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Date: Sun, 20 Sep 2009 20:40:59 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Vegard Nossum <vegard.nossum@gmail.com>, linux-kernel@vger.kernel.org,
       Eric Paris <eparis@redhat.com>, hugh.dickins@tiscali.co.uk
Subject: Re: shmem_fill_super(): WARNING: kmemcheck: Caught 32-bit read
	from uninitialized memory
Message-ID: <20090920184059.GA21154@elte.hu>
References: <20090915080953.GA24958@elte.hu> <20090920072210.GA23787@elte.hu> <19f34abd0909201035t3157948amee532a3a5e96dbab@mail.gmail.com> <alpine.DEB.2.00.0909202046460.7394@melkki.cs.helsinki.fi> <20090920175803.GB23736@elte.hu> <84144f020909201101g2f1dbc22wd2497e8309800d7e@mail.gmail.com>
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* Pekka Enberg <penberg@cs.helsinki.fi> wrote:

> Hi Ingo,
> 
> On Sun, Sep 20, 2009 at 8:58 PM, Ingo Molnar <mingo@elte.hu> wrote:
> >> From: Pekka Enberg <penberg@cs.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@elte.hu>
> >> Signed-off-by: Pekka Enberg <penberg@cs.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?
> 
> Oh, sure. It usually less error prone to use kzalloc() for infrequent 
> allocations such as this.

So in that sense kmemcheck was useful here: it made kernel code a bit 
better.

	Ingo
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