linux-kernel - [PATCH v2 4/4] fork: free thread in copy

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Date:	Thu, 31 Mar 2016 09:55:55 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	akpm@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH v2 4/4] fork: free thread in copy_process on failure

When using this program (as root):
	#include <err.h>
	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>

	#include <sys/io.h>
	#include <sys/types.h>
	#include <sys/wait.h>

	#define ITER 1000
	#define FORKERS 15
	#define THREADS (6000/FORKERS) // 1850 is proc max

	static void fork_100_wait()
	{
		unsigned a, to_wait = 0;

		printf("\t%d forking %d\n", THREADS, getpid());

		for (a = 0; a < THREADS; a++) {
			switch (fork()) {
			case 0:
				usleep(1000);
				exit(0);
				break;
			case -1:
				break;
			default:
				to_wait++;
				break;
			}
		}

		printf("\t%d forked from %d, waiting for %d\n", THREADS, getpid(),
				to_wait);

		for (a = 0; a < to_wait; a++)
			wait(NULL);

		printf("\t%d waited from %d\n", THREADS, getpid());
	}

	static void run_forkers()
	{
		pid_t forkers[FORKERS];
		unsigned a;

		for (a = 0; a < FORKERS; a++) {
			switch ((forkers[a] = fork())) {
			case 0:
				fork_100_wait();
				exit(0);
				break;
			case -1:
				err(1, "DIE fork of %d'th forker", a);
				break;
			default:
				break;
			}
		}

		for (a = 0; a < FORKERS; a++)
			waitpid(forkers[a], NULL, 0);
	}

	int main()
	{
		unsigned a;
		int ret;

		ret = ioperm(10, 20, 0);
		if (ret < 0)
			err(1, "ioperm");

		for (a = 0; a < ITER; a++)
			run_forkers();

		return 0;
	}

kmemleak reports many occurences of this leak:
unreferenced object 0xffff8805917c8000 (size 8192):
  comm "fork-leak", pid 2932, jiffies 4295354292 (age 1871.028s)
  hex dump (first 32 bytes):
    ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  ................
    ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  ................
  backtrace:
    [<ffffffff814cfbf5>] kmemdup+0x25/0x50
    [<ffffffff8103ab43>] copy_thread_tls+0x6c3/0x9a0
    [<ffffffff81150174>] copy_process+0x1a84/0x5790
    [<ffffffff811dc375>] wake_up_new_task+0x2d5/0x6f0
    [<ffffffff8115411d>] _do_fork+0x12d/0x820
...

Make sure the memory is freed when fork fails later in copy_process.
This is done by calling exit_thread with the thread to kill.

Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 kernel/fork.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index d277e83ed3e0..0ac98c9ea15e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1470,7 +1470,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		pid = alloc_pid(p->nsproxy->pid_ns_for_children);
 		if (IS_ERR(pid)) {
 			retval = PTR_ERR(pid);
-			goto bad_fork_cleanup_io;
+			goto bad_fork_cleanup_thread;
 		}
 	}
 
@@ -1632,6 +1632,8 @@ bad_fork_cancel_cgroup:
 bad_fork_free_pid:
 	if (pid != &init_struct_pid)
 		free_pid(pid);
+bad_fork_cleanup_thread:
+	exit_thread(p);
 bad_fork_cleanup_io:
 	if (p->io_context)
 		exit_io_context(p);
-- 
2.7.4