seedrng: re-add fsync after unlink, and explain its purpose

function                                             old     new   delta
seedrng_main                                        1003    1022     +19

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/util-linux/seedrng.c b/util-linux/seedrng.c
index 74bf633a7..b79ce6627 100644
--- a/util-linux/seedrng.c
+++ b/util-linux/seedrng.c
@@ -128,7 +128,7 @@ static void seed_rng(uint8_t *seed, size_t len, bool credit)
                 close(random_fd);
 }
 
-static void seed_from_file_if_exists(const char *filename, bool credit, sha256_ctx_t *hash)
+static void seed_from_file_if_exists(const char *filename, int dfd, bool credit, sha256_ctx_t *hash)
 {
         uint8_t seed[MAX_SEED_LEN];
         ssize_t seed_len;
@@ -141,6 +141,16 @@ static void seed_from_file_if_exists(const char *filename, bool credit, sha256_c
         }
         xunlink(filename);
         if (seed_len != 0) {
+                /* We are going to use this data to seed the RNG:
+                 * we believe it to genuinely containing entropy.
+                 * If this just-unlinked file survives
+                 * (e.g. if machine crashes _right now_)
+                 * and we reuse it after reboot, this assumption
+                 * would be violated. Fsync the directory to
+                 * make sure file is gone:
+                 */
+                fsync(dfd);
+
                 sha256_hash(hash, &seed_len, sizeof(seed_len));
                 sha256_hash(hash, seed, seed_len);
                 printf("Seeding %u bits %s crediting\n",
@@ -193,6 +203,7 @@ int seedrng_main(int argc UNUSED_PARAM, char *argv[])
 
         for (int i = 1; i < 3; ++i) {
                 seed_from_file_if_exists(i == 1 ? NON_CREDITABLE_SEED_NAME : CREDITABLE_SEED_NAME,
+                                        dfd,
                                         i == 1 ? false : !skip_credit,
                                         &hash);
         }