signal: Fix memory leak for PIDFD_SELF* sentinels

Commit f08d0c3 ("pidfd: add PIDFD_SELF* sentinels to refer to own
thread/process") introduced a leak by acquiring a pid reference through
get_task_pid(), which increments pid->count but never drops it with
put_pid().

As a result, kmemleak reports unreferenced pid objects after running
tools/testing/selftests/pidfd/pidfd_test, for example:

  unreferenced object 0xff1100206757a940 (size 160):
    comm "pidfd_test", pid 16965, jiffies 4294853028
    hex dump (first 32 bytes):
      01 00 00 00 00 00 00 00 00 00 00 00 fd 57 50 04  .............WP.
      5e 44 00 00 00 00 00 00 18 de 34 17 01 00 11 ff  ^D........4.....
    backtrace (crc cd8844d4):
      kmem_cache_alloc_noprof+0x2f4/0x3f0
      alloc_pid+0x54/0x3d0
      copy_process+0xd58/0x1740
      kernel_clone+0x99/0x3b0
      __do_sys_clone3+0xbe/0x100
      do_syscall_64+0x7b/0x2c0
      entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fix this by calling put_pid() after do_pidfd_send_signal() returns.

Fixes: f08d0c3 ("pidfd: add PIDFD_SELF* sentinels to refer to own thread/process")
Signed-off-by: Adrian Huang (Lenovo) <adrianhuang0701@gmail.com>
Link: https://lore.kernel.org/20250818134310.12273-1-adrianhuang0701@gmail.com
Tested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: AdrianHuang

kernel/signal.c

@@ -4067,6 +4067,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 {
 	struct pid *pid;
 	enum pid_type type;
+	int ret;
 
 	/* Enforce flags be set to 0 until we add an extension. */
 	if (flags & ~PIDFD_SEND_SIGNAL_FLAGS)
@@ -4108,7 +4109,10 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	}
 	}
 
-	return do_pidfd_send_signal(pid, sig, type, info, flags);
+	ret = do_pidfd_send_signal(pid, sig, type, info, flags);
+	put_pid(pid);
+
+	return ret;
 }
 
 static int