pidfs: Fix memory leak in pidfd_info()

JIRA: https://issues.redhat.com/browse/RHEL-113598
Conflicts:
  A merge conflict due to missing upstream commit 8ec7c82 ("pidfs:
  persist information").

commit 0b2d71a
Author: Adrian Huang (Lenovo) <adrianhuang0701@gmail.com>
Date:   Thu, 14 Aug 2025 17:44:53 +0800

    pidfs: Fix memory leak in pidfd_info()

    After running the program 'ioctl_pidfd03' of Linux Test Project (LTP) or
    the program 'pidfd_info_test' in 'tools/testing/selftests/pidfd' of the
    kernel source, kmemleak reports the following memory leaks:

      # cat /sys/kernel/debug/kmemleak
      unreferenced object 0xff110020e5988000 (size 8216):
        comm "ioctl_pidfd03", pid 10853, jiffies 4294800031
        hex dump (first 32 bytes):
          02 40 00 00 00 00 00 00 10 00 00 00 00 00 00 00  .@..............
          00 00 00 00 af 01 00 00 80 00 00 00 00 00 00 00  ................
        backtrace (crc 69483047):
          kmem_cache_alloc_node_noprof+0x2fb/0x410
          copy_process+0x178/0x1740
          kernel_clone+0x99/0x3b0
          __do_sys_clone3+0xbe/0x100
          do_syscall_64+0x7b/0x2c0
          entry_SYSCALL_64_after_hwframe+0x76/0x7e
      ...
      unreferenced object 0xff11002097b70000 (size 8216):
      comm "pidfd_info_test", pid 11840, jiffies 4294889165
      hex dump (first 32 bytes):
        06 40 00 00 00 00 00 00 10 00 00 00 00 00 00 00  .@..............
        00 00 00 00 b5 00 00 00 80 00 00 00 00 00 00 00  ................
      backtrace (crc a6286bb7):
        kmem_cache_alloc_node_noprof+0x2fb/0x410
        copy_process+0x178/0x1740
        kernel_clone+0x99/0x3b0
        __do_sys_clone3+0xbe/0x100
        do_syscall_64+0x7b/0x2c0
        entry_SYSCALL_64_after_hwframe+0x76/0x7e
      ...

    The leak occurs because pidfd_info() obtains a task_struct via
    get_pid_task() but never calls put_task_struct() to drop the reference,
    leaving task->usage unbalanced.

    Fix the issue by adding '__free(put_task) = NULL' to the local variable
    'task', ensuring that put_task_struct() is automatically invoked when
    the variable goes out of scope.

    Fixes: 7477d7d ("pidfs: allow to retrieve exit information")
    Signed-off-by: Adrian Huang (Lenovo) <adrianhuang0701@gmail.com>
    Link: https://lore.kernel.org/20250814094453.15232-1-adrianhuang0701@gmail.com
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

fs/pidfs.c

@@ -242,13 +242,13 @@ static inline bool pid_in_current_pidns(const struct pid *pid)
 static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 {
 	struct pidfd_info __user *uinfo = (struct pidfd_info __user *)arg;
+	struct task_struct *task __free(put_task) = NULL;
 	struct inode *inode = file_inode(file);
 	struct pid *pid = pidfd_pid(file);
 	size_t usize = _IOC_SIZE(cmd);
 	struct pidfd_info kinfo = {};
 	struct pidfs_exit_info *exit_info;
 	struct user_namespace *user_ns;
-	struct task_struct *task;
 	const struct cred *c;
 	__u64 mask;