Date:	Mon, 21 Sep 2009 11:49:09 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Pekka J Enberg <penberg@...helsinki.fi>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Eric Paris <eparis@...hat.com>
Subject: Re: shmem_fill_super(): WARNING: kmemcheck: Caught 32-bit read from
  uninitialized memory

On Sun, 20 Sep 2009, Pekka J Enberg wrote:
> On Sun, 20 Sep 2009, Vegard Nossum wrote:
> > Thanks for the report.
> >
> > AFAICT it's this line of mm/shmem.c:
> >
> > 2356         inode = shmem_get_inode(sb, S_IFDIR | sbinfo->mode, 0,
> > VM_NORESERVE     );
> >
> > and the loading of sbinfo->mode. It fits with the offset 0x3c(%esi) ==
> > the address reported by kmemcheck and the offset of ->mode:
> >
> > (gdb) p &((struct shmem_sb_info *) 0).mode
> > $1 = (mode_t *) 0x3c
> >
> > Looking for the definition of mode_t, it seems to be defined in x86
> > sources as unsigned short:
> >
> > arch/x86/include/asm/posix_types_32.h:11:typedef unsigned short
> > __kernel_mode_t;
> > include/linux/types.h:typedef __kernel_mode_t           mode_t;
> >
> > And the load was clearly 32-bit (kmemcheck said so) and in my assembly
> > dump it is also so.
> >
> > As I said before, I really don't like the solution of sprinkling the
> > kmemcheck annotations all over the place to cover up field padding
> > inside structs, not in the least because they confuse more than they
> > help, and they are not maintainable -- when somebody changes the
> > struct definitions, anything may happen to the field layout, and then
> > the annotation may have to change too. And it's not exactly obvious.
> >
> > I still vote for patching gcc as the long-term solution. There is
> > -fmudflap, there is -fstack-protector, why not a -fsacred-padding? Of
> > course it has to be implemented too...
> 
> As Ingo already explained, we would need to wait for a year or so for
> "-fscared-padding" to appear in a GCC release and probably one year more for
> it to be picked up by distributions.
> 
> So while we wait for such a thing to appear, how about something like this?
> 
> 			Pekka
> From a7cb569beb2d2fe769d558d1a017b6f5aa05d7eb Mon Sep 17 00:00:00 2001

Ingo, Vegard, Pekka: thanks for doing all the work on this, especially
Vegard for deciphering it (I've added you to the credits below, and
added a further line of explanation to Pekka's comment).  Andrew,
please pick this up into mmotm and send on to Linus - thanks.

I got a bit anxious when I saw that the mode arg to shmem_get_inode()
is declared as an int: was afraid that compiler was then passing a bad
upper half down, which in fact would cause no trouble, but how could it
be sure of that?  However, it looks okay: after doing the 32-bit load,
it goes on to do a movzwl %ax,%eax - seems an odd way to proceed to me,
but I bet it knows a lot more about efficiency of memory loads than I do.

Hugh

From: Pekka Enberg <penberg@...helsinki.fi>
Date: Sun, 20 Sep 2009 20:43:35 +0300
Subject: [PATCH] shmem: initialize struct shmem_sb_info to zero

Fixes the following kmemcheck false positive (the compiler is using
a 32-bit mov to load the 16-bit sbinfo->mode in shmem_fill_super):

[    0.337000] Total of 1 processors activated (3088.38 BogoMIPS).
[    0.352000] CPU0 attaching NULL sched-domain.
[    0.360000] WARNING: kmemcheck: Caught 32-bit read from uninitialized
memory (9f8020fc)
[    0.361000]
a44240820000000041f6998100000000000000000000000000000000ff030000
[    0.368000]  i i i i i i i i i i i i i i i i u u u u i i i i i i i i i i u
u
[    0.375000]                                                          ^
[    0.376000]
[    0.377000] Pid: 9, comm: khelper Not tainted (2.6.31-tip #206) P4DC6
[    0.378000] EIP: 0060:[<810a3a95>] EFLAGS: 00010246 CPU: 0
[    0.379000] EIP is at shmem_fill_super+0xb5/0x120
[    0.380000] EAX: 00000000 EBX: 9f845400 ECX: 824042a4 EDX: 8199f641
[    0.381000] ESI: 9f8020c0 EDI: 9f845400 EBP: 9f81af68 ESP: 81cd6eec
[    0.382000]  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
[    0.383000] CR0: 8005003b CR2: 9f806200 CR3: 01ccd000 CR4: 000006d0
[    0.384000] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
[    0.385000] DR6: ffff4ff0 DR7: 00000400
[    0.386000]  [<810c25fc>] get_sb_nodev+0x3c/0x80
[    0.388000]  [<810a3514>] shmem_get_sb+0x14/0x20
[    0.390000]  [<810c207f>] vfs_kern_mount+0x4f/0x120
[    0.392000]  [<81b2849e>] init_tmpfs+0x7e/0xb0
[    0.394000]  [<81b11597>] do_basic_setup+0x17/0x30
[    0.396000]  [<81b11907>] kernel_init+0x57/0xa0
[    0.398000]  [<810039b7>] kernel_thread_helper+0x7/0x10
[    0.400000]  [<ffffffff>] 0xffffffff
[    0.402000] khelper used greatest stack depth: 2820 bytes left
[    0.407000] calling  init_mmap_min_addr+0x0/0x10 @ 1
[    0.408000] initcall init_mmap_min_addr+0x0/0x10 returned 0 after 0 usecs

Reported-by: Ingo Molnar <mingo@...e.hu>
Analysed-by: Vegard Nossum <vegard.nossum@...il.com>
Signed-off-by: Pekka Enberg <penberg@...helsinki.fi>
Acked-by: Hugh Dickins <hugh.dickins@...cali.co.uk>
---
 mm/shmem.c |    5 +----
 1 files changed, 1 insertions(+), 4 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index d713239..a8f54f3 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2307,17 +2307,14 @@ static int shmem_fill_super(struct super_block *sb,
 	int err = -ENOMEM;

 	/* Round up to L1_CACHE_BYTES to resist false sharing */
-	sbinfo = kmalloc(max((int)sizeof(struct shmem_sb_info),
+	sbinfo = kzalloc(max((int)sizeof(struct shmem_sb_info),
 				L1_CACHE_BYTES), GFP_KERNEL);
 	if (!sbinfo)
 		return -ENOMEM;

-	sbinfo->max_blocks = 0;
-	sbinfo->max_inodes = 0;
 	sbinfo->mode = S_IRWXUGO | S_ISVTX;
 	sbinfo->uid = current_fsuid();
 	sbinfo->gid = current_fsgid();
-	sbinfo->mpol = NULL;
 	sb->s_fs_info = sbinfo;

 #ifdef CONFIG_TMPFS
-- 
1.5.6.4
--
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