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Message-ID: <Pine.LNX.4.64.0909211126080.32504@sister.anvils> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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