X-Git-Url: https://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2Fapply-default-acl.c;h=4a6ef765e10379d213661dacf39d8a08636cb20c;hb=6d396c52c731f260cc261e26c75034bd5007f0b0;hp=9c48b977d77adc90eb59306d12ed2549c24e6348;hpb=56883e0b8096b29cc545702f063f28619b593fe0;p=apply-default-acl.git

diff --git a/src/apply-default-acl.c b/src/apply-default-acl.c
index 9c48b97..4a6ef76 100644
--- a/src/apply-default-acl.c
+++ b/src/apply-default-acl.c
@@ -10,6 +10,7 @@
 #define _GNU_SOURCE
 
 #include <errno.h>
+#include <fcntl.h>  /* AT_FOO constants */
 #include <ftw.h>    /* nftw() et al. */
 #include <getopt.h>
 #include <libgen.h> /* dirname()     */
@@ -33,10 +34,6 @@
 #define ACL_SUCCESS 1
 
 
-/* Command-line options */
-static bool no_exec_mask = false;
-
-
 
 /**
  * @brief Get the mode bits from the given path.
@@ -54,19 +51,44 @@ mode_t get_mode(const char* path) {
   }
 
   struct stat s;
-  int result = stat(path, &s);
+  int result = lstat(path, &s);
 
   if (result == 0) {
     return s.st_mode;
   }
   else {
-    /* errno will be set already by stat() */
+    /* errno will be set already by lstat() */
     return result;
   }
 }
 
 
 
+/**
+ * @brief Determine if the given path might refer to an (unsafe) hard link.
+ *
+ * @param path
+ *   The path to test.
+ *
+ * @return true if we are certain that @c path does not refer to a hard
+ *   link, and false otherwise. In case of error, false is returned,
+ *   because we are not sure that @c path is not a hard link.
+ */
+bool is_hardlink_safe(const char* path) {
+  if (path == NULL) {
+    return false;
+  }
+  struct stat s;
+  int result = lstat(path, &s);
+  if (result == 0) {
+    return (s.st_nlink == 1 || S_ISDIR(s.st_mode));
+  }
+  else {
+    return false;
+  }
+}
+
+
 /**
  * @brief Determine whether or not the given path is a regular file.
  *
@@ -81,7 +103,7 @@ bool is_regular_file(const char* path) {
   }
 
   struct stat s;
-  int result = stat(path, &s);
+  int result = lstat(path, &s);
   if (result == 0) {
     return S_ISREG(s.st_mode);
   }
@@ -92,6 +114,42 @@ bool is_regular_file(const char* path) {
 
 
 
+/**
+ * @brief Determine whether or not the given path is accessible.
+ *
+ * @param path
+ *   The path to test.
+ *
+ * @return true if @c path is accessible to the current effective
+ *   user/group, false otherwise.
+ */
+bool path_accessible(const char* path) {
+  if (path == NULL) {
+    return false;
+  }
+
+  /*  Test for access using the effective user and group rather than
+      the real one. */
+  int flags = AT_EACCESS;
+
+  /* Don't follow symlinks when checking for a path's existence,
+     since we won't follow them to set its ACLs either. */
+  flags |= AT_SYMLINK_NOFOLLOW;
+
+  /* If the path is relative, interpret it relative to the current
+     working directory (just like the access() system call). */
+  int result = faccessat(AT_FDCWD, path, F_OK, flags);
+
+  if (result == 0) {
+    return true;
+  }
+  else {
+    return false;
+  }
+}
+
+
+
 /**
  * @brief Determine whether or not the given path is a directory.
  *
@@ -100,13 +158,13 @@ bool is_regular_file(const char* path) {
  *
  * @return true if @c path is a directory, false otherwise.
  */
-bool is_directory(const char* path) {
+bool is_path_directory(const char* path) {
   if (path == NULL) {
     return false;
   }
 
   struct stat s;
-  int result = stat(path, &s);
+  int result = lstat(path, &s);
   if (result == 0) {
     return S_ISDIR(s.st_mode);
   }
@@ -125,9 +183,9 @@ bool is_directory(const char* path) {
  * on each path before applying the default to it.
  *
  * The assumption that there are no extended entries makes things much
- * simpler. For example, we only have to update the ACL_USER_OBJ,
- * ACL_GROUP_OBJ, and ACL_OTHER entries -- all others can simply be
- * created anew. This means we don't have to fool around comparing
+ * simpler. For example, we only have to update the @c ACL_USER_OBJ,
+ * @c ACL_GROUP_OBJ, and @c ACL_OTHER entries -- all others can simply
+ * be created anew. This means we don't have to fool around comparing
  * named-user/group entries.
  *
  * @param aclp
@@ -137,8 +195,8 @@ bool is_directory(const char* path) {
  *   The new entry. If @c entry contains a user/group/other entry, we
  *   update the existing one. Otherwise we create a new entry.
  *
- * @return If there is an unexpected library error, ACL_ERROR is
- * returned. Otherwise, ACL_SUCCESS.
+ * @return If there is an unexpected library error, @c ACL_ERROR is
+ *   returned. Otherwise, @c ACL_SUCCESS.
  *
  */
 int acl_set_entry(acl_t* aclp,
@@ -211,7 +269,13 @@ int acl_set_entry(acl_t* aclp,
      ACL. */
   acl_entry_t new_entry;
 
-  /* We allocate memory here that we should release! */
+  /* The acl_create_entry() function can allocate new memory and/or
+   * change the location of the ACL structure entirely. When that
+   * happens, the value pointed to by aclp is updated, which means
+   * that a new acl_t gets "passed out" to our caller, eventually to
+   * be fed to acl_free(). In other words, we should still be freeing
+   * the right thing, even if the value pointed to by aclp changes.
+   */
   int c_result = acl_create_entry(aclp, &new_entry);
   if (c_result == ACL_ERROR) {
     perror("acl_set_entry (acl_create_entry)");
@@ -254,20 +318,20 @@ int acl_set_entry(acl_t* aclp,
  * @brief Determine the number of entries in the given ACL.
  *
  * @param acl
- *   A pointer to an acl_t structure.
+ *   The ACL to inspect.
  *
  * @return Either the non-negative number of entries in @c acl, or
- *   ACL_ERROR on error.
+ *   @c ACL_ERROR on error.
  */
-int acl_entry_count(acl_t* acl) {
+int acl_entry_count(acl_t acl) {
 
   acl_entry_t entry;
   int entry_count = 0;
-  int result = acl_get_entry(*acl, ACL_FIRST_ENTRY, &entry);
+  int result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);
 
   while (result == ACL_SUCCESS) {
     entry_count++;
-    result = acl_get_entry(*acl, ACL_NEXT_ENTRY, &entry);
+    result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
   }
 
   if (result == ACL_ERROR) {
@@ -286,14 +350,14 @@ int acl_entry_count(acl_t* acl) {
  * An ACL is minimal if it has fewer than four entries.
  *
  * @param acl
- *   A pointer to an acl_t structure.
+ *   The ACL whose minimality is in question.
  *
  * @return
- *   - ACL_SUCCESS - @c acl is minimal
- *   - ACL_FAILURE - @c acl is not minimal
- *   - ACL_ERROR - Unexpected library error
+ *   - @c ACL_SUCCESS - @c acl is minimal
+ *   - @c ACL_FAILURE - @c acl is not minimal
+ *   - @c ACL_ERROR - Unexpected library error
  */
-int acl_is_minimal(acl_t* acl) {
+int acl_is_minimal(acl_t acl) {
 
   int ec = acl_entry_count(acl);
 
@@ -313,29 +377,17 @@ int acl_is_minimal(acl_t* acl) {
 
 
 /**
- * @brief Determine whether the given path has an ACL whose mask
- *   denies execute.
+ * @brief Determine whether the given ACL's mask denies execute.
  *
- * @param path
- *   The path to check.
+ * @param acl
+ *   The ACL whose mask we want to check.
  *
  * @return
- *   - ACL_SUCCESS - @c path has a mask which denies execute.
- *   - ACL_FAILURE - The ACL for @c path does not deny execute,
- *     or @c path has no extended ACL at all.
- *   - ACL_ERROR - Unexpected library error.
+ *   - @c ACL_SUCCESS - The @c acl has a mask which denies execute.
+ *   - @c ACL_FAILURE - The @c acl has a mask which does not deny execute.
+ *   - @c ACL_ERROR - Unexpected library error.
  */
-int acl_execute_masked(const char* path) {
-
-  acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
-
-  if (acl == (acl_t)NULL) {
-    perror("acl_execute_masked (acl_get_file)");
-    return ACL_ERROR;
-  }
-
-  /* Our return value. */
-  int result = ACL_FAILURE;
+int acl_execute_masked(acl_t acl) {
 
   acl_entry_t entry;
   int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);
@@ -346,8 +398,7 @@ int acl_execute_masked(const char* path) {
 
     if (tag_result == ACL_ERROR) {
        perror("acl_execute_masked (acl_get_tag_type)");
-       result = ACL_ERROR;
-       goto cleanup;
+       return ACL_ERROR;
     }
 
     if (tag == ACL_MASK) {
@@ -358,15 +409,13 @@ int acl_execute_masked(const char* path) {
       int ps_result = acl_get_permset(entry, &permset);
       if (ps_result == ACL_ERROR) {
 	perror("acl_execute_masked (acl_get_permset)");
-	result = ACL_ERROR;
-	goto cleanup;
+	return ACL_ERROR;
       }
 
       int gp_result = acl_get_perm(permset, ACL_EXECUTE);
       if (gp_result == ACL_ERROR) {
 	perror("acl_execute_masked (acl_get_perm)");
-	result = ACL_ERROR;
-	goto cleanup;
+	return ACL_ERROR;
       }
 
       if (gp_result == ACL_FAILURE) {
@@ -378,12 +427,11 @@ int acl_execute_masked(const char* path) {
     ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
   }
 
- cleanup:
-  acl_free(acl);
-  return result;
+  return ACL_FAILURE;
 }
 
 
+
 /**
  * @brief Determine whether @c path is executable (by anyone) or a
  *   directory.
@@ -400,14 +448,15 @@ int acl_execute_masked(const char* path) {
  *   The path to check.
  *
  * @return
- *   - ACL_SUCCESS - @c path is a directory, or someone has effective
+ *   - @c ACL_SUCCESS - @c path is a directory, or someone has effective
        execute permissions.
- *   - ACL_FAILURE - @c path is a regular file and nobody can execute it.
- *   - ACL_ERROR - Unexpected library error.
+ *   - @c ACL_FAILURE - @c path is a regular file and nobody can execute
+       it.
+ *   - @c ACL_ERROR - Unexpected library error.
  */
 int any_can_execute_or_dir(const char* path) {
 
-  if (is_directory(path)) {
+  if (is_path_directory(path)) {
     /* That was easy... */
     return ACL_SUCCESS;
   }
@@ -422,7 +471,7 @@ int any_can_execute_or_dir(const char* path) {
   /* Our return value. */
   int result = ACL_FAILURE;
 
-  if (acl_is_minimal(&acl)) {
+  if (acl_is_minimal(acl)) {
     mode_t mode = get_mode(path);
     if (mode & (S_IXUSR | S_IXOTH | S_IXGRP)) {
       result = ACL_SUCCESS;
@@ -438,6 +487,23 @@ int any_can_execute_or_dir(const char* path) {
   int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);
 
   while (ge_result == ACL_SUCCESS) {
+    /* The first thing we do is check to see if this is a mask
+       entry. If it is, we skip it entirely. */
+    acl_tag_t tag = ACL_UNDEFINED_TAG;
+    int tag_result = acl_get_tag_type(entry, &tag);
+
+    if (tag_result == ACL_ERROR) {
+      perror("any_can_execute_or_dir (acl_get_tag_type)");
+      result = ACL_ERROR;
+      goto cleanup;
+    }
+
+    if (tag == ACL_MASK) {
+      ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
+      continue;
+    }
+
+    /* Ok, so it's not a mask entry. Check the execute perms. */
     acl_permset_t permset;
 
     int ps_result = acl_get_permset(entry, &permset);
@@ -455,8 +521,8 @@ int any_can_execute_or_dir(const char* path) {
     }
 
     if (gp_result == ACL_SUCCESS) {
-      /* Only return one if this execute bit is not masked. */
-      if (acl_execute_masked(path) != ACL_SUCCESS) {
+      /* Only return ACL_SUCCESS if this execute bit is not masked. */
+      if (acl_execute_masked(acl) != ACL_SUCCESS) {
 	result = ACL_SUCCESS;
 	goto cleanup;
       }
@@ -479,78 +545,74 @@ int any_can_execute_or_dir(const char* path) {
 
 
 /**
- * @brief Inherit the default ACL from @c parent to @c path.
- *
- * The @c parent parameter does not necessarily need to be the parent
- * of @c path, although that will usually be the case. This overwrites
- * any existing default ACL on @c path.
+ * @brief Set @c acl as the default ACL on @c path if it's a directory.
  *
- * @param parent
- *   The parent directory whose ACL we want to inherit.
+ * This overwrites any existing default ACL on @c path. If no default
+ * ACL exists, then one is created. If @c path is not a directory, we
+ * return ACL_FAILURE but no error is raised.
  *
  * @param path
- *   The target directory whose ACL we wish to overwrite (or create).
+ *   The target directory whose ACL we wish to replace or create.
+ *
+  * @param acl
+ *   The ACL to set as default on @c path.
  *
  * @return
- *   - ACL_SUCCESS - The default ACL was inherited successfully.
- *   - ACL_FAILURE - Either @c parent or @c path is not a directory.
- *   - ACL_ERROR - Unexpected library error.
+ *   - @c ACL_SUCCESS - The default ACL was assigned successfully.
+ *   - @c ACL_FAILURE - If @c path is not a directory.
+ *   - @c ACL_ERROR - Unexpected library error.
  */
-int inherit_default_acl(const char* path, const char* parent) {
-  /* Inherit the default ACL from parent to path. This overwrites any
-   * existing default ACL. Returns 1 for success, 0 for failure, and
-   * -1 on error.
-   */
-
-  /* Our return value. */
-  int result = ACL_SUCCESS;
+int assign_default_acl(const char* path, acl_t acl) {
 
   if (path == NULL) {
     errno = ENOENT;
     return ACL_ERROR;
   }
 
-  if (!is_directory(path) || !is_directory(parent)) {
+  if (!is_path_directory(path)) {
     return ACL_FAILURE;
   }
 
-  acl_t parent_acl = acl_get_file(parent, ACL_TYPE_DEFAULT);
-  if (parent_acl == (acl_t)NULL) {
-    perror("inherit_default_acl (acl_get_file)");
-    return ACL_ERROR;
-  }
-
-  acl_t path_acl = acl_dup(parent_acl);
+    /* Our return value; success unless something bad happens. */
+  int result = ACL_SUCCESS;
+  acl_t path_acl = acl_dup(acl);
 
   if (path_acl == (acl_t)NULL) {
     perror("inherit_default_acl (acl_dup)");
-    acl_free(parent_acl);
-    return ACL_ERROR;
+    return ACL_ERROR; /* Nothing to clean up in this case. */
   }
 
   int sf_result = acl_set_file(path, ACL_TYPE_DEFAULT, path_acl);
   if (sf_result == -1) {
     perror("inherit_default_acl (acl_set_file)");
     result = ACL_ERROR;
-    goto cleanup;
   }
 
- cleanup:
   acl_free(path_acl);
   return result;
 }
 
 
+
+/**
+ * @brief Remove @c ACL_USER, @c ACL_GROUP, and @c ACL_MASK entries
+ *   from the given path.
+ *
+ * @param path
+ *   The path whose ACLs we want to wipe.
+ *
+ * @return
+ *   - @c ACL_SUCCESS - The ACLs were wiped successfully, or none
+ *     existed in the first place.
+ *   - @c ACL_ERROR - Unexpected library error.
+ */
 int wipe_acls(const char* path) {
-  /* Remove ACL_USER, ACL_GROUP, and ACL_MASK entries from
-     path. Returns 1 for success, 0 for failure, and -1 on error. */
 
   if (path == NULL) {
     errno = ENOENT;
     return ACL_ERROR;
   }
 
-  /* Finally, remove individual named/mask entries. */
   acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
   if (acl == (acl_t)NULL) {
     perror("wipe_acls (acl_get_file)");
@@ -595,17 +657,58 @@ int wipe_acls(const char* path) {
 }
 
 
-int apply_default_acl(const char* path) {
-  /* Really apply the default ACL by looping through it. Returns one
-   * for success, zero for failure (i.e. no ACL), and -1 on unexpected
-   * errors.
 
-   */
+/**
+ * @brief Apply parent default ACL to a path.
+ *
+ * This overwrites any existing ACLs on @c path.
+ *
+ * @param path
+ *   The path whose ACL we would like to reset to its default.
+ *
+ * @param no_exec_mask
+ *   The value (either true or false) of the --no-exec-mask flag.
+ *
+ * @return
+ *   - @c ACL_SUCCESS - The parent default ACL was inherited successfully.
+ *   - @c ACL_FAILURE - The target path is not a regular file/directory,
+ *     or the parent of @c path is not a directory.
+ *   - @c ACL_ERROR - Unexpected library error.
+ */
+int apply_default_acl(const char* path, bool no_exec_mask) {
+
   if (path == NULL) {
+    errno = ENOENT;
+    return ACL_ERROR;
+  }
+
+  /* Refuse to operate on hard links, which can be abused by an
+   * attacker to trick us into changing the ACL on a file we didn't
+   * intend to; namely the "target" of the hard link. To truly prevent
+   * that sort of mischief, we should be using file descriptors for
+   * the target and its parent directory. Then modulo a tiny race
+   * condition, we would be sure that "path" and "parent" don't change
+   * their nature between the time that we test them and when we
+   * utilize them. For contrast, the same attacker is free to replace
+   * "path" with a hard link after is_hardlink_safe() has returned
+   * "true" below.
+   *
+   * Unfortunately, our API is lacking in this area. For example,
+   * acl_set_fd() is only capable of setting the ACL_TYPE_ACCESS list,
+   * and not the ACL_TYPE_DEFAULT. Apparently the only way to operate
+   * on default ACLs is through the path name, which is inherently
+   * unreliable since the acl_*_file() calls themselves might follow
+   * links (both hard and symbolic).
+   *
+   * Some improvement could still be made by using descriptors where
+   * possible -- this would shrink the exploit window -- but for now
+   * we use a naive implementation that only keeps honest men honest.
+  */
+  if (!is_hardlink_safe(path)) {
     return ACL_FAILURE;
   }
 
-  if (!is_regular_file(path) && !is_directory(path)) {
+  if (!is_regular_file(path) && !is_path_directory(path)) {
     return ACL_FAILURE;
   }
 
@@ -615,7 +718,7 @@ int apply_default_acl(const char* path) {
   path_copy[PATH_MAX-1] = 0;
 
   char* parent = dirname(path_copy);
-  if (!is_directory(parent)) {
+  if (!is_path_directory(parent)) {
     /* Make sure dirname() did what we think it did. */
     return ACL_FAILURE;
   }
@@ -629,9 +732,10 @@ int apply_default_acl(const char* path) {
     int ace_result = any_can_execute_or_dir(path);
 
     if (ace_result == ACL_ERROR) {
-    perror("apply_default_acl (any_can_execute_or_dir)");
-    return ACL_ERROR;
-  }
+      perror("apply_default_acl (any_can_execute_or_dir)");
+      return ACL_ERROR;
+    }
+
     allow_exec = (bool)ace_result;
   }
 
@@ -657,11 +761,12 @@ int apply_default_acl(const char* path) {
   acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
   if (acl == (acl_t)NULL) {
     perror("apply_default_acl (acl_get_file)");
-    return ACL_ERROR;
+    result = ACL_ERROR;
+    goto cleanup;
   }
 
   /* If it's a directory, inherit the parent's default. */
-  int inherit_result = inherit_default_acl(path, parent);
+  int inherit_result = assign_default_acl(path, defacl);
   if (inherit_result == ACL_ERROR) {
     perror("apply_default_acl (inherit_acls)");
     result = ACL_ERROR;
@@ -717,7 +822,22 @@ int apply_default_acl(const char* path) {
       }
     }
 
-    /* Finally, add the permset to the access ACL. */
+    /* Finally, add the permset to the access ACL. It's actually
+     * important that we pass in the address of "acl" here, and not
+     * "acl" itself. Why? The call to acl_create_entry() within
+     * acl_set_entry() can allocate new memory for the entry.
+     * Sometimes that can be done in-place, in which case everything
+     * is cool and the new memory gets released when we call
+     * acl_free(acl).
+     *
+     * But occasionally, the whole ACL structure will have to be moved
+     * in order to allocate the extra space. When that happens,
+     * acl_create_entry() modifies the pointer it was passed (in this
+     * case, &acl) to point to the new location. We want to call
+     * acl_free() on the new location, and since acl_free() gets
+     * called right here, we need acl_create_entry() to update the
+     * value of "acl". To do that, it needs the address of "acl".
+     */
     int set_result = acl_set_entry(&acl, entry);
     if (set_result == ACL_ERROR) {
       perror("apply_default_acl (acl_set_entry)");
@@ -749,10 +869,15 @@ int apply_default_acl(const char* path) {
 }
 
 
-void usage(char* program_name) {
-  /*
-   * Print usage information.
-   */
+
+/**
+ * @brief Display program usage information.
+ *
+ * @param program_name
+ *   The program name to use in the output.
+ *
+ */
+void usage(const char* program_name) {
   printf("Apply any applicable default ACLs to the given files or "
 	 "directories.\n\n");
   printf("Usage: %s [flags] <target1> [<target2> [ <target3>...]]\n\n",
@@ -766,16 +891,24 @@ void usage(char* program_name) {
 }
 
 
+/**
+ * @brief Wrapper around @c apply_default_acl() for use with @c nftw().
+ *
+ * For parameter information, see the @c nftw man page.
+ *
+ * @return If the ACL was applied to @c target successfully, we return
+ *   @c FTW_CONTINUE to signal to @ nftw() that we should proceed onto
+ *   the next file or directory. Otherwise, we return @c FTW_STOP to
+ *   signal failure.
+ *
+ */
 int apply_default_acl_nftw(const char *target,
 			   const struct stat *s,
 			   int info,
 			   struct FTW *ftw) {
-  /* A wrapper around the apply_default_acl() function for use with
-   * nftw(). We need to adjust the return value so that nftw() doesn't
-   * think we've failed.
-   */
-  bool reapp_result = apply_default_acl(target);
-  if (reapp_result) {
+
+  bool app_result = apply_default_acl(target, false);
+  if (app_result) {
     return FTW_CONTINUE;
   }
   else {
@@ -784,25 +917,72 @@ int apply_default_acl_nftw(const char *target,
 }
 
 
-bool apply_default_acl_recursive(const char *target) {
-  /* Attempt to apply default ACLs recursively. If target is a
-   * directory, we recurse through its entries. If not, we just
-   * apply the default ACL to target.
-   *
-   * We ignore symlinks for consistency with chmod -r.
-   *
-   */
-  if (!is_directory(target)) {
-    return apply_default_acl(target);
+
+/**
+ * @brief Wrapper around @c apply_default_acl() for use with @c nftw().
+ *
+ * This is identical to @c apply_default_acl_nftw(), except it passes
+ * @c true to @c apply_default_acl() as its no_exec_mask argument.
+ *
+ */
+int apply_default_acl_nftw_x(const char *target,
+			     const struct stat *s,
+			     int info,
+			     struct FTW *ftw) {
+
+  bool app_result = apply_default_acl(target, true);
+  if (app_result) {
+    return FTW_CONTINUE;
+  }
+  else {
+    return FTW_STOP;
+  }
+}
+
+
+
+/**
+ * @brief Recursive version of @c apply_default_acl().
+ *
+ * If @c target is a directory, we use @c nftw() to call @c
+ * apply_default_acl() recursively on all of its children. Otherwise,
+ * we just delegate to @c apply_default_acl().
+ *
+ * We ignore symlinks for consistency with chmod -r.
+ *
+ * @param target
+ *   The root (path) of the recursive application.
+ *
+ * @param no_exec_mask
+ *   The value (either true or false) of the --no-exec-mask flag.
+ *
+ * @return
+ *   If @c target is not a directory, we return the result of
+ *   calling @c apply_default_acl() on @c target. Otherwise, we convert
+ *   the return value of @c nftw(). If @c nftw() succeeds (returns 0),
+ *   then we return @c true. Otherwise, we return @c false.
+ *   \n\n
+ *   If there is an error, it will be reported via @c perror, but
+ *   we still return @c false.
+ */
+bool apply_default_acl_recursive(const char *target, bool no_exec_mask) {
+
+  if (!is_path_directory(target)) {
+    return apply_default_acl(target, no_exec_mask);
   }
 
   int max_levels = 256;
   int flags = FTW_PHYS; /* Don't follow links. */
 
-  int nftw_result = nftw(target,
-			 apply_default_acl_nftw,
-			 max_levels,
-			 flags);
+  /* There are two separate functions that could be passed to
+     nftw(). One passes no_exec_mask = true to apply_default_acl(),
+     and the other passes no_exec_mask = false. Since the function we
+     pass to nftw() cannot have parameters, we have to create separate
+     options and make the decision here. */
+  int (*fn)(const char *, const struct stat *, int, struct FTW *) = NULL;
+  fn = no_exec_mask ? apply_default_acl_nftw_x : apply_default_acl_nftw;
+
+  int nftw_result = nftw(target, fn, max_levels, flags);
 
   if (nftw_result == 0) {
     /* Success */
@@ -821,17 +1001,24 @@ bool apply_default_acl_recursive(const char *target) {
 }
 
 
+
+/**
+ * @brief Call apply_default_acl (possibly recursively) on each
+ * command-line argument.
+ *
+ * @return Either @c EXIT_FAILURE or @c EXIT_SUCCESS. If everything
+ *   goes as expected, we return @c EXIT_SUCCESS. Otherwise, we return
+ *   @c EXIT_FAILURE.
+ */
 int main(int argc, char* argv[]) {
-  /*
-   * Call apply_default_acl on each command-line argument.
-   */
+
   if (argc < 2) {
     usage(argv[0]);
     return EXIT_FAILURE;
   }
 
   bool recursive = false;
-  /* bool no_exec_mask is declared static/global */
+  bool no_exec_mask = false;
 
   struct option long_options[] = {
     /* These options set a flag. */
@@ -867,12 +1054,23 @@ int main(int argc, char* argv[]) {
     const char* target = argv[arg_index];
     bool reapp_result = false;
 
+    /* Make sure we can access the given path before we go out of our
+     * way to please it. Doing this check outside of
+     * apply_default_acl() lets us spit out a better error message for
+     * typos, too.
+     */
+    if (!path_accessible(target)) {
+      fprintf(stderr, "%s: %s: No such file or directory\n", argv[0], target);
+      result = EXIT_FAILURE;
+      continue;
+    }
+
     if (recursive) {
-      reapp_result = apply_default_acl_recursive(target);
+      reapp_result = apply_default_acl_recursive(target, no_exec_mask);
     }
     else {
-      /* It's either normal file, or we're not operating recursively. */
-      reapp_result = apply_default_acl(target);
+      /* It's either a normal file, or we're not operating recursively. */
+      reapp_result = apply_default_acl(target, no_exec_mask);
     }
 
     if (!reapp_result) {