diff --git a/mk/tests.mk b/mk/tests.mk
index f53f1cc..dc3ac5a 100644
--- a/mk/tests.mk
+++ b/mk/tests.mk
@@ -6,6 +6,7 @@ TEST_DIR   = tests/unit
 TEST_SRCS  = $(TEST_DIR)/test-runner.c \
              $(TEST_DIR)/test-fd-table.c \
              $(TEST_DIR)/test-path.c \
+             $(TEST_DIR)/test-mount.c \
              $(TEST_DIR)/test-cli.c \
              $(TEST_DIR)/test-identity.c \
              $(TEST_DIR)/test-syscall-nr.c \
@@ -30,6 +31,8 @@ endif
 # Unit tests link only the pure-computation sources (no LKL)
 TEST_SUPPORT_SRCS = $(SRC_DIR)/fd-table.c \
                     $(SRC_DIR)/path.c \
+                    $(SRC_DIR)/mount.c \
+                    $(TEST_DIR)/test-mount-stubs.c \
                     $(SRC_DIR)/cli.c \
                     $(SRC_DIR)/identity.c \
                     $(SRC_DIR)/syscall-nr.c \
diff --git a/src/mount.c b/src/mount.c
index f149d36..e5ad92d 100644
--- a/src/mount.c
+++ b/src/mount.c
@@ -10,13 +10,19 @@
 #include <errno.h>
 #include <stdio.h>
 #include <string.h>
+#include <sys/stat.h>
 
 #include "kbox/mount.h"
 #include "lkl-wrap.h"
+#include "syscall-nr.h"
 
 /* MS_BIND from <linux/mount.h> without pulling the full header. */
 #define KBOX_MS_BIND 0x1000
 
+/* asm-generic O_* values for LKL openat (same on x86_64 and aarch64). */
+#define LKL_O_CREAT 0100
+#define LKL_O_EXCL 0200
+
 /* Bind-mount spec parser. */
 
 int kbox_parse_bind_spec(const char *spec, struct kbox_bind_spec *out)
@@ -118,7 +124,90 @@ int kbox_apply_recommended_mounts(const struct kbox_sysnrs *s,
     return 0;
 }
 
-/* Bind mounts. */
+/* Bind mounts.
+ *
+ * The source is a host path; stat it to determine whether the bind-mount
+ * target inside LKL should be a directory or a regular file. Anything
+ * other than a regular file or directory is rejected up front.
+ */
+
+/* Verify that the existing inode at target has the expected type. */
+static int verify_existing_target(const struct kbox_sysnrs *s,
+                                  const char *target,
+                                  unsigned int expected_mode_type)
+{
+    struct kbox_lkl_stat lkl_st;
+    long ret;
+
+    ret = kbox_lkl_newfstatat(s, AT_FDCWD_LINUX, target, &lkl_st, 0);
+    if (ret < 0) {
+        fprintf(stderr, "stat(%s): %s\n", target, kbox_err_text(ret));
+        return -1;
+    }
+    if ((lkl_st.st_mode & S_IFMT) != expected_mode_type) {
+        fprintf(stderr,
+                "bind mount: target %s exists but has wrong type "
+                "(expected 0%o, got 0%o)\n",
+                target, expected_mode_type, lkl_st.st_mode & S_IFMT);
+        return -1;
+    }
+    return 0;
+}
+
+static int create_bind_target(const struct kbox_sysnrs *s,
+                              const char *source,
+                              const char *target)
+{
+    struct stat st;
+    long ret;
+
+    if (lstat(source, &st) < 0) {
+        fprintf(stderr, "bind mount: cannot stat source %s: %s\n", source,
+                strerror(errno));
+        return -1;
+    }
+
+    if (S_ISLNK(st.st_mode)) {
+        fprintf(stderr, "bind mount: source %s must not be a symlink\n",
+                source);
+        return -1;
+    }
+
+    if (S_ISDIR(st.st_mode)) {
+        ret = kbox_lkl_mkdir(s, target, 0755);
+        if (ret == -EEXIST)
+            return verify_existing_target(s, target, S_IFDIR);
+        if (ret < 0) {
+            fprintf(stderr, "mkdir(%s): %s\n", target, kbox_err_text(ret));
+            return -1;
+        }
+        return 0;
+    }
+
+    if (S_ISREG(st.st_mode)) {
+        long cret;
+
+        ret = kbox_lkl_openat(s, AT_FDCWD_LINUX, target,
+                              LKL_O_CREAT | LKL_O_EXCL, 0644);
+        if (ret == -EEXIST)
+            return verify_existing_target(s, target, S_IFREG);
+        if (ret < 0) {
+            fprintf(stderr, "creat(%s): %s\n", target, kbox_err_text(ret));
+            return -1;
+        }
+        cret = kbox_lkl_close(s, ret);
+        if (cret < 0) {
+            fprintf(stderr, "close(%s): %s\n", target, kbox_err_text(cret));
+            return -1;
+        }
+        return 0;
+    }
+
+    fprintf(stderr,
+            "bind mount: source %s is neither a regular file nor a directory\n",
+            source);
+    return -1;
+}
 
 int kbox_apply_bind_mounts(const struct kbox_sysnrs *s,
                            const struct kbox_bind_spec *specs,
@@ -127,13 +216,12 @@ int kbox_apply_bind_mounts(const struct kbox_sysnrs *s,
     int i;
     long ret;
 
+    if (!s || (!specs && count > 0) || count < 0)
+        return -1;
+
     for (i = 0; i < count; i++) {
-        ret = kbox_lkl_mkdir(s, specs[i].target, 0755);
-        if (ret < 0 && ret != -EEXIST) {
-            fprintf(stderr, "mkdir(%s): %s\n", specs[i].target,
-                    kbox_err_text(ret));
+        if (create_bind_target(s, specs[i].source, specs[i].target) < 0)
             return -1;
-        }
 
         ret = kbox_lkl_mount(s, specs[i].source, specs[i].target, NULL,
                              KBOX_MS_BIND, NULL);
diff --git a/tests/unit/test-mount-stubs.c b/tests/unit/test-mount-stubs.c
new file mode 100644
index 0000000..6c485b8
--- /dev/null
+++ b/tests/unit/test-mount-stubs.c
@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: MIT */
+/* Stubs for LKL functions referenced by mount.c but not exercised
+ * in the unit tests (only kbox_parse_bind_spec is tested).
+ */
+
+#include <errno.h>
+
+#include "lkl-wrap.h"
+#include "syscall-nr.h"
+
+long kbox_lkl_mkdir(const struct kbox_sysnrs *s, const char *path, int mode)
+{
+    (void) s;
+    (void) path;
+    (void) mode;
+    return -ENOSYS;
+}
+
+long kbox_lkl_mount(const struct kbox_sysnrs *s,
+                    const char *src,
+                    const char *target,
+                    const char *fstype,
+                    long flags,
+                    const void *data)
+{
+    (void) s;
+    (void) src;
+    (void) target;
+    (void) fstype;
+    (void) flags;
+    (void) data;
+    return -ENOSYS;
+}
+
+long kbox_lkl_openat(const struct kbox_sysnrs *s,
+                     long dirfd,
+                     const char *path,
+                     long flags,
+                     long mode)
+{
+    (void) s;
+    (void) dirfd;
+    (void) path;
+    (void) flags;
+    (void) mode;
+    return -ENOSYS;
+}
+
+long kbox_lkl_close(const struct kbox_sysnrs *s, long fd)
+{
+    (void) s;
+    (void) fd;
+    return 0;
+}
+
+long kbox_lkl_newfstatat(const struct kbox_sysnrs *s,
+                         long dirfd,
+                         const char *path,
+                         void *buf,
+                         long flags)
+{
+    (void) s;
+    (void) dirfd;
+    (void) path;
+    (void) buf;
+    (void) flags;
+    return -ENOSYS;
+}
+
+const char *kbox_err_text(long code)
+{
+    (void) code;
+    return "stub";
+}
diff --git a/tests/unit/test-mount.c b/tests/unit/test-mount.c
new file mode 100644
index 0000000..dd6af93
--- /dev/null
+++ b/tests/unit/test-mount.c
@@ -0,0 +1,103 @@
+/* SPDX-License-Identifier: MIT */
+#include <string.h>
+
+#include "kbox/mount.h"
+#include "test-runner.h"
+
+static void fill_char(char *buf, size_t len, char c)
+{
+    memset(buf, c, len);
+    buf[len] = '\0';
+}
+
+/* --- kbox_parse_bind_spec tests --- */
+
+static void test_parse_basic(void)
+{
+    struct kbox_bind_spec spec;
+    ASSERT_EQ(kbox_parse_bind_spec("/host/dir:/guest/dir", &spec), 0);
+    ASSERT_STREQ(spec.source, "/host/dir");
+    ASSERT_STREQ(spec.target, "/guest/dir");
+}
+
+static void test_parse_null_spec(void)
+{
+    struct kbox_bind_spec spec;
+    ASSERT_EQ(kbox_parse_bind_spec(NULL, &spec), -1);
+}
+
+static void test_parse_null_out(void)
+{
+    ASSERT_EQ(kbox_parse_bind_spec("/a:/b", NULL), -1);
+}
+
+static void test_parse_no_colon(void)
+{
+    struct kbox_bind_spec spec;
+    ASSERT_EQ(kbox_parse_bind_spec("/no/colon/here", &spec), -1);
+}
+
+static void test_parse_empty_source(void)
+{
+    struct kbox_bind_spec spec;
+    ASSERT_EQ(kbox_parse_bind_spec(":/guest", &spec), -1);
+}
+
+static void test_parse_empty_target(void)
+{
+    struct kbox_bind_spec spec;
+    ASSERT_EQ(kbox_parse_bind_spec("/host:", &spec), -1);
+}
+
+static void test_parse_colon_in_middle(void)
+{
+    struct kbox_bind_spec spec;
+    ASSERT_EQ(kbox_parse_bind_spec("a:b", &spec), 0);
+    ASSERT_STREQ(spec.source, "a");
+    ASSERT_STREQ(spec.target, "b");
+}
+
+static void test_parse_multiple_colons(void)
+{
+    struct kbox_bind_spec spec;
+    ASSERT_EQ(kbox_parse_bind_spec("/a:/b:c", &spec), 0);
+    ASSERT_STREQ(spec.source, "/a");
+    ASSERT_STREQ(spec.target, "/b:c");
+}
+
+static void test_parse_source_too_long(void)
+{
+    struct kbox_bind_spec spec;
+    /* source component >= sizeof(spec.source) must be rejected */
+    char big[4096 + 1 + 2]; /* 4096 'x' + ':' + '/' + NUL */
+    fill_char(big, 4096, 'x');
+    big[4096] = ':';
+    big[4097] = '/';
+    big[4098] = '\0';
+    ASSERT_EQ(kbox_parse_bind_spec(big, &spec), -1);
+}
+
+static void test_parse_target_too_long(void)
+{
+    struct kbox_bind_spec spec;
+    /* target component >= sizeof(spec.target) must be rejected */
+    char big[2 + 4096 + 1]; /* '/' + ':' + 4096 'y' + NUL */
+    big[0] = '/';
+    big[1] = ':';
+    fill_char(big + 2, 4096, 'y');
+    ASSERT_EQ(kbox_parse_bind_spec(big, &spec), -1);
+}
+
+void test_mount_init(void)
+{
+    TEST_REGISTER(test_parse_basic);
+    TEST_REGISTER(test_parse_null_spec);
+    TEST_REGISTER(test_parse_null_out);
+    TEST_REGISTER(test_parse_no_colon);
+    TEST_REGISTER(test_parse_empty_source);
+    TEST_REGISTER(test_parse_empty_target);
+    TEST_REGISTER(test_parse_colon_in_middle);
+    TEST_REGISTER(test_parse_multiple_colons);
+    TEST_REGISTER(test_parse_source_too_long);
+    TEST_REGISTER(test_parse_target_too_long);
+}
diff --git a/tests/unit/test-runner.c b/tests/unit/test-runner.c
index ed105c1..52d3b33 100644
--- a/tests/unit/test-runner.c
+++ b/tests/unit/test-runner.c
@@ -91,6 +91,7 @@ void test_pass(void)
 /* Portable test suites (all hosts) */
 extern void test_fd_table_init(void);
 extern void test_path_init(void);
+extern void test_mount_init(void);
 extern void test_cli_init(void);
 extern void test_identity_init(void);
 extern void test_syscall_nr_init(void);
@@ -120,6 +121,7 @@ int main(int argc, char *argv[])
     /* Portable suites */
     test_fd_table_init();
     test_path_init();
+    test_mount_init();
     test_cli_init();
     test_identity_init();
     test_syscall_nr_init();