[PATCH mptcp-next v14 01/12] selftests: mptcp: sockopt: reseed RNG after fork

From: Geliang Tang <[email protected]>

xfork() does not reseed the RNG in the child, so server and client
processes use the same rand() sequence. Restore the per-fork reseed
from mptcp_inq.c by calling init_rng() in the child, ensuring
independent random values for buffer contents and transfer sizes.

init_rng() is now called from each forked child (server, client) so that
rand() sequences don't diverge from a single parent seed. But the fallback
path did 'srand(time(NULL))' - and time(NULL) has 1-second resolution, so
the two children spawned back-to-back in main() almost always land in the
same second and end up with the same seed, defeating the per-child re-seed.

Mix in getpid() (per-process unique, kernel-guaranteed) with time(NULL)
(cross-second entropy) so the fallback produces an independent seed for
each child even when /dev/urandom is unavailable.

Also hoist the 'foo' local to function scope and collapse the two srand()
callsites into a single one at the end, so both branches consistently write
the seed into 'foo' first.

Signed-off-by: Geliang Tang <[email protected]>
---
 tools/testing/selftests/net/mptcp/mptcp_sockopt.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
index b6e58d936ebe..6c3d896d4a1b 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
@@ -128,6 +128,8 @@ struct so_state {
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
 #endif
 
+static void init_rng(void);
+
 static void __noreturn die_perror(const char *msg)
 {
 	perror(msg);
@@ -799,6 +801,8 @@ static pid_t xfork(void)
 
 	if (p < 0)
 		die_perror("fork");
+	else if (p == 0)
+		init_rng();
 
 	return p;
 }
@@ -822,9 +826,9 @@ static int rcheck(int wstatus, const char *what)
 static void init_rng(void)
 {
 	int fd = open("/dev/urandom", O_RDONLY);
+	unsigned int foo;
 
 	if (fd >= 0) {
-		unsigned int foo;
 		ssize_t ret;
 
 		/* can't fail */
@@ -832,10 +836,10 @@ static void init_rng(void)
 		assert(ret == sizeof(foo));
 
 		close(fd);
-		srand(foo);
 	} else {
-		srand(time(NULL));
+		foo = getpid() ^ time(NULL);
 	}
+	srand(foo);
 }
 
 int main(int argc, char *argv[])
-- 
2.53.0