Subject: [PATCH 03/16] mm: check rb_subtree_gap correctness

From: [email-masked] (Sasha Levin)
Date: Fri, 09 Nov 2012 09:13:26 -0500

On 11/05/2012 05:47 PM, Michel Lespinasse wrote:
> When CONFIG_DEBUG_VM_RB is enabled, check that rb_subtree_gap is
> correctly set for every vma and that mm->highest_vm_end is also correct.
> 
> Also add an explicit 'bug' variable to track if browse_rb() detected any
> invalid condition.
> 
> Signed-off-by: Michel Lespinasse <[email-masked]>
> Reviewed-by: Rik van Riel <[email-masked]>
> 
> ---

Hi all,

While fuzzing with trinity inside a KVM tools (lkvm) guest, using today's -next
kernel, I'm getting these:

[  117.007714] free gap 7fba0dd1c000, correct 7fba0dcfb000
[  117.019773] map_count 750 rb -1
[  117.028362] ------------[ cut here ]------------
[  117.029813] kernel BUG at mm/mmap.c:439!
[  117.031024] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[  117.032933] Dumping ftrace buffer:
[  117.033972]    (ftrace buffer empty)
[  117.035085] CPU 4
[  117.035676] Pid: 6859, comm: trinity-child46 Tainted: G        W    3.7.0-rc4-next-20121109-sasha-00013-g9407f3c #124
[  117.038217] RIP: 0010:[<ffffffff81236687>]  [<ffffffff81236687>] validate_mm+0x297/0x2c0
[  117.041056] RSP: 0018:ffff880016a4fdf8  EFLAGS: 00010296
[  117.041056] RAX: 0000000000000013 RBX: 00000000ffffffff RCX: 0000000000000006
[  117.041056] RDX: 0000000000005270 RSI: ffff880024120910 RDI: 0000000000000286
[  117.052131] RBP: ffff880016a4fe48 R08: 0000000000000000 R09: 0000000000000000
[  117.052131] R10: 0000000000000001 R11: 0000000000000000 R12: 00000000000002ee
[  117.052131] R13: 00007fffea1fc000 R14: ffff88002412c000 R15: 0000000000000000
[  117.052131] FS:  00007fba129db700(0000) GS:ffff880063600000(0000) knlGS:0000000000000000
[  117.052131] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  117.052131] CR2: 0000000003323288 CR3: 00000000169b2000 CR4: 00000000000406e0
[  117.052131] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  117.052131] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  117.052131] Process trinity-child46 (pid: 6859, threadinfo ffff880016a4e000, task ffff880024120000)
[  117.052131] Stack:
[  117.052131]  ffffffff8489e201 ffffffff81235aa0 ffff88000885cac8 0000000100000000
[  117.052131]  ffffffff812361b9 ffff88002412c000 ffff88000885cac8 ffff88000885cdc8
[  117.052131]  ffff88000885cdd0 ffff88002412c000 ffff880016a4fe98 ffffffff812367b4
[  117.052131] Call Trace:
[  117.052131]  [<ffffffff81235aa0>] ? vma_compute_subtree_gap+0x40/0x40
[  117.052131]  [<ffffffff812361b9>] ? vma_gap_update+0x19/0x30
[  117.052131]  [<ffffffff812367b4>] vma_link+0x94/0xe0
[  117.052131]  [<ffffffff812386c4>] do_brk+0x2c4/0x380
[  117.052131]  [<ffffffff812387bf>] ? sys_brk+0x3f/0x190
[  117.052131]  [<ffffffff812388ce>] sys_brk+0x14e/0x190
[  117.052131]  [<ffffffff83be2618>] tracesys+0xe1/0xe6
[  117.052131] Code: d8 41 8b 76 60 39 de 74 1b 89 da 48 c7 c7 c6 d9 89 84 31 c0 e8 01 76 94 02 eb 10 66 0f 1f 84 00 00 00 00 00
8b 45 c8 85 c0 74 18 <0f> 0b 4c 8d 48 e0 48 8b 70 e0 31 db c7 45 cc 00 00 00 00 e9 f4
[  117.052131] RIP  [<ffffffff81236687>] validate_mm+0x297/0x2c0
[  117.052131]  RSP <ffff880016a4fdf8>
[  117.136092] ---[ end trace 5ce250e0bf6d040c ]---

Note that they are very easy to reproduce.

Also, I see that lots of the code there has a local variable named 'bug' thats tracking
whether we should BUG() later on. Why does it work that way and the BUG() isn't immediate?

Thanks,
Sasha