X-Git-Url: https://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2Fapply-default-acl.c;h=993b1f7649e14c50300174da1f5d87fcb534d20a;hb=f22034c7b75b7096e6ef26de7a5bc8e12a3f0b07;hp=4f7b3b06a0e1f2cc835038f5701550c57c696859;hpb=1504070cadf122c8d3de0d15a0e2fd35f81eb576;p=apply-default-acl.git

diff --git a/src/apply-default-acl.c b/src/apply-default-acl.c
index 4f7b3b0..993b1f7 100644
--- a/src/apply-default-acl.c
+++ b/src/apply-default-acl.c
@@ -1,11 +1,19 @@
+/**
+ * @file apply-default-acl.c
+ *
+ * @brief The entire implementation.
+ *
+ */
+
 /* On Linux, ftw.h needs this special voodoo to work. */
 #define _XOPEN_SOURCE 500
+#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()     */
-#include <limits.h> /* PATH_MAX      */
+#include <libgen.h> /* basename(), dirname() */
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -18,44 +26,85 @@
 #include <sys/types.h>
 #include <sys/acl.h>
 
-
-/* Command-line options */
-static bool no_exec_mask = false;
-
-
-mode_t get_mode(const char* path) {
-  /*
-   * Get the mode bits from path.
-   */
-  if (path == NULL) {
+/* Most of the libacl functions return 1 for success, 0 for failure,
+   and -1 on error */
+#define ACL_ERROR -1
+#define ACL_FAILURE 0
+#define ACL_SUCCESS 1
+
+
+
+/**
+ * @brief Get the mode bits from the given file descriptor.
+ *
+ * @param fd
+ *   The file descriptor (which may reference a directory) whose
+ *   mode we want.
+ *
+ * @return A mode_t (st_mode) structure containing the mode bits.
+ *   See sys/stat.h for details.
+ */
+mode_t get_mode(int fd) {
+  if (fd <= 0) {
     errno = ENOENT;
-    return -1;
+    return ACL_ERROR;
   }
 
   struct stat s;
-  int result = stat(path, &s);
+  int result = fstat(fd, &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;
   }
 }
 
 
-bool is_regular_file(const char* path) {
-  /*
-   * Returns true if path is a regular file, false otherwise.
-   */
-  if (path == NULL) {
+
+/**
+ * @brief Determine if the given file descriptor might refer to an
+ *   (unsafe) hard link.
+ *
+ * @param fd
+ *   The file descriptor whose link count we want to investigate.
+ *
+ * @return true if we are certain that @c fd does not describe a hard
+ *   link, and false otherwise. In case of error, false is returned,
+ *   because we are not sure that @c fd is not a hard link.
+ */
+bool is_hardlink_safe(int fd) {
+  if (fd <= 0) {
+    return false;
+  }
+  struct stat s;
+  if (fstat(fd, &s) == 0) {
+    return (s.st_nlink == 1 || S_ISDIR(s.st_mode));
+  }
+  else {
+    return false;
+  }
+}
+
+
+/**
+ * @brief Determine whether or not the given file descriptor is for
+ *   a regular file.
+ *
+ * @param fd
+ *   The file descriptor to test for regular-fileness.
+ *
+ * @return true if @c fd describes a regular file, and false otherwise.
+ */
+bool is_regular_file(int fd) {
+  if (fd <= 0) {
     return false;
   }
 
   struct stat s;
-  int result = stat(path, &s);
-  if (result == 0) {
+  if (fstat(fd, &s) == 0) {
     return S_ISREG(s.st_mode);
   }
   else {
@@ -63,17 +112,57 @@ bool is_regular_file(const char* path) {
   }
 }
 
-bool is_directory(const char* path) {
-  /*
-   * Returns true if path is a directory, false otherwise.
-   */
+
+
+/**
+ * @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). */
+  if (faccessat(AT_FDCWD, path, F_OK, flags) == 0) {
+    return true;
+  }
+  else {
+    return false;
+  }
+}
+
+
+
+/**
+ * @brief Determine whether or not the given path is a directory.
+ *
+ * @param path
+ *   The path to test.
+ *
+ * @return true if @c path is a directory, false otherwise.
+ */
+bool is_path_directory(const char* path) {
   if (path == NULL) {
     return false;
   }
 
   struct stat s;
-  int result = stat(path, &s);
-  if (result == 0) {
+  if (lstat(path, &s) == 0) {
     return S_ISDIR(s.st_mode);
   }
   else {
@@ -82,38 +171,80 @@ bool is_directory(const char* path) {
 }
 
 
+/**
+ * @brief Determine whether or not the given file descriptor is for
+ *   a directory.
+ *
+ * @param fd
+ *   The file descriptor whose directoryness is in question.
+ *
+ * @return true if @c fd describes a directory, and false otherwise.
+ */
+bool is_directory(int fd) {
+  if (fd <= 0) {
+    return false;
+  }
 
+  struct stat s;
+  if (fstat(fd, &s) == 0) {
+    return S_ISDIR(s.st_mode);
+  }
+  else {
+    return false;
+  }
+}
+
+
+
+/**
+ * @brief Update (or create) an entry in an @b minimal ACL.
+ *
+ * This function will not work if @c aclp contains extended
+ * entries. This is fine for our purposes, since we call @c wipe_acls
+ * 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 @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
+ *   A pointer to the acl_t structure whose entry we want to modify.
+ *
+ * @param entry
+ *   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, @c ACL_ERROR is
+ *   returned. Otherwise, @c ACL_SUCCESS.
+ *
+ */
 int acl_set_entry(acl_t* aclp,
 		  acl_entry_t entry) {
-  /*
-   * Update or create the given entry.
-   */
 
   acl_tag_t entry_tag;
-  int gt_result = acl_get_tag_type(entry, &entry_tag);
-  if (gt_result == -1) {
+  if (acl_get_tag_type(entry, &entry_tag) == ACL_ERROR) {
     perror("acl_set_entry (acl_get_tag_type)");
-    return -1;
+    return ACL_ERROR;
   }
 
   acl_permset_t entry_permset;
-  int ps_result = acl_get_permset(entry, &entry_permset);
-  if (ps_result == -1) {
+  if (acl_get_permset(entry, &entry_permset) == ACL_ERROR) {
     perror("acl_set_entry (acl_get_permset)");
-    return -1;
+    return ACL_ERROR;
   }
 
   acl_entry_t existing_entry;
   /* Loop through the given ACL looking for matching entries. */
   int result = acl_get_entry(*aclp, ACL_FIRST_ENTRY, &existing_entry);
 
-  while (result == 1) {
+  while (result == ACL_SUCCESS) {
     acl_tag_t existing_tag = ACL_UNDEFINED_TAG;
-    int tag_result = acl_get_tag_type(existing_entry, &existing_tag);
 
-    if (tag_result == -1) {
+    if (acl_get_tag_type(existing_entry, &existing_tag) == ACL_ERROR) {
        perror("set_acl_tag_permset (acl_get_tag_type)");
-       return -1;
+       return ACL_ERROR;
     }
 
     if (existing_tag == entry_tag) {
@@ -121,21 +252,21 @@ int acl_set_entry(acl_t* aclp,
 	  entry_tag == ACL_GROUP_OBJ ||
 	  entry_tag == ACL_OTHER) {
 	/* Only update for these three since all other tags will have
-	   been wiped. */
+	   been wiped. These three are guaranteed to exist, so if we
+	   match one of them, we're allowed to return ACL_SUCCESS
+	   below and bypass the rest of the function. */
 	acl_permset_t existing_permset;
-	int gep_result = acl_get_permset(existing_entry, &existing_permset);
-	if (gep_result == -1) {
+	if (acl_get_permset(existing_entry, &existing_permset) == ACL_ERROR) {
 	  perror("acl_set_entry (acl_get_permset)");
-	  return -1;
+	  return ACL_ERROR;
 	}
 
-	int s_result = acl_set_permset(existing_entry, entry_permset);
-	if (s_result == -1) {
+	if (acl_set_permset(existing_entry, entry_permset) == ACL_ERROR) {
 	  perror("acl_set_entry (acl_set_permset)");
-	  return -1;
+	  return ACL_ERROR;
 	}
 
-	return 1;
+	return ACL_SUCCESS;
       }
 
     }
@@ -145,32 +276,35 @@ int acl_set_entry(acl_t* aclp,
 
   /* This catches both the initial acl_get_entry and the ones at the
      end of the loop. */
-  if (result == -1) {
+  if (result == ACL_ERROR) {
     perror("acl_set_entry (acl_get_entry)");
-    return -1;
+    return ACL_ERROR;
   }
 
   /* If we've made it this far, we need to add a new entry to the
      ACL. */
   acl_entry_t new_entry;
 
-  /* We allocate memory here that we should release! */
-  int c_result = acl_create_entry(aclp, &new_entry);
-  if (c_result == -1) {
+  /* 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.
+   */
+  if (acl_create_entry(aclp, &new_entry) == ACL_ERROR) {
     perror("acl_set_entry (acl_create_entry)");
-    return -1;
+    return ACL_ERROR;
   }
 
-  int st_result = acl_set_tag_type(new_entry, entry_tag);
-  if (st_result == -1) {
+  if (acl_set_tag_type(new_entry, entry_tag) == ACL_ERROR) {
     perror("acl_set_entry (acl_set_tag_type)");
-    return -1;
+    return ACL_ERROR;
   }
 
-  int s_result = acl_set_permset(new_entry, entry_permset);
-  if (s_result == -1) {
+  if (acl_set_permset(new_entry, entry_permset) == ACL_ERROR) {
     perror("acl_set_entry (acl_set_permset)");
-    return -1;
+    return ACL_ERROR;
   }
 
   if (entry_tag == ACL_USER || entry_tag == ACL_GROUP) {
@@ -178,88 +312,104 @@ int acl_set_entry(acl_t* aclp,
     void* entry_qual = acl_get_qualifier(entry);
     if (entry_qual == (void*)NULL) {
       perror("acl_set_entry (acl_get_qualifier)");
-      return -1;
+      return ACL_ERROR;
     }
 
-    int sq_result = acl_set_qualifier(new_entry, entry_qual);
-    if (sq_result == -1) {
+    if (acl_set_qualifier(new_entry, entry_qual) == ACL_ERROR) {
       perror("acl_set_entry (acl_set_qualifier)");
-      return -1;
+      return ACL_ERROR;
     }
   }
 
-  return 1;
+  return ACL_SUCCESS;
 }
 
 
 
-int acl_entry_count(acl_t* acl) {
-  /*
-   * Return the number of entries in acl, or -1 on error.
-   */
+/**
+ * @brief Determine the number of entries in the given ACL.
+ *
+ * @param acl
+ *   The ACL to inspect.
+ *
+ * @return Either the non-negative number of entries in @c acl, or
+ *   @c ACL_ERROR on error.
+ */
+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 == 1) {
+  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 == -1) {
-    perror("acl_is_minimal (acl_get_entry)");
-    return -1;
+  if (result == ACL_ERROR) {
+    perror("acl_entry_count (acl_get_entry)");
+    return ACL_ERROR;
   }
 
   return entry_count;
 }
 
 
-int acl_is_minimal(acl_t* acl) {
-  /* An ACL is minimal if it has fewer than four entries. Return 0 for
-   * false, 1 for true, and -1 on error.
-   */
+
+/**
+ * @brief Determine whether or not the given ACL is minimal.
+ *
+ * An ACL is minimal if it has fewer than four entries.
+ *
+ * @param acl
+ *   The ACL whose minimality is in question.
+ *
+ * @return
+ *   - @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 ec = acl_entry_count(acl);
-  if (ec == -1) {
+
+  if (ec == ACL_ERROR) {
     perror("acl_is_minimal (acl_entry_count)");
-    return -1;
+    return ACL_ERROR;
   }
 
   if (ec < 4) {
-    return 1;
+    return ACL_SUCCESS;
   }
   else {
-    return 0;
+    return ACL_FAILURE;
   }
 }
 
 
-int acl_execute_masked(const char* path) {
-  /* Returns 1 i the given path has an ACL mask which denies
-     execute. Returns 0 if it does not (or if it has no ACL/mask at
-     all), and -1 on error. */
-
-  acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
-
-  if (acl == (acl_t)NULL) {
-    return 0;
-  }
 
-  /* Our return value. */
-  int result = 0;
+/**
+ * @brief Determine whether the given ACL's mask denies execute.
+ *
+ * @param acl
+ *   The ACL whose mask we want to check.
+ *
+ * @return
+ *   - @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(acl_t acl) {
 
   acl_entry_t entry;
   int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);
 
-  while (ge_result == 1) {
+  while (ge_result == ACL_SUCCESS) {
     acl_tag_t tag = ACL_UNDEFINED_TAG;
-    int tag_result = acl_get_tag_type(entry, &tag);
 
-    if (tag_result == -1) {
+    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
        perror("acl_execute_masked (acl_get_tag_type)");
-       result = -1;
-       goto cleanup;
+       return ACL_ERROR;
     }
 
     if (tag == ACL_MASK) {
@@ -267,68 +417,78 @@ int acl_execute_masked(const char* path) {
 	 execute is specified. */
       acl_permset_t permset;
 
-      int ps_result = acl_get_permset(entry, &permset);
-      if (ps_result == -1) {
+      if (acl_get_permset(entry, &permset) == ACL_ERROR) {
 	perror("acl_execute_masked (acl_get_permset)");
-	result = -1;
-	goto cleanup;
+	return ACL_ERROR;
       }
 
       int gp_result = acl_get_perm(permset, ACL_EXECUTE);
-      if (gp_result == -1) {
+      if (gp_result == ACL_ERROR) {
 	perror("acl_execute_masked (acl_get_perm)");
-	result = -1;
-	goto cleanup;
+	return ACL_ERROR;
       }
 
-      if (gp_result == 0) {
+      if (gp_result == ACL_FAILURE) {
 	/* No execute bit set in the mask; execute not allowed. */
-	return 1;
+	return ACL_SUCCESS;
       }
     }
 
     ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
   }
 
- cleanup:
-  acl_free(acl);
-  return result;
+  return ACL_FAILURE;
 }
 
 
-int any_can_execute_or_dir(const char* path) {
-  /* If the given path is a directory, returns 1. Otherwise, returns 1
-   * if any ACL entry has EFFECTIVE execute access, 0 if none do, and
-   * -1 on error.
-   *
-   * This is meant to somewhat mimic setfacl's handling of the capital
-   * 'X' perm, which allows execute access if the target is a
-   * directory or someone can already execute it. We differ in that we
-   * check the effective execute rather than just the execute bits.
-   */
 
-  if (is_directory(path)) {
+/**
+ * @brief Determine whether @c fd is executable (by anyone) or a
+ *   directory.
+ *
+ * This is used as part of the heuristic to determine whether or not
+ * we should mask the execute bit when inheriting an ACL. If @c fd
+ * describes a directory, the answer is a clear-cut yes. This behavior
+ * is modeled after the capital 'X' perms of setfacl.
+ *
+ * If @c fd describes a file, we check the @a effective permissions,
+ * contrary to what setfacl does.
+ *
+ * @param fd
+ *   The file descriptor to check.
+ *
+ * @return
+ *   - @c ACL_SUCCESS - @c fd describes a directory, or someone has effective
+       execute permissions.
+ *   - @c ACL_FAILURE - @c fd describes a regular file and nobody can execute
+       it.
+ *   - @c ACL_ERROR - Unexpected library error.
+ */
+int any_can_execute_or_dir(int fd) {
+
+  if (is_directory(fd)) {
     /* That was easy... */
-    return 1;
+    return ACL_SUCCESS;
   }
 
-  acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
+  acl_t acl = acl_get_fd(fd);
 
   if (acl == (acl_t)NULL) {
-    return 0;
+    perror("any_can_execute_or_dir (acl_get_file)");
+    return ACL_ERROR;
   }
 
   /* Our return value. */
-  int result = 0;
+  int result = ACL_FAILURE;
 
-  if (acl_is_minimal(&acl)) {
-    mode_t mode = get_mode(path);
+  if (acl_is_minimal(acl)) {
+    mode_t mode = get_mode(fd);
     if (mode & (S_IXUSR | S_IXOTH | S_IXGRP)) {
-      result = 1;
+      result = ACL_SUCCESS;
       goto cleanup;
     }
     else {
-      result = 0;
+      result = ACL_FAILURE;
       goto cleanup;
     }
   }
@@ -336,27 +496,42 @@ int any_can_execute_or_dir(const char* path) {
   acl_entry_t entry;
   int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);
 
-  while (ge_result == 1) {
+  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;
+
+    if (acl_get_tag_type(entry, &tag) == 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);
-    if (ps_result == -1) {
+    if (acl_get_permset(entry, &permset) == ACL_ERROR) {
       perror("any_can_execute_or_dir (acl_get_permset)");
-      result = -1;
+      result = ACL_ERROR;
       goto cleanup;
     }
 
     int gp_result = acl_get_perm(permset, ACL_EXECUTE);
-    if (gp_result == -1) {
+    if (gp_result == ACL_ERROR) {
       perror("any_can_execute_or_dir (acl_get_perm)");
-      result = -1;
+      result = ACL_ERROR;
       goto cleanup;
     }
 
-    if (gp_result == 1) {
-      /* Only return one if this execute bit is not masked. */
-      if (acl_execute_masked(path) != 1) {
-	result = 1;
+    if (gp_result == 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;
       }
     }
@@ -364,9 +539,9 @@ int any_can_execute_or_dir(const char* path) {
     ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
   }
 
-  if (ge_result == -1) {
+  if (ge_result == ACL_ERROR) {
     perror("any_can_execute_or_dir (acl_get_entry)");
-    result = -1;
+    result = ACL_ERROR;
     goto cleanup;
   }
 
@@ -376,127 +551,166 @@ int any_can_execute_or_dir(const char* path) {
 }
 
 
-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 = 1;
+/**
+ * @brief Set @c acl as the default ACL on @c path if it's a directory.
+ *
+ * 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 replace or create.
+ *
+  * @param acl
+ *   The ACL to set as default on @c path.
+ *
+ * @return
+ *   - @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 assign_default_acl(const char* path, acl_t acl) {
 
   if (path == NULL) {
     errno = ENOENT;
-    return -1;
-  }
-
-  if (!is_directory(path) || !is_directory(parent)) {
-    return 0;
+    return ACL_ERROR;
   }
 
-  acl_t parent_acl = acl_get_file(parent, ACL_TYPE_DEFAULT);
-  if (parent_acl == (acl_t)NULL) {
-    return 0;
+  if (!is_path_directory(path)) {
+    return ACL_FAILURE;
   }
 
-  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 -1;
+    perror("assign_default_acl (acl_dup)");
+    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 = -1;
-    goto cleanup;
+  if (acl_set_file(path, ACL_TYPE_DEFAULT, path_acl) == ACL_ERROR) {
+    perror("assign_default_acl (acl_set_file)");
+    result = ACL_ERROR;
   }
 
- cleanup:
   acl_free(path_acl);
   return result;
 }
 
 
-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 -1;
+/**
+ * @brief Remove all @c ACL_TYPE_ACCESS entries from the given file
+ *   descriptor, leaving the UNIX permission bits.
+ *
+ * @param fd
+ *   The file descriptor 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(int fd) {
+  /* Initialize an empty ACL, and then overwrite the one on "fd" with it. */
+  acl_t empty_acl = acl_init(0);
+
+  if (empty_acl == (acl_t)NULL) {
+    perror("wipe_acls (acl_init)");
+    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)");
-    return -1;
+  if (acl_set_fd(fd, empty_acl) == ACL_ERROR) {
+    perror("wipe_acls (acl_set_fd)");
+    acl_free(empty_acl);
+    return ACL_ERROR;
   }
 
-  /* Our return value. */
-  int result = 1;
+  acl_free(empty_acl);
+  return ACL_SUCCESS;
+}
 
-  acl_entry_t entry;
-  int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);
 
-  while (ge_result == 1) {
-    int d_result = acl_delete_entry(acl, entry);
-    if (d_result == -1) {
-      perror("wipe_acls (acl_delete_entry)");
-      result = -1;
-      goto cleanup;
-    }
 
-    ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
-  }
+/**
+ * @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) {
 
-  /* Catches the first acl_get_entry as well as the ones at the end of
-     the loop. */
-  if (ge_result == -1) {
-    perror("wipe_acls (acl_get_entry)");
-    result = -1;
-    goto cleanup;
+  if (path == NULL) {
+    errno = ENOENT;
+    return ACL_ERROR;
   }
 
-  int sf_result = acl_set_file(path, ACL_TYPE_ACCESS, acl);
-  if (sf_result == -1) {
-    perror("wipe_acls (acl_set_file)");
-    result = -1;
-    goto cleanup;
-  }
+  /* Define these next three variables here because we may have to
+   * jump to the cleanup routine which expects them to exist.
+   */
 
- cleanup:
-  acl_free(acl);
-  return result;
-}
+  /* Our return value. */
+  int result = ACL_SUCCESS;
 
+  /* The default ACL on path's parent directory */
+  acl_t defacl = (acl_t)NULL;
 
-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.
+  /* The file descriptor corresponding to "path" */
+  int fd = 0;
 
+  /* Split "path" into base/dirname parts to be used with openat().
+   * We duplicate the strings involved because dirname/basename mangle
+   * their arguments.
    */
-  if (path == NULL) {
-    return 0;
+  char* path_copy = strdup(path);
+  if (path_copy == NULL) {
+    perror("apply_default_acl (strdup)");
+    return ACL_ERROR;
   }
+  char* parent = dirname(path_copy);
 
-  if (!is_regular_file(path) && !is_directory(path)) {
-    return 0;
+  fd = open(path, O_NOFOLLOW);
+  if (fd == -1) {
+    if (errno == ELOOP) {
+      result = ACL_FAILURE; /* hit a symlink */
+      goto cleanup;
+    }
+    else {
+      perror("apply_default_acl (open fd)");
+      result = ACL_ERROR;
+      goto cleanup;
+    }
   }
 
-  /* dirname mangles its argument */
-  char path_copy[PATH_MAX];
-  strncpy(path_copy, path, PATH_MAX-1);
-  path_copy[PATH_MAX-1] = 0;
 
-  char* parent = dirname(path_copy);
-  if (!is_directory(parent)) {
-    /* Make sure dirname() did what we think it did. */
-    return 0;
+  /* 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. There is TOCTOU
+   * race condition here, but the window is as small as possible
+   * between when we open the file descriptor (look above) and when we
+   * fstat it.
+  */
+  if (!is_hardlink_safe(fd)) {
+    result = ACL_FAILURE;
+    goto cleanup;
+  }
+
+  if (!is_regular_file(fd) && !is_directory(fd)) {
+    result = ACL_FAILURE;
+    goto cleanup;
   }
 
   /* Default to not masking the exec bit; i.e. applying the default
@@ -505,68 +719,65 @@ int apply_default_acl(const char* path) {
   bool allow_exec = true;
 
   if (!no_exec_mask) {
-    int ace_result = any_can_execute_or_dir(path);
+    int ace_result = any_can_execute_or_dir(fd);
+
+    if (ace_result == ACL_ERROR) {
+      perror("apply_default_acl (any_can_execute_or_dir)");
+      result = ACL_ERROR;
+      goto cleanup;
+    }
 
-    if (ace_result == -1) {
-    perror("apply_default_acl (any_can_execute_or_dir)");
-    return -1;
-  }
     allow_exec = (bool)ace_result;
   }
 
-  acl_t defacl = acl_get_file(parent, ACL_TYPE_DEFAULT);
+  defacl = acl_get_file(parent, ACL_TYPE_DEFAULT);
 
   if (defacl == (acl_t)NULL) {
     perror("apply_default_acl (acl_get_file)");
-    return -1;
+    result = ACL_ERROR;
+    goto cleanup;
   }
 
-  /* Our return value. */
-  int result = 1;
-
-  int wipe_result = wipe_acls(path);
-  if (wipe_result == -1) {
+  if (wipe_acls(fd) == ACL_ERROR) {
     perror("apply_default_acl (wipe_acls)");
-    result = -1;
+    result = ACL_ERROR;
     goto cleanup;
   }
 
   /* Do this after wipe_acls(), otherwise we'll overwrite the wiped
      ACL with this one. */
-  acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
+  acl_t acl = acl_get_fd(fd);
   if (acl == (acl_t)NULL) {
-    perror("apply_default_acl (acl_get_file)");
-    return -1;
+    perror("apply_default_acl (acl_get_fd)");
+    result = ACL_ERROR;
+    goto cleanup;
   }
 
   /* If it's a directory, inherit the parent's default. */
-  int inherit_result = inherit_default_acl(path, parent);
-  if (inherit_result == -1) {
-    perror("apply_default_acl (inherit_acls)");
-    result = -1;
+  if (assign_default_acl(path, defacl) == ACL_ERROR) {
+    perror("apply_default_acl (assign_default_acl)");
+    result = ACL_ERROR;
     goto cleanup;
   }
 
   acl_entry_t entry;
   int ge_result = acl_get_entry(defacl, ACL_FIRST_ENTRY, &entry);
 
-  while (ge_result == 1) {
+  while (ge_result == ACL_SUCCESS) {
     acl_tag_t tag = ACL_UNDEFINED_TAG;
-    int tag_result = acl_get_tag_type(entry, &tag);
 
-    if (tag_result == -1) {
+    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
        perror("apply_default_acl (acl_get_tag_type)");
-       result = -1;
+       result = ACL_ERROR;
        goto cleanup;
     }
 
 
     /* We've got an entry/tag from the default ACL. Get its permset. */
     acl_permset_t permset;
-    int ps_result = acl_get_permset(entry, &permset);
-    if (ps_result == -1) {
+    if (acl_get_permset(entry, &permset) == ACL_ERROR) {
       perror("apply_default_acl (acl_get_permset)");
-      result = -1;
+      result = ACL_ERROR;
       goto cleanup;
     }
 
@@ -580,27 +791,39 @@ int apply_default_acl(const char* path) {
 	/* The mask doesn't affect acl_user_obj, acl_group_obj (in
 	   minimal ACLs) or acl_other entries, so if execute should be
 	   masked, we have to do it manually. */
-	int d_result = acl_delete_perm(permset, ACL_EXECUTE);
-	if (d_result == -1) {
+	if (acl_delete_perm(permset, ACL_EXECUTE) == ACL_ERROR) {
 	  perror("apply_default_acl (acl_delete_perm)");
-	  result = -1;
+	  result = ACL_ERROR;
 	  goto cleanup;
 	}
 
-	int sp_result = acl_set_permset(entry, permset);
-	if (sp_result == -1) {
+	if (acl_set_permset(entry, permset) == ACL_ERROR) {
 	  perror("apply_default_acl (acl_set_permset)");
-	  result = -1;
+	  result = ACL_ERROR;
 	  goto cleanup;
 	}
       }
     }
 
-    /* Finally, add the permset to the access ACL. */
-    int set_result = acl_set_entry(&acl, entry);
-    if (set_result == -1) {
+    /* 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".
+     */
+    if (acl_set_entry(&acl, entry) == ACL_ERROR) {
       perror("apply_default_acl (acl_set_entry)");
-      result = -1;
+      result = ACL_ERROR;
       goto cleanup;
     }
 
@@ -609,29 +832,40 @@ int apply_default_acl(const char* path) {
 
   /* Catches the first acl_get_entry as well as the ones at the end of
      the loop. */
-  if (ge_result == -1) {
+  if (ge_result == ACL_ERROR) {
     perror("apply_default_acl (acl_get_entry)");
-    result = -1;
+    result = ACL_ERROR;
     goto cleanup;
   }
 
-  int sf_result = acl_set_file(path, ACL_TYPE_ACCESS, acl);
-  if (sf_result == -1) {
-    perror("apply_default_acl (acl_set_file)");
-    result = -1;
+  if (acl_set_fd(fd, acl) == ACL_ERROR) {
+    perror("apply_default_acl (acl_set_fd)");
+    result = ACL_ERROR;
     goto cleanup;
   }
 
  cleanup:
-  acl_free(defacl);
+  free(path_copy);
+  if (defacl != (acl_t)NULL) {
+    acl_free(defacl);
+  }
+  if (fd >= 0 && close(fd) == -1) {
+    perror("apply_default_acl (close)");
+    result = ACL_ERROR;
+  }
   return result;
 }
 
 
-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",
@@ -645,43 +879,96 @@ 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) {
-    return 0;
+
+  if (apply_default_acl(target, false)) {
+    return FTW_CONTINUE;
   }
   else {
-    return 1;
+    return FTW_STOP;
   }
 }
 
 
-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) {
+
+  if (apply_default_acl(target, true)) {
+    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 */
@@ -700,17 +987,24 @@ 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. */
@@ -746,12 +1040,23 @@ 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) {