[PATCH] seedrng: limit poolsize to 256 bytes and document flock() usage

[Jason A. Donenfeld]

Rather than having getrandom() be called in a loop that handles EINTR --
which would require more code bloat -- we just limit the maximum seed
size to 256 bytes, which the kernel guarantees won't be interrupted.
Additionally document the flock() usage so that somebody doesn't remove
it.

Cc: Denys Vlasenko <vda.linux at googlemail.com>
Cc: Bernhard Reutner-Fischer <rep.dot.nop at gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
---
 util-linux/seedrng.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/util-linux/seedrng.c b/util-linux/seedrng.c
index c42274759..1257cd941 100644
--- a/util-linux/seedrng.c
+++ b/util-linux/seedrng.c
@@ -56,7 +56,7 @@
 
 enum {
 	MIN_SEED_LEN = SHA256_OUTSIZE,
-	MAX_SEED_LEN = 512
+	MAX_SEED_LEN = 256 /* Maximum size of getrandom() call without EINTR. */
 };
 
 static size_t determine_optimal_seed_len(void)
@@ -190,6 +190,8 @@ int seedrng_main(int argc UNUSED_PARAM, char *argv[])
 	if (mkdir(seed_dir, 0700) < 0 && errno != EEXIST)
 		bb_perror_msg_and_die("can't %s seed directory", "create");
 	dfd = open(seed_dir, O_DIRECTORY | O_RDONLY);
+	/* The flock() here is absolutely necessary, as the consistency of this
+	 * program breaks down with concurrent uses. */
 	if (dfd < 0 || flock(dfd, LOCK_EX) < 0)
 		bb_perror_msg_and_die("can't %s seed directory", "lock");
 	xfchdir(dfd);
-- 
2.35.1