fs-util: teach unlinkat_deallocate() a simple scheme for overwriting for erasing

With that it becomes useful for deleting password files and such.

src/basic/fs-util.c

@@ -23,6 +23,7 @@
 #include "parse-util.h"
 #include "path-util.h"
 #include "process-util.h"
+#include "random-util.h"
 #include "stat-util.h"
 #include "stdio-util.h"
 #include "string-util.h"
@@ -1303,11 +1304,13 @@ void unlink_tempfilep(char (*p)[]) {
                 (void) unlink_noerrno(*p);
 }
 
-int unlinkat_deallocate(int fd, const char *name, int flags) {
+int unlinkat_deallocate(int fd, const char *name, UnlinkDeallocateFlags flags) {
         _cleanup_close_ int truncate_fd = -1;
         struct stat st;
         off_t l, bs;
 
+        assert((flags & ~(UNLINK_REMOVEDIR|UNLINK_ERASE)) == 0);
+
         /* Operates like unlinkat() but also deallocates the file contents if it is a regular file and there's no other
          * link to it. This is useful to ensure that other processes that might have the file open for reading won't be
          * able to keep the data pinned on disk forever. This call is particular useful whenever we execute clean-up
@@ -1324,7 +1327,7 @@ int unlinkat_deallocate(int fd, const char *name, int flags) {
          * Note that we attempt deallocation, but failure to succeed with that is not considered fatal, as long as the
          * primary job – to delete the file – is accomplished. */
 
-        if ((flags & AT_REMOVEDIR) == 0) {
+        if (!FLAGS_SET(flags, UNLINK_REMOVEDIR)) {
                 truncate_fd = openat(fd, name, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW|O_NONBLOCK);
                 if (truncate_fd < 0) {
 
@@ -1340,7 +1343,7 @@ int unlinkat_deallocate(int fd, const char *name, int flags) {
                 }
         }
 
-        if (unlinkat(fd, name, flags) < 0)
+        if (unlinkat(fd, name, FLAGS_SET(flags, UNLINK_REMOVEDIR) ? AT_REMOVEDIR : 0) < 0)
                 return -errno;
 
         if (truncate_fd < 0) /* Don't have a file handle, can't do more ☹️ */
@@ -1351,7 +1354,45 @@ int unlinkat_deallocate(int fd, const char *name, int flags) {
                 return 0;
         }
 
-        if (!S_ISREG(st.st_mode) || st.st_blocks == 0 || st.st_nlink > 0)
+        if (!S_ISREG(st.st_mode))
+                return 0;
+
+        if (FLAGS_SET(flags, UNLINK_ERASE) && st.st_size > 0 && st.st_nlink == 0) {
+                uint64_t left = st.st_size;
+                char buffer[64 * 1024];
+
+                /* If erasing is requested, let's overwrite the file with random data once before deleting
+                 * it. This isn't going to give you shred(1) semantics, but hopefully should be good enough
+                 * for stuff backed by tmpfs at least.
+                 *
+                 * Note that we only erase like this if the link count of the file is zero. If it is higer it
+                 * is still linked by someone else and we'll leave it to them to remove it securely
+                 * eventually! */
+
+                random_bytes(buffer, sizeof(buffer));
+
+                while (left > 0) {
+                        ssize_t n;
+
+                        n = write(truncate_fd, buffer, MIN(sizeof(buffer), left));
+                        if (n < 0) {
+                                log_debug_errno(errno, "Failed to erase data in file '%s', ignoring.", name);
+                                break;
+                        }
+
+                        assert(left >= (size_t) n);
+                        left -= n;
+                }
+
+                /* Let's refresh metadata */
+                if (fstat(truncate_fd, &st) < 0) {
+                        log_debug_errno(errno, "Failed to stat file '%s' for deallocation, ignoring: %m", name);
+                        return 0;
+                }
+        }
+
+        /* Don't dallocate if there's nothing to deallocate or if the file is linked elsewhere */
+        if (st.st_blocks == 0 || st.st_nlink > 0)
                 return 0;
 
         /* If this is a regular file, it actually took up space on disk and there are no other links it's time to

src/basic/fs-util.h

@@ -113,7 +113,13 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(char*, unlink_and_free);
 int access_fd(int fd, int mode);
 
 void unlink_tempfilep(char (*p)[]);
-int unlinkat_deallocate(int fd, const char *name, int flags);
+
+typedef enum UnlinkDeallocateFlags {
+        UNLINK_REMOVEDIR = 1 << 0,
+        UNLINK_ERASE     = 1 << 1,
+} UnlinkDeallocateFlags;
+
+int unlinkat_deallocate(int fd, const char *name, UnlinkDeallocateFlags flags);
 
 int fsync_directory_of_file(int fd);
 int fsync_full(int fd);

src/test/test-fs-util.c

@@ -670,7 +670,7 @@ static void test_unlinkat_deallocate(void) {
         assert_se(st.st_blocks > 0);
         assert_se(st.st_nlink == 1);
 
-        assert_se(unlinkat_deallocate(AT_FDCWD, p, 0) >= 0);
+        assert_se(unlinkat_deallocate(AT_FDCWD, p, UNLINK_ERASE) >= 0);
 
         assert_se(fstat(fd, &st) >= 0);
         assert_se(IN_SET(st.st_size, 0, 6)); /* depending on whether hole punching worked the size will be 6