linux-kernel - Re: shmem_fill_super(): WARNING: kmemcheck: Caught 32-bit read from uninitialized memory

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Date:	Sun, 20 Sep 2009 19:58:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	linux-kernel@...r.kernel.org, Eric Paris <eparis@...hat.com>,
	hugh.dickins@...cali.co.uk
Subject: Re: shmem_fill_super(): WARNING: kmemcheck: Caught 32-bit read
	from uninitialized memory


* Pekka J Enberg <penberg@...helsinki.fi> 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
> 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:
>
> [    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>
> Signed-off-by: Pekka Enberg <penberg@...helsinki.fi>
> ---
>  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;

That looks like a step forward even without kmemcheck considered, right?

	Ingo
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