[3.15-rc1 slab] Oops when reading /proc/slab_allocators

[Tetsuo Handa]

Hello.

I found that

  $ cat /proc/slab_allocators

causes an oops.

---------- dmesg start ----------
[   22.719620] BUG: unable to handle kernel paging request at ffff8800389b7ff8
[   22.719742] IP: [<ffffffff811b751d>] handle_slab+0x8d/0x1a0
[   22.719742] PGD 2d64067 PUD 2d65067 PMD 3fc20067 PTE 80000000389b7060
[   22.719742] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
[   22.719742] Modules linked in:
[   22.719742] CPU: 0 PID: 1 Comm: init Not tainted 3.14.0-12041-g75ff24f #297
[   22.719742] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2007
[   22.719742] task: ffff88003decc010 ti: ffff88003ded0000 task.ti: ffff88003ded0000
[   22.719742] RIP: 0010:[<ffffffff811b751d>]  [<ffffffff811b751d>] handle_slab+0x8d/0x1a0
[   22.719742] RSP: 0018:ffff88003ded1d88  EFLAGS: 00000002
[   22.719742] RAX: 0000000000000001 RBX: 0000000000000001 RCX: ffff88003f625c80
[   22.719742] RDX: ffffea0000e26dc0 RSI: 0000000000000000 RDI: ffff88001763f000
[   22.719742] RBP: ffff88003ded1dd8 R08: ffff88001763f000 R09: ffff88001763f008
[   22.719742] R10: ffff88001763f010 R11: 0000000000001000 R12: ffff880017616f00
[   22.719742] R13: 0000000000000000 R14: ffff8800389b7000 R15: ffffea0000e26dc0
[   22.719742] FS:  00000000015b7860(0063) GS:ffff88003fa00000(0000) knlGS:0000000000000000
[   22.719742] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   22.719742] CR2: ffff8800389b7ff8 CR3: 0000000017619000 CR4: 00000000000006f0
[   22.719742] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   22.719742] DR3: 0000000000000000 DR6: 0000000000000000 DR7: 0000000000000000
[   22.719742] Stack:
[   22.719742]  ffff88001763f010 ffff88003f625c80 ffff88001763f000 ffff88001763f008
[   22.719742]  ffff88003ded1dd8 ffff88001763f000 ffff880017616f00 ffff88003f625c80
[   22.719742]  0000000000000400 ffffea0000e26dc0 ffff88003ded1e48 ffffffff811bcbd6
[   22.719742] Call Trace:
[   22.719742]  [<ffffffff811bcbd6>] leaks_show+0xf6/0x270
[   22.719742]  [<ffffffff811f529f>] seq_read+0x2af/0x440
[   22.719742]  [<ffffffff8123e213>] proc_reg_read+0x43/0x70
[   22.719742]  [<ffffffff811ccdcd>] vfs_read+0xad/0x190
[   22.719742]  [<ffffffff811ccf99>] SyS_read+0x59/0xd0
[   22.719742]  [<ffffffff81658412>] system_call_fastpath+0x16/0x1b
[   22.719742] Code: 75 18 e9 df 00 00 00 0f 1f 80 00 00 00 00 48 63 d0 3b 34 97 0f 84 cc 00 00 00 83 c0 01 39 c3 77 ed f6 41 1a 01 0f 84 02 01 00 00 <4f> 8b 64 1e f8 4d 85 e4 0f 84 ad 00 00 00 49 8b 39 4c 89 d3 89
[   22.719742] RIP  [<ffffffff811b751d>] handle_slab+0x8d/0x1a0
[   22.719742]  RSP <ffff88003ded1d88>
[   22.719742] CR2: ffff8800389b7ff8
[   22.719742] ---[ end trace b3602f55c39df20e ]---
---------- dmesg end ----------

Kernel config is at http://I-love.SAKURA.ne.jp/tmp/config-pre-3.15-rc1 .

The location is

  if (!add_caller(n, (unsigned long)*dbg_userword(c, p)))

in handle_slab() in mm/slab.c , and dbg_userword(c, p) is returning
an undereferencable value (0xffff8800389b7ff8) when
n=ffff880017645000, c=ffff88003f625c80, p=ffff8800389b7000,
c->flags=0x80010c00 and c->size=4096.

  static void **dbg_userword(struct kmem_cache *cachep, void *objp)
  {
          BUG_ON(!(cachep->flags & SLAB_STORE_USER));
          return (void **)(objp + cachep->size - BYTES_PER_WORD);
  }

Bisection reached commit b1cb0982 "slab: change the management method
of free objects of the slab".

  bad commit b1cb0982bdd6f57fed690f796659733350bb2cae
  good commit a57a49887eb3398d31bfaa8009531f7121d6537d
  untestable commit 56f295ef0dfa7e1d0be18deebe0c15fb6b2d9d5b
  untestable commit 0c3aa83e00a9cd93f08e7aa42fba01924aa5f2fc
  good commit 73293c2f900d0adbb6a415b312cd57976d5ae242
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