X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2Fapply-default-acl.c;h=b7ef3a82c951f83e167c9dd00653402f7b74ef95;hb=b0131edac01695a533c4ffd33774215aec9cd03a;hp=a4961957b7487fa10591f87e93ee54f41d3d2d31;hpb=f0c1a9ee7c7a6acbfc0c4270f9167022504a6a45;p=apply-default-acl.git

diff --git a/src/apply-default-acl.c b/src/apply-default-acl.c
index a496195..b7ef3a8 100644
--- a/src/apply-default-acl.c
+++ b/src/apply-default-acl.c
@@ -13,8 +13,7 @@
 #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>
@@ -35,84 +34,6 @@
 
 
 
-/**
- * @brief Get the mode bits from the given path.
- *
- * @param path
- *   The path (file or 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(const char* path) {
-  if (path == NULL) {
-    errno = ENOENT;
-    return -1;
-  }
-
-  struct stat s;
-  int result = lstat(path, &s);
-
-  if (result == 0) {
-    return s.st_mode;
-  }
-  else {
-    /* 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.
- *
- * @param path
- *   The path to test.
- *
- * @return true if @c path is a regular file, false otherwise.
- */
-bool is_regular_file(const char* path) {
-  if (path == NULL) {
-    return false;
-  }
-
-  struct stat s;
-  int result = lstat(path, &s);
-  if (result == 0) {
-    return S_ISREG(s.st_mode);
-  }
-  else {
-    return false;
-  }
-}
-
-
 
 /**
  * @brief Determine whether or not the given path is accessible.
@@ -138,9 +59,7 @@ bool path_accessible(const char* path) {
 
   /* 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) {
+  if (faccessat(AT_FDCWD, path, F_OK, flags) == 0) {
     return true;
   }
   else {
@@ -150,31 +69,6 @@ bool path_accessible(const char* path) {
 
 
 
-/**
- * @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_directory(const char* path) {
-  if (path == NULL) {
-    return false;
-  }
-
-  struct stat s;
-  int result = lstat(path, &s);
-  if (result == 0) {
-    return S_ISDIR(s.st_mode);
-  }
-  else {
-    return false;
-  }
-}
-
-
-
 /**
  * @brief Update (or create) an entry in an @b minimal ACL.
  *
@@ -199,19 +93,16 @@ bool is_directory(const char* path) {
  *   returned. Otherwise, @c ACL_SUCCESS.
  *
  */
-int acl_set_entry(acl_t* aclp,
-		  acl_entry_t entry) {
+int acl_set_entry(acl_t* aclp, acl_entry_t entry) {
 
   acl_tag_t entry_tag;
-  int gt_result = acl_get_tag_type(entry, &entry_tag);
-  if (gt_result == ACL_ERROR) {
+  if (acl_get_tag_type(entry, &entry_tag) == ACL_ERROR) {
     perror("acl_set_entry (acl_get_tag_type)");
     return ACL_ERROR;
   }
 
   acl_permset_t entry_permset;
-  int ps_result = acl_get_permset(entry, &entry_permset);
-  if (ps_result == ACL_ERROR) {
+  if (acl_get_permset(entry, &entry_permset) == ACL_ERROR) {
     perror("acl_set_entry (acl_get_permset)");
     return ACL_ERROR;
   }
@@ -222,9 +113,8 @@ int acl_set_entry(acl_t* aclp,
 
   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 == ACL_ERROR) {
+    if (acl_get_tag_type(existing_entry, &existing_tag) == ACL_ERROR) {
        perror("set_acl_tag_permset (acl_get_tag_type)");
        return ACL_ERROR;
     }
@@ -238,14 +128,12 @@ int acl_set_entry(acl_t* aclp,
 	   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 == ACL_ERROR) {
+	if (acl_get_permset(existing_entry, &existing_permset) == ACL_ERROR) {
 	  perror("acl_set_entry (acl_get_permset)");
 	  return ACL_ERROR;
 	}
 
-	int s_result = acl_set_permset(existing_entry, entry_permset);
-	if (s_result == ACL_ERROR) {
+	if (acl_set_permset(existing_entry, entry_permset) == ACL_ERROR) {
 	  perror("acl_set_entry (acl_set_permset)");
 	  return ACL_ERROR;
 	}
@@ -269,21 +157,24 @@ int acl_set_entry(acl_t* aclp,
      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 == ACL_ERROR) {
+  /* 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 ACL_ERROR;
   }
 
-  int st_result = acl_set_tag_type(new_entry, entry_tag);
-  if (st_result == ACL_ERROR) {
+  if (acl_set_tag_type(new_entry, entry_tag) == ACL_ERROR) {
     perror("acl_set_entry (acl_set_tag_type)");
     return ACL_ERROR;
   }
 
-  int s_result = acl_set_permset(new_entry, entry_permset);
-  if (s_result == ACL_ERROR) {
+  if (acl_set_permset(new_entry, entry_permset) == ACL_ERROR) {
     perror("acl_set_entry (acl_set_permset)");
     return ACL_ERROR;
   }
@@ -296,8 +187,7 @@ int acl_set_entry(acl_t* aclp,
       return ACL_ERROR;
     }
 
-    int sq_result = acl_set_qualifier(new_entry, entry_qual);
-    if (sq_result == ACL_ERROR) {
+    if (acl_set_qualifier(new_entry, entry_qual) == ACL_ERROR) {
       perror("acl_set_entry (acl_set_qualifier)");
       return ACL_ERROR;
     }
@@ -312,20 +202,20 @@ int acl_set_entry(acl_t* aclp,
  * @brief Determine the number of entries in the given ACL.
  *
  * @param acl
- *   A pointer to an @c acl_t structure.
+ *   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) {
+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) {
@@ -344,14 +234,14 @@ 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
  *   - @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);
 
@@ -371,41 +261,27 @@ 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
- *   - @c ACL_SUCCESS - @c path has a mask which denies execute.
- *   - @c ACL_FAILURE - The ACL for @c path does not deny execute,
- *     or @c path has no extended ACL at all.
+ *   - @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);
 
   while (ge_result == ACL_SUCCESS) {
     acl_tag_t tag = ACL_UNDEFINED_TAG;
-    int tag_result = acl_get_tag_type(entry, &tag);
 
-    if (tag_result == ACL_ERROR) {
+    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
        perror("acl_execute_masked (acl_get_tag_type)");
-       result = ACL_ERROR;
-       goto cleanup;
+       return ACL_ERROR;
     }
 
     if (tag == ACL_MASK) {
@@ -413,18 +289,15 @@ 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 == ACL_ERROR) {
+      if (acl_get_permset(entry, &permset) == 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) {
@@ -436,55 +309,44 @@ 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.
+ * @brief Determine whether @c fd is executable by anyone.
+ *
  *
  * 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 path
- * is a directory, the answer is a clear-cut yes. This behavior is
- * modeled after the capital 'X' perms of setfacl.
+ * we should mask the execute bit when inheriting an ACL. If @c fd
+ * describes a file, we check the @a effective permissions, contrary
+ * to what setfacl does.
  *
- * If @c path is a file, we check the @a effective permissions,
- * contrary to what setfacl does.
+ * @param fd
+ *   The file descriptor to check.
  *
- * @param path
- *   The path to check.
+ * @param sp
+ *   A pointer to a stat structure for @c fd.
  *
  * @return
- *   - @c ACL_SUCCESS - @c path is a directory, or someone has effective
-       execute permissions.
- *   - @c ACL_FAILURE - @c path is a regular file and nobody can execute
-       it.
+ *   - @c ACL_SUCCESS - Someone has effective execute permissions on @c fd.
+ *   - @c ACL_FAILURE - Nobody can execute @c fd.
  *   - @c ACL_ERROR - Unexpected library error.
  */
-int any_can_execute_or_dir(const char* path) {
-
-  if (is_directory(path)) {
-    /* That was easy... */
-    return ACL_SUCCESS;
-  }
-
-  acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
+int any_can_execute(int fd, const struct stat* sp) {
+  acl_t acl = acl_get_fd(fd);
 
   if (acl == (acl_t)NULL) {
-    perror("any_can_execute_or_dir (acl_get_file)");
+    perror("any_can_execute (acl_get_file)");
     return ACL_ERROR;
   }
 
   /* Our return value. */
   int result = ACL_FAILURE;
 
-  if (acl_is_minimal(&acl)) {
-    mode_t mode = get_mode(path);
-    if (mode & (S_IXUSR | S_IXOTH | S_IXGRP)) {
+  if (acl_is_minimal(acl)) {
+    if (sp->st_mode & (S_IXUSR | S_IXOTH | S_IXGRP)) {
       result = ACL_SUCCESS;
       goto cleanup;
     }
@@ -501,10 +363,9 @@ int any_can_execute_or_dir(const char* path) {
     /* 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)");
+    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
+      perror("any_can_execute_or (acl_get_tag_type)");
       result = ACL_ERROR;
       goto cleanup;
     }
@@ -517,23 +378,22 @@ int any_can_execute_or_dir(const char* path) {
     /* 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 == ACL_ERROR) {
-      perror("any_can_execute_or_dir (acl_get_permset)");
+    if (acl_get_permset(entry, &permset) == ACL_ERROR) {
+      perror("any_can_execute_or (acl_get_permset)");
       result = ACL_ERROR;
       goto cleanup;
     }
 
     int gp_result = acl_get_perm(permset, ACL_EXECUTE);
     if (gp_result == ACL_ERROR) {
-      perror("any_can_execute_or_dir (acl_get_perm)");
+      perror("any_can_execute (acl_get_perm)");
       result = ACL_ERROR;
       goto cleanup;
     }
 
     if (gp_result == ACL_SUCCESS) {
       /* Only return ACL_SUCCESS if this execute bit is not masked. */
-      if (acl_execute_masked(path) != ACL_SUCCESS) {
+      if (acl_execute_masked(acl) != ACL_SUCCESS) {
 	result = ACL_SUCCESS;
 	goto cleanup;
       }
@@ -543,7 +403,7 @@ int any_can_execute_or_dir(const char* path) {
   }
 
   if (ge_result == ACL_ERROR) {
-    perror("any_can_execute_or_dir (acl_get_entry)");
+    perror("any_can_execute (acl_get_entry)");
     result = ACL_ERROR;
     goto cleanup;
   }
@@ -556,11 +416,10 @@ int any_can_execute_or_dir(const char* path) {
 
 
 /**
- * @brief Set @c acl as the default ACL on @c path if it's a directory.
+ * @brief Set @c acl as the default ACL on @c path.
  *
- * 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.
+ * This overwrites any existing default ACL on @c path. If @c path is
+ * not a directory, we return ACL_ERROR and @c errno is set.
  *
  * @param path
  *   The target directory whose ACL we wish to replace or create.
@@ -570,32 +429,27 @@ int any_can_execute_or_dir(const char* 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;
+    errno = EINVAL;
+    perror("assign_default_acl (args)");
     return ACL_ERROR;
   }
 
-  if (!is_directory(path)) {
-    return ACL_FAILURE;
-  }
-
-    /* Our return value; success unless something bad happens. */
+  /* 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)");
+    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)");
+  if (acl_set_file(path, ACL_TYPE_DEFAULT, path_acl) == ACL_ERROR) {
+    perror("assign_default_acl (acl_set_file)");
     result = ACL_ERROR;
   }
 
@@ -606,65 +460,34 @@ int assign_default_acl(const char* path, acl_t acl) {
 
 
 /**
- * @brief Remove @c ACL_USER, @c ACL_GROUP, and @c ACL_MASK entries
- *   from the given path.
+ * @brief Remove all @c ACL_TYPE_ACCESS entries from the given file
+ *   descriptor, leaving the UNIX permission bits.
  *
- * @param path
- *   The path whose ACLs we want to wipe.
+ * @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(const char* path) {
+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 (path == NULL) {
-    errno = ENOENT;
+  if (empty_acl == (acl_t)NULL) {
+    perror("wipe_acls (acl_init)");
     return ACL_ERROR;
   }
 
-  acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS);
-  if (acl == (acl_t)NULL) {
-    perror("wipe_acls (acl_get_file)");
+  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 = ACL_SUCCESS;
-
-  acl_entry_t entry;
-  int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry);
-
-  while (ge_result == ACL_SUCCESS) {
-    int d_result = acl_delete_entry(acl, entry);
-    if (d_result == ACL_ERROR) {
-      perror("wipe_acls (acl_delete_entry)");
-      result = ACL_ERROR;
-      goto cleanup;
-    }
-
-    ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry);
-  }
-
-  /* Catches the first acl_get_entry as well as the ones at the end of
-     the loop. */
-  if (ge_result == ACL_ERROR) {
-    perror("wipe_acls (acl_get_entry)");
-    result = ACL_ERROR;
-    goto cleanup;
-  }
-
-  int sf_result = acl_set_file(path, ACL_TYPE_ACCESS, acl);
-  if (sf_result == ACL_ERROR) {
-    perror("wipe_acls (acl_set_file)");
-    result = ACL_ERROR;
-    goto cleanup;
-  }
-
- cleanup:
-  acl_free(acl);
-  return result;
+  acl_free(empty_acl);
+  return ACL_SUCCESS;
 }
 
 
@@ -677,6 +500,10 @@ int wipe_acls(const char* path) {
  * @param path
  *   The path whose ACL we would like to reset to its default.
  *
+ * @param sp
+ *   A pointer to a stat structure for @c path, or @c NULL if you don't
+ *   have one handy.
+ *
  * @param no_exec_mask
  *   The value (either true or false) of the --no-exec-mask flag.
  *
@@ -686,82 +513,111 @@ int wipe_acls(const char* path) {
  *     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) {
+int apply_default_acl(const char* path,
+		      const struct stat* sp,
+		      bool no_exec_mask) {
 
   if (path == NULL) {
-    errno = ENOENT;
+    errno = EINVAL;
+    perror("apply_default_acl (args)");
+    return ACL_ERROR;
+  }
+
+  /* Define these next three variables here because we may have to
+   * jump to the cleanup routine which expects them to exist.
+   */
+
+  /* Our return value. */
+  int result = ACL_SUCCESS;
+
+  /* The default ACL on path's parent directory */
+  acl_t defacl = (acl_t)NULL;
+
+  /* The file descriptor corresponding to "path" */
+  int fd = 0;
+
+  /* Get the parent directory of "path" with dirname(), which happens
+   * to murder its argument and necessitates a path_copy.
+   */
+  char* path_copy = strdup(path);
+  if (path_copy == NULL) {
+    perror("apply_default_acl (strdup)");
     return ACL_ERROR;
   }
+  char* parent = dirname(path_copy);
+
+  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;
+    }
+  }
+
 
   /* 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.
+   * 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.
    *
-   * 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.
+   * Note: we only need to call fstat ourselves if we weren't passed a
+   * valid pointer to a stat structure (nftw does that).
   */
-  if (!is_hardlink_safe(path)) {
-    return ACL_FAILURE;
-  }
+  if (sp == NULL) {
+    struct stat s;
+    if (fstat(fd, &s) == -1) {
+      perror("apply_default_acl (fstat)");
+      goto cleanup;
+    }
 
-  if (!is_regular_file(path) && !is_directory(path)) {
-    return ACL_FAILURE;
+    sp = &s;
   }
 
-  /* 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 ACL_FAILURE;
+  if (!S_ISDIR(sp->st_mode)) {
+    /* If it's not a directory, make sure it's a regular,
+       non-hard-linked file. */
+    if (!S_ISREG(sp->st_mode) || sp->st_nlink != 1) {
+      result = ACL_FAILURE;
+      goto cleanup;
+    }
   }
 
+
   /* Default to not masking the exec bit; i.e. applying the default
      ACL literally. If --no-exec-mask was not specified, then we try
-     to "guess" whether or not to mask the exec bit. */
+     to "guess" whether or not to mask the exec bit. This behavior
+     is modeled after the capital 'X' perms of setfacl. */
   bool allow_exec = true;
 
   if (!no_exec_mask) {
-    int ace_result = any_can_execute_or_dir(path);
+    /* Never mask the execute bit on directories. */
+    int ace_result = any_can_execute(fd,sp) || S_ISDIR(sp->st_mode);
 
     if (ace_result == ACL_ERROR) {
-      perror("apply_default_acl (any_can_execute_or_dir)");
-      return ACL_ERROR;
+      perror("apply_default_acl (any_can_execute)");
+      result = ACL_ERROR;
+      goto cleanup;
     }
 
     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 ACL_ERROR;
+    result = ACL_ERROR;
+    goto cleanup;
   }
 
-  /* Our return value. */
-  int result = ACL_SUCCESS;
-
-  int wipe_result = wipe_acls(path);
-  if (wipe_result == ACL_ERROR) {
+  if (wipe_acls(fd) == ACL_ERROR) {
     perror("apply_default_acl (wipe_acls)");
     result = ACL_ERROR;
     goto cleanup;
@@ -769,16 +625,21 @@ int apply_default_acl(const char* path, bool no_exec_mask) {
 
   /* 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 ACL_ERROR;
+    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 = assign_default_acl(path, defacl);
-  if (inherit_result == ACL_ERROR) {
-    perror("apply_default_acl (inherit_acls)");
+  /* If it's a directory, inherit the parent's default. We sure hope
+   * that "path" still points to the same thing that "fd" and this
+   * "sp" describe. If not, we may wind up trying to set a default ACL
+   * on a file, and this will throw an error. I guess that's what we
+   * want to do?
+   */
+  if (S_ISDIR(sp->st_mode) && assign_default_acl(path, defacl) == ACL_ERROR) {
+    perror("apply_default_acl (assign_default_acl)");
     result = ACL_ERROR;
     goto cleanup;
   }
@@ -788,9 +649,8 @@ int apply_default_acl(const char* path, bool no_exec_mask) {
 
   while (ge_result == ACL_SUCCESS) {
     acl_tag_t tag = ACL_UNDEFINED_TAG;
-    int tag_result = acl_get_tag_type(entry, &tag);
 
-    if (tag_result == ACL_ERROR) {
+    if (acl_get_tag_type(entry, &tag) == ACL_ERROR) {
        perror("apply_default_acl (acl_get_tag_type)");
        result = ACL_ERROR;
        goto cleanup;
@@ -799,8 +659,7 @@ int apply_default_acl(const char* path, bool no_exec_mask) {
 
     /* 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 == ACL_ERROR) {
+    if (acl_get_permset(entry, &permset) == ACL_ERROR) {
       perror("apply_default_acl (acl_get_permset)");
       result = ACL_ERROR;
       goto cleanup;
@@ -816,15 +675,13 @@ int apply_default_acl(const char* path, bool no_exec_mask) {
 	/* 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 == ACL_ERROR) {
+	if (acl_delete_perm(permset, ACL_EXECUTE) == ACL_ERROR) {
 	  perror("apply_default_acl (acl_delete_perm)");
 	  result = ACL_ERROR;
 	  goto cleanup;
 	}
 
-	int sp_result = acl_set_permset(entry, permset);
-	if (sp_result == ACL_ERROR) {
+	if (acl_set_permset(entry, permset) == ACL_ERROR) {
 	  perror("apply_default_acl (acl_set_permset)");
 	  result = ACL_ERROR;
 	  goto cleanup;
@@ -832,9 +689,23 @@ int apply_default_acl(const char* path, bool no_exec_mask) {
       }
     }
 
-    /* Finally, add the permset to the access ACL. */
-    int set_result = acl_set_entry(&acl, entry);
-    if (set_result == ACL_ERROR) {
+    /* 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 = ACL_ERROR;
       goto cleanup;
@@ -851,15 +722,21 @@ int apply_default_acl(const char* path, bool no_exec_mask) {
     goto cleanup;
   }
 
-  int sf_result = acl_set_file(path, ACL_TYPE_ACCESS, acl);
-  if (sf_result == ACL_ERROR) {
-    perror("apply_default_acl (acl_set_file)");
+  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;
 }
 
@@ -898,12 +775,11 @@ void usage(const char* program_name) {
  *
  */
 int apply_default_acl_nftw(const char *target,
-			   const struct stat *s,
+			   const struct stat *sp,
 			   int info,
 			   struct FTW *ftw) {
 
-  bool app_result = apply_default_acl(target, false);
-  if (app_result) {
+  if (apply_default_acl(target, sp, false)) {
     return FTW_CONTINUE;
   }
   else {
@@ -921,12 +797,11 @@ int apply_default_acl_nftw(const char *target,
  *
  */
 int apply_default_acl_nftw_x(const char *target,
-			     const struct stat *s,
+			     const struct stat *sp,
 			     int info,
 			     struct FTW *ftw) {
 
-  bool app_result = apply_default_acl(target, true);
-  if (app_result) {
+  if (apply_default_acl(target, sp, true)) {
     return FTW_CONTINUE;
   }
   else {
@@ -961,11 +836,6 @@ int apply_default_acl_nftw_x(const char *target,
  *   we still return @c false.
  */
 bool apply_default_acl_recursive(const char *target, bool no_exec_mask) {
-
-  if (!is_directory(target)) {
-    return apply_default_acl(target, no_exec_mask);
-  }
-
   int max_levels = 256;
   int flags = FTW_PHYS; /* Don't follow links. */
 
@@ -1065,7 +935,7 @@ int main(int argc, char* argv[]) {
     }
     else {
       /* It's either a normal file, or we're not operating recursively. */
-      reapp_result = apply_default_acl(target, no_exec_mask);
+      reapp_result = apply_default_acl(target, NULL, no_exec_mask);
     }
 
     if (!reapp_result) {