Merge pull request #31731 from poettering/stat-is-set

stat-util: add explicit helpers for checking if stat/statx is initialized

src/basic/stat-util.c

@@ -364,8 +364,7 @@ bool stat_inode_same(const struct stat *a, const struct stat *b) {
         /* Returns if the specified stat structure references the same (though possibly modified) inode. Does
          * a thorough check, comparing inode nr, backing device and if the inode is still of the same type. */
 
-        return a && b &&
-                a->st_dev != 0 && /* is the structure ever initialized? */
+        return stat_is_set(a) && stat_is_set(b) &&
                 ((a->st_mode ^ b->st_mode) & S_IFMT) == 0 &&  /* same inode type */
                 a->st_dev == b->st_dev &&
                 a->st_ino == b->st_ino;
@@ -393,7 +392,7 @@ bool statx_inode_same(const struct statx *a, const struct statx *b) {
 
         /* Same as stat_inode_same() but for struct statx */
 
-        return a && b &&
+        return statx_is_set(a) && statx_is_set(b) &&
                 FLAGS_SET(a->stx_mask, STATX_TYPE|STATX_INO) && FLAGS_SET(b->stx_mask, STATX_TYPE|STATX_INO) &&
                 ((a->stx_mode ^ b->stx_mode) & S_IFMT) == 0 &&
                 a->stx_dev_major == b->stx_dev_major &&
@@ -402,7 +401,7 @@ bool statx_inode_same(const struct statx *a, const struct statx *b) {
 }
 
 bool statx_mount_same(const struct new_statx *a, const struct new_statx *b) {
-        if (!a || !b)
+        if (!new_statx_is_set(a) || !new_statx_is_set(b))
                 return false;
 
         /* if we have the mount ID, that's all we need */
@@ -536,6 +535,8 @@ const char* inode_type_to_string(mode_t m) {
                 return "sock";
         }
 
+        /* Note anonmyous inodes in the kernel will have a zero type. Hence fstat() of an eventfd() will
+         * return an .st_mode where we'll return NULL here! */
         return NULL;
 }
 

src/basic/stat-util.h

@@ -9,6 +9,7 @@
 #include <sys/types.h>
 #include <sys/vfs.h>
 
+#include "fs-util.h"
 #include "macro.h"
 #include "missing_stat.h"
 #include "siphash24.h"
@@ -126,3 +127,18 @@ extern const struct hash_ops inode_hash_ops;
 
 const char* inode_type_to_string(mode_t m);
 mode_t inode_type_from_string(const char *s);
+
+/* Macros that check whether the stat/statx structures have been initialized already. For "struct stat" we
+ * use a check for .st_dev being non-zero, since the kernel unconditionally fills that in, mapping the file
+ * to its originating superblock, regardless if the fs is block based or virtual (we also check for .st_mode
+ * being MODE_INVALID, since we use that as an invalid marker for separate mode_t fields). For "struct statx"
+ * we use the .stx_mask field, which must be non-zero if any of the fields have already been initialized. */
+static inline bool stat_is_set(const struct stat *st) {
+        return st && st->st_dev != 0 && st->st_mode != MODE_INVALID;
+}
+static inline bool statx_is_set(const struct statx *sx) {
+        return sx && sx->stx_mask != 0;
+}
+static inline bool new_statx_is_set(const struct new_statx *sx) {
+        return sx && sx->stx_mask != 0;
+}

src/resolve/resolved-etc-hosts.c

@@ -342,7 +342,7 @@ static int manager_etc_hosts_read(Manager *m) {
 
         m->etc_hosts_last = ts;
 
-        if (m->etc_hosts_stat.st_mode != 0) {
+        if (stat_is_set(&m->etc_hosts_stat)) {
                 if (stat("/etc/hosts", &st) < 0) {
                         if (errno != ENOENT)
                                 return log_error_errno(errno, "Failed to stat /etc/hosts: %m");

src/test/test-stat-util.c

@@ -3,6 +3,7 @@
 #include <fcntl.h>
 #include <linux/magic.h>
 #include <sched.h>
+#include <sys/eventfd.h>
 #include <unistd.h>
 
 #include "alloc-util.h"
@@ -195,6 +196,21 @@ TEST(inode_type_from_string) {
                 assert_se(inode_type_from_string(inode_type_to_string(*m)) == *m);
 }
 
+TEST(anonymous_inode) {
+        _cleanup_close_ int fd = -EBADF;
+
+        fd = eventfd(0, EFD_CLOEXEC);
+        assert_se(fd >= 0);
+
+        /* Verify that we handle anonymous inodes correctly, i.e. those which have no file type */
+
+        struct stat st;
+        assert_se(fstat(fd, &st) >= 0);
+        assert_se((st.st_mode & S_IFMT) == 0);
+
+        assert_se(!inode_type_to_string(st.st_mode));
+}
+
 TEST(fd_verify_linked) {
         _cleanup_(rm_rf_physical_and_freep) char *t = NULL;
         _cleanup_close_ int tfd = -EBADF, fd = -EBADF;