diff --git a/nvx-rs/src/host/filesystem.rs b/nvx-rs/src/host/filesystem.rs
index a8df69f37..8d5fe8071 100644
--- a/nvx-rs/src/host/filesystem.rs
+++ b/nvx-rs/src/host/filesystem.rs
@@ -781,8 +781,9 @@ struct Mapped {
     directory: bool,
 }
 
-/// Returns whether `path` is `directory` or one of its descendants.
-fn within(path: &str, directory: &str) -> bool {
+/// Returns whether the guest path `path` is `directory` or one of its descendants, comparing
+/// whole components: `/run/nvx` is within `/run`, but `/runner` is not.
+pub(crate) fn within(path: &str, directory: &str) -> bool {
     path == directory
         || path
             .strip_prefix(directory)
@@ -951,6 +952,26 @@ mod tests {
         assert_eq!(guest_path(Path::new("..")), None);
     }
 
+    /// Verifies that a guest path is within itself and its ancestors only, comparing whole
+    /// components, as the reserved targets here and the sandbox's share checks require.
+    #[test]
+    fn guest_paths_are_within_themselves_and_their_ancestors() {
+        for (path, directory) in [
+            ("/run", "/run"),
+            ("/run/nvx", "/run"),
+            ("/run/nvx/t", "/run"),
+        ] {
+            assert!(within(path, directory), "{path} {directory}");
+        }
+        for (path, directory) in [
+            ("/runner", "/run"),
+            ("/run", "/run/nvx"),
+            ("/srv/run", "/run"),
+        ] {
+            assert!(!within(path, directory), "{path} {directory}");
+        }
+    }
+
     /// The layout of microsoft/nvx#281: directories that share no directory but the volume's
     /// root, each with its own access.
     #[test]
diff --git a/nvx-rs/src/host/sandbox/mount.rs b/nvx-rs/src/host/sandbox/mount.rs
index ce6528df5..c90e30184 100644
--- a/nvx-rs/src/host/sandbox/mount.rs
+++ b/nvx-rs/src/host/sandbox/mount.rs
@@ -20,6 +20,7 @@ use sha2::{Digest, Sha256};
 
 use super::refused;
 use crate::error::{Error, Result};
+use crate::host::filesystem::within;
 use crate::host::python::builtins::{is_space, repr};
 use crate::host::python::fs;
 use crate::host::python::path::{Flavor, PurePath};
@@ -310,11 +311,7 @@ pub fn validate_mount_target(target: &str) -> Result<()> {
         )));
     }
     for reserved in RESERVED_MOUNT_TARGETS {
-        if target == reserved
-            || target
-                .strip_prefix(reserved)
-                .is_some_and(|rest| rest.starts_with('/'))
-        {
+        if within(target, reserved) {
             return Err(refused(format!(
                 "sandbox mount target {target} overlaps the reserved {reserved} tree"
             )));
@@ -349,11 +346,7 @@ pub(crate) fn require_mount_owner_supported_on(owner: MountOwner, windows: bool)
 
 /// Returns whether two guest targets overlap: whether they are equal, or one contains the other.
 pub(crate) fn targets_overlap(left: &str, right: &str) -> bool {
-    let inside = |path: &str, directory: &str| {
-        path.strip_prefix(directory)
-            .is_some_and(|rest| rest.starts_with('/'))
-    };
-    left == right || inside(left, right) || inside(right, left)
+    within(left, right) || within(right, left)
 }
 
 /// Returns the kind of share that `host_path` requests: a file for a regular file, and
diff --git a/nvx-rs/src/host/sandbox/options.rs b/nvx-rs/src/host/sandbox/options.rs
index dfdb1afb3..470c88c63 100644
--- a/nvx-rs/src/host/sandbox/options.rs
+++ b/nvx-rs/src/host/sandbox/options.rs
@@ -22,6 +22,7 @@ use super::layer::LayerRole;
 use super::mount::{
     AGGREGATE_MOUNT_TARGET, MountKind, MountOwner, MountPolicy, POLICY_KINDS, PolicyKind,
     SandboxMount, has_forbidden_character, has_irregular_component, mount_kind, os_error,
+    targets_overlap,
 };
 use super::{SandboxLaunch, refused};
 use crate::error::Result;
@@ -335,11 +336,7 @@ pub fn validate_run_share_targets(targets: &[&str]) -> Result<()> {
             )));
         }
         for other in std::iter::once(&AGGREGATE_MOUNT_TARGET).chain(&targets[..index]) {
-            let inside = |path: &str, directory: &str| {
-                path.strip_prefix(directory)
-                    .is_some_and(|rest| rest.starts_with('/'))
-            };
-            if target == other || inside(target, other) || inside(other, target) {
+            if targets_overlap(target, other) {
                 return Err(refused(format!(
                     "--mount targets {other} and {target} overlap; a share cannot hide another"
                 )));