linux-kernel - Re: kmemleak splat on copy

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Date:   Tue, 7 Feb 2017 02:37:02 +0100
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Mateusz Guzik <mguzik@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: kmemleak splat on copy_process()

On Mon, Feb 06, 2017 at 10:47:41AM +0100, Michal Hocko wrote:
> On Fri 03-02-17 13:06:04, Luis R. Rodriguez wrote:
> > On next-20170125 running some kselftest not yet upstream I eventually
> > get a kmemleak splat:
> > 
> > unreferenced object 0xffffa7b1034b4000 (size 16384):
> >   comm "driver_data.sh", pid 6506, jiffies 4295068366 (age 1697.272s)
> >   hex dump (first 32 bytes):
> >     9d 6e ac 57 00 00 00 00 74 2d 64 72 69 76 65 72  .n.W....t-driver
> >     5f 64 61 74 61 2e 62 69 6e 0a 00 00 00 00 00 00  _data.bin.......
> >   backtrace:
> >     [<ffffffff9005f7fa>] kmemleak_alloc+0x4a/0xa0
> >     [<ffffffff8fbe7006>] __vmalloc_node_range+0x206/0x2a0
> >     [<ffffffff8fa7f3e9>] copy_process.part.36+0x609/0x1cc0
> >     [<ffffffff8fa80c77>] _do_fork+0xd7/0x390
> >     [<ffffffff8fa80fd9>] SyS_clone+0x19/0x20
> >     [<ffffffff8fa03b4b>] do_syscall_64+0x5b/0xc0
> >     [<ffffffff9006b3af>] return_from_SYSCALL_64+0x0/0x6a
> >     [<ffffffffffffffff>] 0xffffffffffffffff
> > 
> > As per gdb:
> > 
> > (gdb) l *(copy_process+0x609)
> > 0xffffffff8107f3e9 is in copy_process (kernel/fork.c:204).
> > warning: Source file is more recent than executable.
> > 199             /*
> > 200              * We can't call find_vm_area() in interrupt context, and
> > 201              * free_thread_stack() can be called in interrupt context,
> > 202              * so cache the vm_struct.
> > 203              */
> > 204             if (stack) {
> > 205                     tsk->stack_vm_area = find_vm_area(stack);
> > 206             }
> > 207             return stack;
> > 208     #else
> 
> Could you check the state of the above process (pid 6506)? Does it still
> own its stack? 

Although I can reproduce the splat on kmemleak, getting it to trigger
at a point I can stop the kernel and inspect the process seems rather hard.

> From a quick check I do not see any leak there either.

Then in that case what about:

diff --git a/kernel/fork.c b/kernel/fork.c
index 937ba59709c9..3c96aafa1f82 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -196,6 +196,7 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
 				     PAGE_KERNEL,
 				     0, node, __builtin_return_address(0));
 
+	kmemleak_ignore(stack);
 	/*
 	 * We can't call find_vm_area() in interrupt context, and
 	 * free_thread_stack() can be called in interrupt context,

I no longer get the spurious splats from kmemleak after this.

  Luis