directory. For symlinks, req.file_mode is
+///
+/// If this function returns an error, the filesystem may be in an unexpected state. This is not
+/// significant for the caller, since errors are almost certainly not retriable. The runtime should
+/// abandon this VM instead.
+fn do_copy_file(req: &CopyFileRequest, shared_dir: &PathBuf) -> Result<()> {
+    let insecure_full_path = PathBuf::from(req.path.as_str());
+    let path = insecure_full_path
+        .strip_prefix(&shared_dir)
+        .context(format!(
+            "removing {:?} prefix from {}",
+            shared_dir, req.path
+        ))?;
 
-    if !path.starts_with(CONTAINER_BASE) {
-        return Err(anyhow!(
-            "Path {:?} does not start with {}",
-            path,
-            CONTAINER_BASE
-        ));
-    }
+    // The shared directory might not exist yet, but we need to create it in order to open the root.
+    std::fs::create_dir_all(shared_dir)?;
+    let root = pathrs::Root::open(shared_dir)?;
+
+    // Remove anything that might already exist at the target location.
+    // This is safe even for a symlink leaf, remove_all removes the named inode in its parent dir.
+    root.remove_all(path).or_else(|e| match e.kind() {
+        pathrs::error::ErrorKind::OsError(Some(errno)) if errno == libc::ENOENT => Ok(()),
+        _ => Err(e),
+    })?;
 
     // Create parent directories if missing
     if let Some(parent) = path.parent() {
-        if !parent.exists() {
-            let dir = parent.to_path_buf();
-            // Attempt to create directory, ignore AlreadyExists errors
-            if let Err(e) = fs::create_dir_all(&dir) {
-                if e.kind() != std::io::ErrorKind::AlreadyExists {
-                    return Err(e.into());
-                }
-            }
+        let dir = root
+            .mkdir_all(
+                parent,
+                &std::fs::Permissions::from_mode(req.dir_mode & IMPLICIT_DIRECTORY_PERMISSION_MASK),
+            )
+            .context("mkdir_all parent")?
+            .reopen(OpenFlags::O_DIRECTORY)
+            .context("reopen parent")?;
 
-            // Set directory permissions and ownership
-            std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(req.dir_mode))?;
-            unistd::chown(
-                &dir,
-                Some(Uid::from_raw(req.uid as u32)),
-                Some(Gid::from_raw(req.gid as u32)),
-            )?;
-        }
+        // TODO(burgerdev): why are we only applying this to the immediate parent?
+        unistd::fchown(
+            dir.as_raw_fd(),
+            Some(Uid::from_raw(req.uid as u32)),
+            Some(Gid::from_raw(req.gid as u32)),
+        )
+        .context("fchown parent")?
     }
 
     let sflag = stat::SFlag::from_bits_truncate(req.file_mode);
 
     if sflag.contains(stat::SFlag::S_IFDIR) {
-        // Remove existing non-directory file if present
-        if path.exists() && !path.is_dir() {
-            fs::remove_file(&path)?;
-        }
+        // mkdir_all does not support the setuid/setgid/sticky bits, so we first create the
+        // directory with the stricter mask and then change permissions with the correct mask.
+        let dir = root
+            .mkdir_all(
+                &path,
+                &std::fs::Permissions::from_mode(req.file_mode & IMPLICIT_DIRECTORY_PERMISSION_MASK),
+            )
+            .context("mkdir_all dir")?
+            .reopen(OpenFlags::O_DIRECTORY)
+            .context("reopen dir")?;
+        dir.set_permissions(std::fs::Permissions::from_mode(req.file_mode & EXPLICIT_DIRECTORY_PERMISSION_MASK))?;
 
-        fs::create_dir(&path).or_else(|e| {
-            if e.kind() != std::io::ErrorKind::AlreadyExists {
-                return Err(e);
-            }
-            Ok(())
-        })?;
-
-        std::fs::set_permissions(&path, std::fs::Permissions::from_mode(req.file_mode))?;
-
-        unistd::chown(
-            &path,
+        unistd::fchown(
+            dir.as_raw_fd(),
             Some(Uid::from_raw(req.uid as u32)),
             Some(Gid::from_raw(req.gid as u32)),
-        )?;
+        )
+        .context("fchown dir")?;
 
         return Ok(());
     }
 
     // Handle symlink creation
     if sflag.contains(stat::SFlag::S_IFLNK) {
-        // Clean up existing path (whether symlink, dir, or file)
-        if path.exists() || path.is_symlink() {
-            // Use appropriate removal method based on path type
-            if path.is_symlink() {
-                unistd::unlink(&path)?;
-            } else if path.is_dir() {
-                fs::remove_dir_all(&path)?;
-            } else {
-                fs::remove_file(&path)?;
-            }
-        }
-
         // Create new symbolic link
         let src = PathBuf::from(OsStr::from_bytes(&req.data));
-        unistd::symlinkat(&src, None, &path)?;
-
-        // Set symlink ownership (permissions not supported for symlinks)
-        let path_str = CString::new(path.as_os_str().as_bytes())?;
-
-        let ret = unsafe { libc::lchown(path_str.as_ptr(), req.uid as u32, req.gid as u32) };
-        Errno::result(ret).map(drop)?;
-
+        root.create(path, &pathrs::InodeType::Symlink(src))
+            .context("create symlink")?;
+        // Symlinks don't have permissions on Linux!
         return Ok(());
     }
 
-    let mut tmpfile = path.clone();
-    tmpfile.set_extension("tmp");
+    // Write file content.
+    let flags = if req.offset == 0 {
+        OpenFlags::O_RDWR | OpenFlags::O_CREAT | OpenFlags::O_TRUNC
+    } else {
+        OpenFlags::O_RDWR | OpenFlags::O_CREAT
+    };
+    let file = root
+        .create_file(path, flags, &std::fs::Permissions::from_mode(req.file_mode & FILE_PERMISSION_MASK))
+        .context("create_file")?;
+    file.write_all_at(req.data.as_slice(), req.offset as u64)
+        .context("write_all_at")?;
+    // Things like umask can change the permissions after create, make sure that they stay
+    file.set_permissions(std::fs::Permissions::from_mode(req.file_mode & FILE_PERMISSION_MASK))
+        .context("set_permissions")?;
 
-    let file = OpenOptions::new()
-        .write(true)
-        .create(true)
-        .truncate(req.offset == 0) // Only truncate when offset is 0
-        .open(&tmpfile)?;
-
-    file.write_all_at(req.data.as_slice(), req.offset as u64)?;
-    let st = stat::stat(&tmpfile)?;
-
-    if st.st_size != req.file_size {
-        return Ok(());
-    }
-
-    file.set_permissions(std::fs::Permissions::from_mode(req.file_mode))?;
-
-    unistd::chown(
-        &tmpfile,
+    unistd::fchown(
+        file.as_raw_fd(),
         Some(Uid::from_raw(req.uid as u32)),
         Some(Gid::from_raw(req.gid as u32)),
-    )?;
-
-    // Remove existing target path before rename
-    if path.exists() || path.is_symlink() {
-        if path.is_dir() {
-            fs::remove_dir_all(&path)?;
-        } else {
-            fs::remove_file(&path)?;
-        }
-    }
-
-    fs::rename(tmpfile, path)?;
+    )
+    .context("fchown")?;
 
     Ok(())
 }
@@ -2444,6 +2451,7 @@ mod tests {
 
     use super::*;
     use crate::{namespace::Namespace, protocols::agent_ttrpc_async::AgentService as _};
+    use anyhow::ensure;
     use nix::mount;
     use nix::sched::{unshare, CloneFlags};
     use oci::{
@@ -3465,4 +3473,274 @@ COMMIT
             assert_eq!(d.result, result, "{msg}");
         }
     }
+
+    #[tokio::test]
+    async fn test_do_copy_file() {
+        let temp_dir = tempdir().expect("creating temp dir failed");
+        // We start one directory deeper such that we catch problems when the shared directory does
+        // not exist yet.
+        let base = temp_dir.path().join("shared");
+
+        struct TestCase {
+            name: String,
+            request: CopyFileRequest,
+            assertions: Box<dyn Fn(&Path) -> Result<()>>,
+            should_fail: bool,
+        }
+
+        let tests = [
+            TestCase {
+                name: "Create a top-level file".into(),
+                request: CopyFileRequest {
+                    path: base.join("f").to_string_lossy().into(),
+                    file_mode: 0o644 | libc::S_IFREG,
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let f = base.join("f");
+                    let f_stat = fs::metadata(&f).context("stat ./f failed")?;
+                    ensure!(f_stat.is_file());
+                    ensure!(0o644 == f_stat.permissions().mode() & 0o777);
+                    let content = std::fs::read_to_string(&f).context("read ./f failed")?;
+                    ensure!(content.is_empty());
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Replace a top-level file".into(),
+                request: CopyFileRequest {
+                    path: base.join("f").to_string_lossy().into(),
+                    file_mode: 0o600 | libc::S_IFREG,
+                    data: b"Hello!".to_vec(),
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let f = base.join("f");
+                    let f_stat = fs::metadata(&f).context("stat ./f failed")?;
+                    ensure!(f_stat.is_file());
+                    ensure!(0o600 == f_stat.permissions().mode() & 0o777);
+                    let content = std::fs::read_to_string(&f).context("read ./f failed")?;
+                    ensure!("Hello!" == content);
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a file and its parent directory".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/b").to_string_lossy().into(),
+                    dir_mode: 0o755 | libc::S_IFDIR,
+                    file_mode: 0o644 | libc::S_IFREG,
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let a_stat = fs::metadata(base.join("a")).context("stat ./a failed")?;
+                    ensure!(a_stat.is_dir());
+                    ensure!(0o755 == a_stat.permissions().mode() & 0o777);
+                    let b_stat = fs::metadata(base.join("a/b")).context("stat ./a/b failed")?;
+                    ensure!(b_stat.is_file());
+                    ensure!(0o644 == b_stat.permissions().mode() & 0o777);
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a file within an existing directory".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/c").to_string_lossy().into(),
+                    dir_mode: 0o700 | libc::S_IFDIR, // Test that existing directories are not touched - we expect this to stay 0o755.
+                    file_mode: 0o621 | libc::S_IFREG,
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let a_stat = fs::metadata(base.join("a")).context("stat ./a failed")?;
+                    ensure!(a_stat.is_dir());
+                    ensure!(0o755 == a_stat.permissions().mode() & 0o777);
+                    let c_stat = fs::metadata(base.join("a/c")).context("stat ./a/c failed")?;
+                    ensure!(c_stat.is_file());
+                    ensure!(0o621 == c_stat.permissions().mode() & 0o777);
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a directory".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/d").to_string_lossy().into(),
+                    dir_mode: 0o700 | libc::S_IFDIR, // Test that the permissions are taken from file_mode.
+                    file_mode: 0o755 | libc::S_IFDIR,
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let a_stat = fs::metadata(base.join("a")).context("stat ./a failed")?;
+                    ensure!(a_stat.is_dir());
+                    ensure!(0o755 == a_stat.permissions().mode() & 0o777);
+                    let d_stat = fs::metadata(base.join("a/d")).context("stat ./a/d failed")?;
+                    ensure!(d_stat.is_dir());
+                    ensure!(0o755 == d_stat.permissions().mode() & 0o777);
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a dir onto an existing file".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/b").to_string_lossy().into(),
+                    dir_mode: 0o700 | libc::S_IFDIR, // Test that the permissions are taken from file_mode.
+                    file_mode: 0o755 | libc::S_IFDIR,
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let b_stat = fs::metadata(base.join("a/b")).context("stat ./a/b failed")?;
+                    ensure!(b_stat.is_dir());
+                    ensure!(0o755 == b_stat.permissions().mode() & 0o777);
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a file onto an existing dir".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/b").to_string_lossy().into(),
+                    dir_mode: 0o755 | libc::S_IFDIR,
+                    file_mode: 0o644 | libc::S_IFREG,
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let b_stat = fs::metadata(base.join("a/b")).context("stat ./a/b failed")?;
+                    ensure!(b_stat.is_file());
+                    ensure!(0o644 == b_stat.permissions().mode() & 0o777);
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a symlink".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/link").to_string_lossy().into(),
+                    dir_mode: 0o700 | libc::S_IFDIR, // Test that the permissions are taken from file_mode.
+                    file_mode: 0o755 | libc::S_IFLNK,
+                    data: b"/etc/passwd".to_vec(),
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    let a_stat = fs::metadata(base.join("a")).context("stat ./a failed")?;
+                    ensure!(a_stat.is_dir());
+                    ensure!(0o755 == a_stat.permissions().mode() & 0o777);
+                    let link = base.join("a/link");
+                    let link_stat = nix::sys::stat::lstat(&link).context("stat ./a/link failed")?;
+                    // Linux symlinks have no permissions!
+                    ensure!(0o777 | libc::S_IFLNK == link_stat.st_mode);
+                    let target = fs::read_link(&link).context("read_link ./a/link failed")?;
+                    ensure!(target.to_string_lossy() == "/etc/passwd");
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a directory with setgid and sticky bit".into(),
+                request: CopyFileRequest {
+                    path: base.join("x/y").to_string_lossy().into(),
+                    dir_mode: 0o3755 | libc::S_IFDIR,
+                    file_mode: 0o3770 | libc::S_IFDIR,
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    // Implicitly created directories should not get a sticky bit.
+                    let x_stat = fs::metadata(base.join("x")).context("stat ./x failed")?;
+                    ensure!(x_stat.is_dir());
+                    ensure!(0o755 == x_stat.permissions().mode() & 0o7777);
+                    // Explicitly created directories should.
+                    let y_stat = fs::metadata(base.join("x/y")).context("stat ./x/y failed")?;
+                    ensure!(y_stat.is_dir());
+                    ensure!(0o3770 == y_stat.permissions().mode() & 0o7777);
+                    Ok(())
+                }),
+            },
+            // =================================
+            // Below are some adversarial tests.
+            // =================================
+            TestCase {
+                name: "Malicious intermediate directory is a symlink".into(),
+                request: CopyFileRequest {
+                    path: base
+                        .join("a/link/this-could-just-be-shadow-but-I-am-not-risking-it")
+                        .to_string_lossy()
+                        .into(),
+                    dir_mode: 0o700 | libc::S_IFDIR, // Test that the permissions are taken from file_mode.
+                    file_mode: 0o755 | libc::S_IFLNK,
+                    data: b"root:password:19000:0:99999:7:::\n".to_vec(),
+                    ..Default::default()
+                },
+                should_fail: true,
+                assertions: Box::new(|base| -> Result<()> {
+                    let link_stat = nix::sys::stat::lstat(&base.join("a/link"))
+                        .context("stat ./a/link failed")?;
+                    ensure!(0o777 | libc::S_IFLNK == link_stat.st_mode);
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a symlink onto an existing symlink".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/link").to_string_lossy().into(),
+                    dir_mode: 0o700 | libc::S_IFDIR, // Test that the permissions are taken from file_mode.
+                    file_mode: 0o755 | libc::S_IFLNK,
+                    data: b"/etc".to_vec(),
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    // The symlink should be created at the same place (not followed), with the new content.
+                    let a_stat = fs::metadata(base.join("a")).context("stat ./a failed")?;
+                    ensure!(a_stat.is_dir());
+                    ensure!(0o755 == a_stat.permissions().mode() & 0o777);
+                    let link = base.join("a/link");
+                    let link_stat = nix::sys::stat::lstat(&link).context("stat ./a/link failed")?;
+                    // Linux symlinks have no permissions!
+                    ensure!(0o777 | libc::S_IFLNK == link_stat.st_mode);
+                    let target = fs::read_link(&link).context("read_link ./a/link failed")?;
+                    ensure!(target.to_string_lossy() == "/etc");
+                    Ok(())
+                }),
+            },
+            TestCase {
+                name: "Create a file onto an existing symlink".into(),
+                request: CopyFileRequest {
+                    path: base.join("a/link").to_string_lossy().into(),
+                    file_mode: 0o600 | libc::S_IFREG,
+                    data: b"Hello!".to_vec(),
+                    ..Default::default()
+                },
+                should_fail: false,
+                assertions: Box::new(|base| -> Result<()> {
+                    // The symlink itself should be replaced with the file, not followed.
+                    let link = base.join("a/link");
+                    let link_stat = nix::sys::stat::lstat(&link).context("stat ./a/link failed")?;
+                    ensure!(0o600 | libc::S_IFREG == link_stat.st_mode);
+                    let content = std::fs::read_to_string(&link).context("read ./a/link failed")?;
+                    ensure!("Hello!" == content);
+                    Ok(())
+                }),
+            },
+        ];
+
+        let uid = unistd::getuid().as_raw() as i32;
+        let gid = unistd::getgid().as_raw() as i32;
+
+        for mut tc in tests {
+            println!("Running test case: {}", tc.name);
+            // Since we're in a unit test, using root ownership causes issues with cleaning the temp dir.
+            tc.request.uid = uid;
+            tc.request.gid = gid;
+
+            let res = do_copy_file(&tc.request, (&base).into());
+            if tc.should_fail != res.is_err() {
+                panic!("{}: unexpected do_copy_file result: {:?}", tc.name, res)
+            }
+            (tc.assertions)(&base).context(tc.name).unwrap()
+        }
+    }
 }