From: "Paul E. McKenney" <paulmck@kernel.org>
To: Jinliang Zheng <alexjlzheng@gmail.com>
Cc: raven@themaw.net, alexjlzheng@tencent.com, bfoster@redhat.com,
david@fromorbit.com, djwong@kernel.org,
linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
rcu@vger.kernel.org
Subject: Re: About the conflict between XFS inode recycle and VFS rcu-walk
Date: Sun, 26 May 2024 10:21:52 -0700 [thread overview]
Message-ID: <3d7074a2-83e9-4282-8c8a-d90176653868@paulmck-laptop> (raw)
In-Reply-To: <20240526150414.1419898-1-alexjlzheng@tencent.com>
On Sun, May 26, 2024 at 11:04:14PM +0800, Jinliang Zheng wrote:
> On Tue, 21 May 2024 at 10:13:38 +0800, Ian Kent wrote:
> > On 21/5/24 09:35, Ian Kent wrote:
> > > On 21/5/24 01:36, Darrick J. Wong wrote:
> > >> On Thu, May 16, 2024 at 03:23:40PM +0800, Ian Kent wrote:
> > >>> On 16/5/24 15:08, Ian Kent wrote:
> > >>>> On 16/5/24 12:56, Jinliang Zheng wrote:
> > >>>>> On Wed, 15 May 2024 at 23:54:41 +0800, Jinliang Zheng wrote:
> > >>>>>> On Wed, 31 Jan 2024 at 11:30:18 -0800, djwong@kernel.org wrote:
> > >>>>>>> On Wed, Jan 31, 2024 at 02:35:17PM +0800, Jinliang Zheng wrote:
> > >>>>>>>> On Fri, 8 Dec 2023 11:14:32 +1100, david@fromorbit.com wrote:
> > >>>>>>>>> On Tue, Dec 05, 2023 at 07:38:33PM +0800,
> > >>>>>>>>> alexjlzheng@gmail.com wrote:
> > >>>>>>>>>> Hi, all
> > >>>>>>>>>>
> > >>>>>>>>>> I would like to ask if the conflict between xfs
> > >>>>>>>>>> inode recycle and vfs rcu-walk
> > >>>>>>>>>> which can lead to null pointer references has been resolved?
> > >>>>>>>>>>
> > >>>>>>>>>> I browsed through emails about the following
> > >>>>>>>>>> patches and their discussions:
> > >>>>>>>>>> -
> > >>>>>>>>>> https://lore.kernel.org/linux-xfs/20220217172518.3842951-2-bfoster@redhat.com/
> > >>>>>>>>>> -
> > >>>>>>>>>> https://lore.kernel.org/linux-xfs/20220121142454.1994916-1-bfoster@redhat.com/
> > >>>>>>>>>> -
> > >>>>>>>>>> https://lore.kernel.org/linux-xfs/164180589176.86426.501271559065590169.stgit@mickey.themaw.net/
> > >>>>>>>>>>
> > >>>>>>>>>> And then came to the conclusion that this
> > >>>>>>>>>> problem has not been solved, am I
> > >>>>>>>>>> right? Did I miss some patch that could solve this problem?
> > >>>>>>>>> We fixed the known problems this caused by turning off the VFS
> > >>>>>>>>> functionality that the rcu pathwalks kept tripping over. See
> > >>>>>>>>> commit
> > >>>>>>>>> 7b7820b83f23 ("xfs: don't expose internal symlink
> > >>>>>>>>> metadata buffers to
> > >>>>>>>>> the vfs").
> > >>>>>>>> Sorry for the delay.
> > >>>>>>>>
> > >>>>>>>> The problem I encountered in the production environment
> > >>>>>>>> was that during the
> > >>>>>>>> rcu walk process the ->get_link() pointer was NULL,
> > >>>>>>>> which caused a crash.
> > >>>>>>>>
> > >>>>>>>> As far as I know, commit 7b7820b83f23 ("xfs: don't
> > >>>>>>>> expose internal symlink
> > >>>>>>>> metadata buffers to the vfs") first appeared in:
> > >>>>>>>> -
> > >>>>>>>> https://lore.kernel.org/linux-fsdevel/YZvvP9RFXi3%2FjX0q@bfoster/
> > >>>>>>>>
> > >>>>>>>> Does this commit solve the problem of NULL ->get_link()? And how?
> > >>>>>>> I suggest reading the call stack from wherever the VFS enters
> > >>>>>>> the XFS
> > >>>>>>> readlink code. If you have a reliable reproducer, then
> > >>>>>>> apply this patch
> > >>>>>>> to your kernel (you haven't mentioned which one it is) and see
> > >>>>>>> if the
> > >>>>>>> bad dereference goes away.
> > >>>>>>>
> > >>>>>>> --D
> > >>>>>> Sorry for the delay.
> > >>>>>>
> > >>>>>> I encountered the following calltrace:
> > >>>>>>
> > >>>>>> [20213.578756] BUG: kernel NULL pointer dereference, address:
> > >>>>>> 0000000000000000
> > >>>>>> [20213.578785] #PF: supervisor instruction fetch in kernel mode
> > >>>>>> [20213.578799] #PF: error_code(0x0010) - not-present page
> > >>>>>> [20213.578812] PGD 3f01d64067 P4D 3f01d64067 PUD 3f01d65067 PMD 0
> > >>>>>> [20213.578828] Oops: 0010 [#1] SMP NOPTI
> > >>>>>> [20213.578839] CPU: 92 PID: 766 Comm: /usr/local/serv Kdump:
> > >>>>>> loaded Not tainted 5.4.241-1-tlinux4-0017.3 #1
> > >>>>>> [20213.578860] Hardware name: New H3C Technologies Co., Ltd.
> > >>>>>> UniServer R4900 G3/RS33M2C9SA, BIOS 2.00.38P02 04/14/2020
> > >>>>>> [20213.578884] RIP: 0010:0x0
> > >>>>>> [20213.578894] Code: Bad RIP value.
> > >>>>>> [20213.578903] RSP: 0018:ffffc90021ebfc38 EFLAGS: 00010246
> > >>>>>> [20213.578916] RAX: ffffffff82081f40 RBX: ffffc90021ebfce0 RCX:
> > >>>>>> 0000000000000000
> > >>>>>> [20213.578932] RDX: ffffc90021ebfd48 RSI: ffff88bfad8d3890 RDI:
> > >>>>>> 0000000000000000
> > >>>>>> [20213.578948] RBP: ffffc90021ebfc70 R08: 0000000000000001 R09:
> > >>>>>> ffff889b9eeae380
> > >>>>>> [20213.578965] R10: 302d343200000067 R11: 0000000000000001 R12:
> > >>>>>> 0000000000000000
> > >>>>>> [20213.578981] R13: ffff88bfad8d3890 R14: ffff889b9eeae380 R15:
> > >>>>>> ffffc90021ebfd48
> > >>>>>> [20213.578998] FS: 00007f89c534e740(0000)
> > >>>>>> GS:ffff88c07fd00000(0000) knlGS:0000000000000000
> > >>>>>> [20213.579016] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > >>>>>> [20213.579030] CR2: ffffffffffffffd6 CR3: 0000003f01d90001 CR4:
> > >>>>>> 00000000007706e0
> > >>>>>> [20213.579046] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
> > >>>>>> 0000000000000000
> > >>>>>> [20213.579062] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:
> > >>>>>> 0000000000000400
> > >>>>>> [20213.579079] PKRU: 55555554
> > >>>>>> [20213.579087] Call Trace:
> > >>>>>> [20213.579099] trailing_symlink+0x1da/0x260
> > >>>>>> [20213.579112] path_lookupat.isra.53+0x79/0x220
> > >>>>>> [20213.579125] filename_lookup.part.69+0xa0/0x170
> > >>>>>> [20213.579138] ? kmem_cache_alloc+0x3f/0x3f0
> > >>>>>> [20213.579151] ? getname_flags+0x4f/0x1e0
> > >>>>>> [20213.579161] user_path_at_empty+0x3e/0x50
> > >>>>>> [20213.579172] vfs_statx+0x76/0xe0
> > >>>>>> [20213.579182] __do_sys_newstat+0x3d/0x70
> > >>>>>> [20213.579194] ? fput+0x13/0x20
> > >>>>>> [20213.579203] ? ksys_ioctl+0xb0/0x300
> > >>>>>> [20213.579213] ? generic_file_llseek+0x24/0x30
> > >>>>>> [20213.579225] ? fput+0x13/0x20
> > >>>>>> [20213.579233] ? ksys_lseek+0x8d/0xb0
> > >>>>>> [20213.579243] __x64_sys_newstat+0x16/0x20
> > >>>>>> [20213.579256] do_syscall_64+0x4d/0x140
> > >>>>>> [20213.579268] entry_SYSCALL_64_after_hwframe+0x5c/0xc1
> > >>>>>>
> > >>>>>> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> > >>>>>>
> > >>>>>>
> > >>>>> Please note that the kernel version I use is the one maintained by
> > >>>>> Tencent.Inc,
> > >>>>> and the baseline is v5.4. But in fact, in the latest upstream source
> > >>>>> tree,
> > >>>>> although the trailing_symlink() function has been removed, its logic
> > >>>>> has been
> > >>>>> moved to pick_link(), so the problem still exists.
> > >>>>>
> > >>>>> Ian Kent pointed out that try_to_unlazy() was introduced in
> > >>>>> pick_link() in the
> > >>>>> latest upstream source tree, but I don't understand why this can
> > >>>>> solve the NULL
> > >>>>> ->get_link pointer dereference problem, because ->get_link pointer
> > >>>>> will be
> > >>>>> dereferenced before try_to_unlazy().
> > >>>>>
> > >>>>> (I don't understand why Ian Kent's email didn't appear on the
> > >>>>> mailing list.)
> > >>>> It was something about html mail and I think my mail client was at
> > >>>> fault.
> > >>>>
> > >>>> In any case what you say is indeed correct, so the comment isn't
> > >>>> important.
> > >>>>
> > >>>>
> > >>>> Fact is it is still a race between the lockless path walk and inode
> > >>>> eviction
> > >>>>
> > >>>> and xfs recycling. I believe that the xfs recycling code is very
> > >>>> hard to
> > >>>> fix.
> > >>>>
> > >>>>
> > >>>> IIRC correctly putting a NULL check in pick_link() was not considered
> > >>>> acceptable
> > >>>>
> > >>>> but there must be a way that is acceptable to check this and
> > >>>> restart the
> > >>>> walk.
> > >>>>
> > >>>> Maybe there was a reluctance to suffer the overhead of restarting the
> > >>>> walk when
> > >>>>
> > >>>> it shouldn't be needed.
> > >>> Or perhaps the worry was that if it can become NULL it could also
> > >>> become a
> > >>> pointer to a
> > >>>
> > >>> different (incorrect) link altogether which could have really
> > >>> odd/unpleasant
> > >>> outcomes.
> > >> Yuck. I think that means that we can't reallocate freed inodes until
> > >> the rcu grace period expires. For inodes that haven't been evicted, I
> > >> think that also means we cannot recycle cached inodes until after an rcu
> > >> grace period expires; or maybe that we cannot reset i_op/i_fop and must
> > >> not leave the incore state in an inconsistent format?
> > >
> > > Yeah, not pretty!
> > >
> > > But shouldn't this case occur only occasionally?
> > >
> > >
> > > So issuing a cache miss shouldn't impact performance too much that was,
> > >
> > > I believe, the concern with waiting for the rcu grace period.
> > >
> > >
> > > Identifying it's happening should be possible, the vfs legitimize_*()
> > >
> > > has this job for various objects but maybe it's using vfs private info.
> > >
> > > (certainly it uses nameidata struct with a seq lock sequence number in
> > >
> > > it) but I assume it can be done somehow.
> >
> > Unfortunately, when you start trying to work out how to do this, it
> > isn't at all
> >
> > obvious how to do it ...
>
> How about adding a synchronize_rcu() in front of xfs_reinit_inode()?
>
> Maybe this will affect performance, but compared to crashing the kernel, this
> performance penalty is completely worth it.
There is always synchronize_rcu_expedited(), especially if this is a
relatively rare operation. The typical synchronize_rcu() delay is tens
of milliseconds, while the typical synchronize_rcu_expedited() delay is
tens to hundreds of microseconds.
The downside of synchronize_rcu_expedited() is higher per-RCU-update
CPU utilization. Plus added IPIs.
> And, perhaps we can gradually take some optimization measures, such as:
> https://lore.kernel.org/linux-xfs/20220217172518.3842951-2-bfoster@redhat.com/
I cannot claim to fully understand this code, but I do agree that the use
of things like poll_state_synchronize_rcu() and cond_synchronize_rcu()
could greatly reduce the number of grace-period waits. In recent kernels,
there is cond_synchronize_rcu_expedited() as well.
Thanx, Paul
> Best Regards,
> Jinliang Zheng
>
> >
> >
> > >
> > >
> > > My question then becomes is it viable/straight forward to not recycle
> > > such
> > >
> > > an inode and discard it instead so it gets re-created, I guess it's
> > > essentially
> > >
> > > a cache miss?
> > >
> > >
> > > Ian
> > >
> > >>
> > >> --D
> > >>
> > >>>>
> > >>>> The alternative would be to find some way to identify when it's unsafe
> > >>>> to reuse
> > >>>>
> > >>>> an inode marked for re-cycle before dropping rcu read, perhaps with
> > >>>> the
> > >>>> reference
> > >>>>
> > >>>> count plus the seqlock. Basically, to reuse inodes xfs will need to
> > >>>> identify when
> > >>>>
> > >>>> the race occurs and let the inode go away under rcu and create a
> > >>>> new one
> > >>>> if a race
> > >>>>
> > >>>> is detected. But possibly that isn't nearly as simple as it sounds?
> > >>>>
> > >>>>
> > >>>>> Thanks,
> > >>>>> Jinliang Zheng
> > >>>>>
> > >>>>>> And I analyzed the disassembly of trailing_symlink() and
> > >>>>>> confirmed that a NULL
> > >>>>>> ->get_link() happened here:
> > >>>>>>
> > >>>>>> 0xffffffff812e4850 <trailing_symlink>: nopl 0x0(%rax,%rax,1)
> > >>>>>> [FTRACE NOP]
> > >>>>>> 0xffffffff812e4855 <trailing_symlink+0x5>: push %rbp
> > >>>>>> 0xffffffff812e4856 <trailing_symlink+0x6>: mov %rsp,%rbp
> > >>>>>> 0xffffffff812e4859 <trailing_symlink+0x9>: push %r15
> > >>>>>> 0xffffffff812e485b <trailing_symlink+0xb>: push %r14
> > >>>>>> 0xffffffff812e485d <trailing_symlink+0xd>: push %r13
> > >>>>>> 0xffffffff812e485f <trailing_symlink+0xf>: push %r12
> > >>>>>> 0xffffffff812e4861 <trailing_symlink+0x11>: push %rbx
> > >>>>>> 0xffffffff812e4862 <trailing_symlink+0x12>: mov
> > >>>>>> %rdi,%rbx # rbx = &nameidate
> > >>>>>> 0xffffffff812e4865 <trailing_symlink+0x15>: sub $0x8,%rsp
> > >>>>>> 0xffffffff812e4869 <trailing_symlink+0x19>: mov
> > >>>>>> 0x1765845(%rip),%edx # 0xffffffff82a4a0b4
> > >>>>>> <sysctl_protected_symlinks>
> > >>>>>> 0xffffffff812e486f <trailing_symlink+0x1f>: mov 0x38(%rdi),%eax
> > >>>>>> 0xffffffff812e4872 <trailing_symlink+0x22>: test %edx,%edx
> > >>>>>> 0xffffffff812e4874 <trailing_symlink+0x24>: je
> > >>>>>> 0xffffffff812e48ac <trailing_symlink+0x5c>
> > >>>>>> 0xffffffff812e4876 <trailing_symlink+0x26>: mov %gs:0x1ad00,%rdx
> > >>>>>> 0xffffffff812e487f <trailing_symlink+0x2f>: mov
> > >>>>>> 0xc8(%rdi),%rcx # rcx = nameidata->link_inode
> > >>>>>> 0xffffffff812e4886 <trailing_symlink+0x36>: mov 0xc18(%rdx),%rdx
> > >>>>>> 0xffffffff812e488d <trailing_symlink+0x3d>: mov
> > >>>>>> 0x4(%rcx),%ecx # ecx = link_inode->uid
> > >>>>>> 0xffffffff812e4890 <trailing_symlink+0x40>: cmp %ecx,0x1c(%rdx)
> > >>>>>> 0xffffffff812e4893 <trailing_symlink+0x43>: je
> > >>>>>> 0xffffffff812e48ac <trailing_symlink+0x5c>
> > >>>>>> 0xffffffff812e4895 <trailing_symlink+0x45>: mov 0x30(%rdi),%rsi
> > >>>>>> 0xffffffff812e4899 <trailing_symlink+0x49>: movzwl (%rsi),%edx
> > >>>>>> 0xffffffff812e489c <trailing_symlink+0x4c>: and $0x202,%dx
> > >>>>>> 0xffffffff812e48a1 <trailing_symlink+0x51>: cmp $0x202,%dx
> > >>>>>> 0xffffffff812e48a6 <trailing_symlink+0x56>: je
> > >>>>>> 0xffffffff812e495f <trailing_symlink+0x10f>
> > >>>>>> 0xffffffff812e48ac <trailing_symlink+0x5c>: or $0x10,%eax
> > >>>>>> 0xffffffff812e48af <trailing_symlink+0x5f>: mov
> > >>>>>> %eax,0x38(%rbx) # nd->flags |= LOOKUP_PARENT
> > >>>>>> 0xffffffff812e48b2 <trailing_symlink+0x62>: mov
> > >>>>>> 0x50(%rbx),%rax # rax = nd->stack
> > >>>>>> 0xffffffff812e48b6 <trailing_symlink+0x66>: movq
> > >>>>>> $0x0,0x20(%rax) # stack[0].name = NULL
> > >>>>>> 0xffffffff812e48be <trailing_symlink+0x6e>: mov
> > >>>>>> 0x48(%rbx),%eax # nd->depth
> > >>>>>> 0xffffffff812e48c1 <trailing_symlink+0x71>: mov
> > >>>>>> 0x50(%rbx),%rdx # nd->stack
> > >>>>>> 0xffffffff812e48c5 <trailing_symlink+0x75>: mov
> > >>>>>> 0xc8(%rbx),%r13 # nd->link_inode
> > >>>>>> 0xffffffff812e48cc <trailing_symlink+0x7c>: lea
> > >>>>>> (%rax,%rax,2),%rax # rax = depth * 3
> > >>>>>> 0xffffffff812e48d0 <trailing_symlink+0x80>: shl
> > >>>>>> $0x4,%rax # rax = rax << 4, sizeof(saved):0x30
> > >>>>>> 0xffffffff812e48d4 <trailing_symlink+0x84>: lea
> > >>>>>> -0x30(%rdx,%rax,1),%r15 # r15 = last
> > >>>>>> 0xffffffff812e48d9 <trailing_symlink+0x89>: mov
> > >>>>>> 0x8(%r15),%r14 # r14 = last->link.dentry
> > >>>>>> 0xffffffff812e48dd <trailing_symlink+0x8d>: testb $0x40,0x38(%rbx)
> > >>>>>> 0xffffffff812e48e1 <trailing_symlink+0x91>: je
> > >>>>>> 0xffffffff812e4950 <trailing_symlink+0x100>
> > >>>>>> 0xffffffff812e48e3 <trailing_symlink+0x93>: mov %r13,%rsi
> > >>>>>> 0xffffffff812e48e6 <trailing_symlink+0x96>: mov %r15,%rdi
> > >>>>>> 0xffffffff812e48e9 <trailing_symlink+0x99>: callq
> > >>>>>> 0xffffffff812f8a00 <atime_needs_update>
> > >>>>>> 0xffffffff812e48ee <trailing_symlink+0x9e>: test %al,%al
> > >>>>>> 0xffffffff812e48f0 <trailing_symlink+0xa0>: jne
> > >>>>>> 0xffffffff812e4a56 <trailing_symlink+0x206>
> > >>>>>> 0xffffffff812e48f6 <trailing_symlink+0xa6>: mov 0x38(%rbx),%edx
> > >>>>>> 0xffffffff812e48f9 <trailing_symlink+0xa9>: mov %r13,%rsi
> > >>>>>> 0xffffffff812e48fc <trailing_symlink+0xac>: mov %r14,%rdi
> > >>>>>> 0xffffffff812e48ff <trailing_symlink+0xaf>: shr $0x6,%edx
> > >>>>>> 0xffffffff812e4902 <trailing_symlink+0xb2>: and $0x1,%edx
> > >>>>>> 0xffffffff812e4905 <trailing_symlink+0xb5>: callq
> > >>>>>> 0xffffffff81424310 <security_inode_follow_link>
> > >>>>>> 0xffffffff812e490a <trailing_symlink+0xba>: movslq %eax,%r12
> > >>>>>> 0xffffffff812e490d <trailing_symlink+0xbd>: test %eax,%eax
> > >>>>>> 0xffffffff812e490f <trailing_symlink+0xbf>: jne
> > >>>>>> 0xffffffff812e4939 <trailing_symlink+0xe9>
> > >>>>>> 0xffffffff812e4911 <trailing_symlink+0xc1>: movl $0x4,0x44(%rbx)
> > >>>>>> 0xffffffff812e4918 <trailing_symlink+0xc8>: mov 0x248(%r13),%r12
> > >>>>>> 0xffffffff812e491f <trailing_symlink+0xcf>: test %r12,%r12
> > >>>>>> 0xffffffff812e4922 <trailing_symlink+0xd2>: je
> > >>>>>> 0xffffffff812e49e5 <trailing_symlink+0x195>
> > >>>>>> 0xffffffff812e4928 <trailing_symlink+0xd8>: movzbl (%r12),%eax
> > >>>>>> 0xffffffff812e492d <trailing_symlink+0xdd>: cmp $0x2f,%al
> > >>>>>> 0xffffffff812e492f <trailing_symlink+0xdf>: je
> > >>>>>> 0xffffffff812e49b7 <trailing_symlink+0x167>
> > >>>>>> 0xffffffff812e4935 <trailing_symlink+0xe5>: test %al,%al
> > >>>>>> 0xffffffff812e4937 <trailing_symlink+0xe7>: je
> > >>>>>> 0xffffffff812e49ae <trailing_symlink+0x15e>
> > >>>>>> 0xffffffff812e4939 <trailing_symlink+0xe9>: test %r12,%r12
> > >>>>>> 0xffffffff812e493c <trailing_symlink+0xec>: je
> > >>>>>> 0xffffffff812e49ae <trailing_symlink+0x15e>
> > >>>>>> 0xffffffff812e493e <trailing_symlink+0xee>: add $0x8,%rsp
> > >>>>>> 0xffffffff812e4942 <trailing_symlink+0xf2>: mov %r12,%rax
> > >>>>>> 0xffffffff812e4945 <trailing_symlink+0xf5>: pop %rbx
> > >>>>>> 0xffffffff812e4946 <trailing_symlink+0xf6>: pop %r12
> > >>>>>> 0xffffffff812e4948 <trailing_symlink+0xf8>: pop %r13
> > >>>>>> 0xffffffff812e494a <trailing_symlink+0xfa>: pop %r14
> > >>>>>> 0xffffffff812e494c <trailing_symlink+0xfc>: pop %r15
> > >>>>>> 0xffffffff812e494e <trailing_symlink+0xfe>: pop %rbp
> > >>>>>> 0xffffffff812e494f <trailing_symlink+0xff>: retq
> > >>>>>> 0xffffffff812e4950 <trailing_symlink+0x100>: mov %r15,%rdi
> > >>>>>> 0xffffffff812e4953 <trailing_symlink+0x103>: callq
> > >>>>>> 0xffffffff812f8ae0 <touch_atime>
> > >>>>>> 0xffffffff812e4958 <trailing_symlink+0x108>: callq
> > >>>>>> 0xffffffff81a26410 <_cond_resched>
> > >>>>>> 0xffffffff812e495d <trailing_symlink+0x10d>: jmp
> > >>>>>> 0xffffffff812e48f6 <trailing_symlink+0xa6>
> > >>>>>> 0xffffffff812e495f <trailing_symlink+0x10f>: mov 0x4(%rsi),%edx
> > >>>>>> 0xffffffff812e4962 <trailing_symlink+0x112>: cmp $0xffffffff,%edx
> > >>>>>> 0xffffffff812e4965 <trailing_symlink+0x115>: je
> > >>>>>> 0xffffffff812e496f <trailing_symlink+0x11f>
> > >>>>>> 0xffffffff812e4967 <trailing_symlink+0x117>: cmp %edx,%ecx
> > >>>>>> 0xffffffff812e4969 <trailing_symlink+0x119>: je
> > >>>>>> 0xffffffff812e48ac <trailing_symlink+0x5c>
> > >>>>>> 0xffffffff812e496f <trailing_symlink+0x11f>: mov
> > >>>>>> $0xfffffffffffffff6,%r12
> > >>>>>> 0xffffffff812e4976 <trailing_symlink+0x126>: test $0x40,%al
> > >>>>>> 0xffffffff812e4978 <trailing_symlink+0x128>: jne
> > >>>>>> 0xffffffff812e493e <trailing_symlink+0xee>
> > >>>>>> 0xffffffff812e497a <trailing_symlink+0x12a>: mov %gs:0x1ad00,%rax
> > >>>>>> 0xffffffff812e4983 <trailing_symlink+0x133>: mov 0xce0(%rax),%rax
> > >>>>>> 0xffffffff812e498a <trailing_symlink+0x13a>: test %rax,%rax
> > >>>>>> 0xffffffff812e498d <trailing_symlink+0x13d>: je
> > >>>>>> 0xffffffff812e4999 <trailing_symlink+0x149>
> > >>>>>> 0xffffffff812e498f <trailing_symlink+0x13f>: mov (%rax),%eax
> > >>>>>> 0xffffffff812e4991 <trailing_symlink+0x141>: test %eax,%eax
> > >>>>>> 0xffffffff812e4993 <trailing_symlink+0x143>: je
> > >>>>>> 0xffffffff812e4a6f <trailing_symlink+0x21f>
> > >>>>>> 0xffffffff812e4999 <trailing_symlink+0x149>: mov
> > >>>>>> $0xffffffff82319b4f,%rdi
> > >>>>>> 0xffffffff812e49a0 <trailing_symlink+0x150>: mov
> > >>>>>> $0xfffffffffffffff3,%r12
> > >>>>>> 0xffffffff812e49a7 <trailing_symlink+0x157>: callq
> > >>>>>> 0xffffffff81161310 <audit_log_link_denied>
> > >>>>>> 0xffffffff812e49ac <trailing_symlink+0x15c>: jmp
> > >>>>>> 0xffffffff812e493e <trailing_symlink+0xee>
> > >>>>>> 0xffffffff812e49ae <trailing_symlink+0x15e>: mov
> > >>>>>> $0xffffffff8230164d,%r12
> > >>>>>> 0xffffffff812e49b5 <trailing_symlink+0x165>: jmp
> > >>>>>> 0xffffffff812e493e <trailing_symlink+0xee>
> > >>>>>> 0xffffffff812e49b7 <trailing_symlink+0x167>: cmpq $0x0,0x20(%rbx)
> > >>>>>> 0xffffffff812e49bc <trailing_symlink+0x16c>: je
> > >>>>>> 0xffffffff812e4a8a <trailing_symlink+0x23a>
> > >>>>>> 0xffffffff812e49c2 <trailing_symlink+0x172>: mov %rbx,%rdi
> > >>>>>> 0xffffffff812e49c5 <trailing_symlink+0x175>: callq
> > >>>>>> 0xffffffff812e2da0 <nd_jump_root>
> > >>>>>> 0xffffffff812e49ca <trailing_symlink+0x17a>: test %eax,%eax
> > >>>>>> 0xffffffff812e49cc <trailing_symlink+0x17c>: jne
> > >>>>>> 0xffffffff812e4a97 <trailing_symlink+0x247>
> > >>>>>> 0xffffffff812e49d2 <trailing_symlink+0x182>: add $0x1,%r12
> > >>>>>> 0xffffffff812e49d6 <trailing_symlink+0x186>: movzbl (%r12),%eax
> > >>>>>> 0xffffffff812e49db <trailing_symlink+0x18b>: cmp $0x2f,%al
> > >>>>>> 0xffffffff812e49dd <trailing_symlink+0x18d>: jne
> > >>>>>> 0xffffffff812e4935 <trailing_symlink+0xe5>
> > >>>>>> 0xffffffff812e49e3 <trailing_symlink+0x193>: jmp
> > >>>>>> 0xffffffff812e49d2 <trailing_symlink+0x182>
> > >>>>>> 0xffffffff812e49e5 <trailing_symlink+0x195>: mov
> > >>>>>> 0x20(%r13),%rax # inode->i_op
> > >>>>>> 0xffffffff812e49e9 <trailing_symlink+0x199>: add $0x10,%r15
> > >>>>>> 0xffffffff812e49ed <trailing_symlink+0x19d>: mov %r13,%rsi
> > >>>>>> 0xffffffff812e49f0 <trailing_symlink+0x1a0>: mov %r15,%rdx
> > >>>>>> 0xffffffff812e49f3 <trailing_symlink+0x1a3>: mov
> > >>>>>> 0x8(%rax),%rcx # inode_operations->get_link
> > >>>>>> 0xffffffff812e49f7 <trailing_symlink+0x1a7>: testb $0x40,0x38(%rbx)
> > >>>>>> 0xffffffff812e49fb <trailing_symlink+0x1ab>: jne
> > >>>>>> 0xffffffff812e4a1f <trailing_symlink+0x1cf>
> > >>>>>> 0xffffffff812e49fd <trailing_symlink+0x1ad>: mov
> > >>>>>> %r14,%rdi # nd->flags & LOOKUP_RCU == 0
> > >>>>>> 0xffffffff812e4a00 <trailing_symlink+0x1b0>: callq
> > >>>>>> 0xffffffff81e00f70 <__x86_indirect_thunk_rcx> # jmpq *%rcx
> > >>>>>> 0xffffffff812e4a05 <trailing_symlink+0x1b5>: mov %rax,%r12
> > >>>>>> 0xffffffff812e4a08 <trailing_symlink+0x1b8>: test %r12,%r12
> > >>>>>> 0xffffffff812e4a0b <trailing_symlink+0x1bb>: je
> > >>>>>> 0xffffffff812e49ae <trailing_symlink+0x15e>
> > >>>>>> 0xffffffff812e4a0d <trailing_symlink+0x1bd>: cmp
> > >>>>>> $0xfffffffffffff000,%r12
> > >>>>>> 0xffffffff812e4a14 <trailing_symlink+0x1c4>: jbe
> > >>>>>> 0xffffffff812e4928 <trailing_symlink+0xd8>
> > >>>>>> 0xffffffff812e4a1a <trailing_symlink+0x1ca>: jmpq
> > >>>>>> 0xffffffff812e493e <trailing_symlink+0xee>
> > >>>>>> 0xffffffff812e4a1f <trailing_symlink+0x1cf>: xor
> > >>>>>> %edi,%edi # nd->flags & LOOKUP_RCU != 0
> > >>>>>> 0xffffffff812e4a21 <trailing_symlink+0x1d1>: mov %rcx,-0x30(%rbp)
> > >>>>>> 0xffffffff812e4a25 <trailing_symlink+0x1d5>: callq
> > >>>>>> 0xffffffff81e00f70 <__x86_indirect_thunk_rcx> # jmpq *%rcx
> > >>>>>> 0xffffffff812e4a2a <trailing_symlink+0x1da>: mov %rax,%r12
> > >>>>>> 0xffffffff812e4a2d <trailing_symlink+0x1dd>: cmp
> > >>>>>> $0xfffffffffffffff6,%rax
> > >>>>>> 0xffffffff812e4a31 <trailing_symlink+0x1e1>: jne
> > >>>>>> 0xffffffff812e4a08 <trailing_symlink+0x1b8>
> > >>>>>> 0xffffffff812e4a33 <trailing_symlink+0x1e3>: mov %rbx,%rdi
> > >>>>>> 0xffffffff812e4a36 <trailing_symlink+0x1e6>: callq
> > >>>>>> 0xffffffff812e3840 <unlazy_walk>
> > >>>>>> 0xffffffff812e4a3b <trailing_symlink+0x1eb>: test %eax,%eax
> > >>>>>> 0xffffffff812e4a3d <trailing_symlink+0x1ed>: jne
> > >>>>>> 0xffffffff812e4a97 <trailing_symlink+0x247>
> > >>>>>> 0xffffffff812e4a3f <trailing_symlink+0x1ef>: mov %r15,%rdx
> > >>>>>> 0xffffffff812e4a42 <trailing_symlink+0x1f2>: mov %r13,%rsi
> > >>>>>> 0xffffffff812e4a45 <trailing_symlink+0x1f5>: mov %r14,%rdi
> > >>>>>> 0xffffffff812e4a48 <trailing_symlink+0x1f8>: mov -0x30(%rbp),%rcx
> > >>>>>> 0xffffffff812e4a4c <trailing_symlink+0x1fc>: callq
> > >>>>>> 0xffffffff81e00f70 <__x86_indirect_thunk_rcx>
> > >>>>>> 0xffffffff812e4a51 <trailing_symlink+0x201>: mov %rax,%r12
> > >>>>>> 0xffffffff812e4a54 <trailing_symlink+0x204>: jmp
> > >>>>>> 0xffffffff812e4a08 <trailing_symlink+0x1b8>
> > >>>>>> 0xffffffff812e4a56 <trailing_symlink+0x206>: mov %rbx,%rdi
> > >>>>>> 0xffffffff812e4a59 <trailing_symlink+0x209>: callq
> > >>>>>> 0xffffffff812e3840 <unlazy_walk>
> > >>>>>> 0xffffffff812e4a5e <trailing_symlink+0x20e>: test %eax,%eax
> > >>>>>> 0xffffffff812e4a60 <trailing_symlink+0x210>: jne
> > >>>>>> 0xffffffff812e4a97 <trailing_symlink+0x247>
> > >>>>>> 0xffffffff812e4a62 <trailing_symlink+0x212>: mov %r15,%rdi
> > >>>>>> 0xffffffff812e4a65 <trailing_symlink+0x215>: callq
> > >>>>>> 0xffffffff812f8ae0 <touch_atime>
> > >>>>>> 0xffffffff812e4a6a <trailing_symlink+0x21a>: jmpq
> > >>>>>> 0xffffffff812e48f6 <trailing_symlink+0xa6>
> > >>>>>> 0xffffffff812e4a6f <trailing_symlink+0x21f>: mov 0x50(%rbx),%rax
> > >>>>>> 0xffffffff812e4a73 <trailing_symlink+0x223>: mov 0xb8(%rbx),%rdi
> > >>>>>> 0xffffffff812e4a7a <trailing_symlink+0x22a>: xor %edx,%edx
> > >>>>>> 0xffffffff812e4a7c <trailing_symlink+0x22c>: mov 0x8(%rax),%rsi
> > >>>>>> 0xffffffff812e4a80 <trailing_symlink+0x230>: callq
> > >>>>>> 0xffffffff811673f0 <__audit_inode>
> > >>>>>> 0xffffffff812e4a85 <trailing_symlink+0x235>: jmpq
> > >>>>>> 0xffffffff812e4999 <trailing_symlink+0x149>
> > >>>>>> 0xffffffff812e4a8a <trailing_symlink+0x23a>: mov %rbx,%rdi
> > >>>>>> 0xffffffff812e4a8d <trailing_symlink+0x23d>: callq
> > >>>>>> 0xffffffff812e4790 <set_root>
> > >>>>>> 0xffffffff812e4a92 <trailing_symlink+0x242>: jmpq
> > >>>>>> 0xffffffff812e49c2 <trailing_symlink+0x172>
> > >>>>>> 0xffffffff812e4a97 <trailing_symlink+0x247>: mov
> > >>>>>> $0xfffffffffffffff6,%r12
> > >>>>>> 0xffffffff812e4a9e <trailing_symlink+0x24e>: jmpq
> > >>>>>> 0xffffffff812e493e <trailing_symlink+0xee>
> > >>>>>>
> > >>>>>> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>> According to my understanding, the problem solved by commit
> > >>>>>> 7b7820b83f23 ("xfs:
> > >>>>>> don't expose internal symlink metadata buffers to the vfs") is a
> > >>>>>> data NULL
> > >>>>>> pointer dereference, but the problem here is an instruction NULL
> > >>>>>> pointer
> > >>>>>> dereference.
> > >>>>>>
> > >>>>>> Further, I analyzed the possible triggering process as follows:
> > >>>>>>
> > >>>>>> rcu_walk do_unlinkat ~~> prune_dcache_sb create
> > >>>>>> rcu_read_lock
> > >>>>>> read_seqcount_retry
> > >>>>>> (the last check) iput_final
> > >>>>>> evict
> > >>>>>> destroy_inode
> > >>>>>> xfs_fs_destroy_inode
> > >>>>>> xfs_inode_set_reclaim_tag xfs_ialloc
> > >>>>>> spin_lock(ip->i_flags_lock) xfs_dialloc
> > >>>>>> set(ip, XFS_IRECLAIMABLE)
> > >>>>>> xfs_iget
> > >>>>>> wakeup(xfs_reclaim_worker) rcu_read_lock
> > >>>>>> spin_unlock(ip->i_flags_lock) xfs_iget_cache_hit
> > >>>>>> spin_lock(ip->i_flags_lock)
> > >>>>>>
> > >>>>>> if (XFS_IRECLAIMABLE && !XFS_IRECLAIM)
> > >>>>>> set(ip, XFS_IRECLAIM)
> > >>>>>> spin_unlock(ip->i_flags_lock)
> > >>>>>> rcu_read_unlock
> > >>>>>> < ------------ >
> > >>>>>>
> > >>>>>> // miss synchronize_rcu()
> > >>>>>> xfs_reinit_inode
> > >>>>>> ->get_link = NULL
> > >>>>>> get_link() // NULL
> > >>>>>>
> > >>>>>> rcu_read_unlock
> > >>>>>>
> > >>>>>> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
> > >>>>>>
> > >>>>>>
> > >>>>>>
> > >>>>>> Therefore, I think that after commit 7b7820b83f23 ("xfs: don't
> > >>>>>> expose internal
> > >>>>>> symlink metadata buffers to the vfs"), we should start
> > >>>>>> processing this NULL
> > >>>>>> ->get_link pointer dereference.
> > >>>>>>
> > >>>>>> Or, am I thinking wrong somewhere?
> > >>>>>>
> > >>>>>> Thanks,
> > >>>>>> Jinliang Zheng
> > >>>>>>
> > >>>>>>>>> Apart from that issue, I'm not aware of any other issues that the
> > >>>>>>>>> XFS inode recycling directly exposes.
> > >>>>>>>>>
> > >>>>>>>>>> According to my understanding, the essence of
> > >>>>>>>>>> this problem is that XFS reuses
> > >>>>>>>>>> the inode evicted by VFS, but VFS rcu-walk
> > >>>>>>>>>> assumes that this will not happen.
> > >>>>>>>>> It assumes that the inode will not change identity during the RCU
> > >>>>>>>>> grace period after the inode has been evicted from cache. We can
> > >>>>>>>>> safely reinstantiate an evicted inode without waiting for an RCU
> > >>>>>>>>> grace period as long as it is the same inode with the same
> > >>>>>>>>> content
> > >>>>>>>>> and same state.
> > >>>>>>>>>
> > >>>>>>>>> Problems *may* arise when we unlink the inode, then evict it,
> > >>>>>>>>> then a
> > >>>>>>>>> new file is created and the old slab cache memory address is used
> > >>>>>>>>> for the new inode. I describe the issue here:
> > >>>>>>>>>
> > >>>>>>>>> https://lore.kernel.org/linux-xfs/20220118232547.GD59729@dread.disaster.area/
> > >>>>>>>>>
> > >>>>>>>>>
> > >>>>>>>> And judging from the relevant emails, the main reason
> > >>>>>>>> why ->get_link() is set
> > >>>>>>>> to NULL should be the lack of synchronize_rcu() before
> > >>>>>>>> xfs_reinit_inode() when
> > >>>>>>>> the inode is chosen to be reused.
> > >>>>>>>>
> > >>>>>>>> However, perhaps due to performance reasons, this
> > >>>>>>>> solution has not been merged
> > >>>>>>>> for a long time. How is it now?
> > >>>>>>>>
> > >>>>>>>> Maybe I am missing something in the threads of mail?
> > >>>>>>>>
> > >>>>>>>> Thank you very much. :)
> > >>>>>>>> Jinliang Zheng
> > >>>>>>>>
> > >>>>>>>>> That said, we have exactly zero evidence that this is actually a
> > >>>>>>>>> problem in production systems. We did get systems tripping
> > >>>>>>>>> over the
> > >>>>>>>>> symlink issue, but there's no evidence that the
> > >>>>>>>>> unlink->close->open(O_CREAT) issues are manifesting in the
> > >>>>>>>>> wild and
> > >>>>>>>>> hence there hasn't been any particular urgency to address it.
> > >>>>>>>>>
> > >>>>>>>>>> Are there any recommended workarounds until an
> > >>>>>>>>>> elegant and efficient solution
> > >>>>>>>>>> can be proposed? After all, causing a crash is
> > >>>>>>>>>> extremely unacceptable in a
> > >>>>>>>>>> production environment.
> > >>>>>>>>> What crashes are you seeing in your production environment?
> > >>>>>>>>>
> > >>>>>>>>> -Dave.
> > >>>>>>>>> --
> > >>>>>>>>> Dave Chinner
> > >>>>>>>>> david@fromorbit.com
> > >
>
next prev parent reply other threads:[~2024-05-26 17:21 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-05 11:38 About the conflict between XFS inode recycle and VFS rcu-walk alexjlzheng
2023-12-08 0:14 ` Dave Chinner
2024-01-31 6:35 ` Jinliang Zheng
2024-01-31 19:30 ` Darrick J. Wong
2024-05-15 15:54 ` alexjlzheng
2024-05-16 4:56 ` Jinliang Zheng
2024-05-16 7:08 ` Ian Kent
2024-05-16 7:23 ` Ian Kent
2024-05-20 17:36 ` Darrick J. Wong
2024-05-21 1:35 ` Ian Kent
2024-05-21 2:13 ` Ian Kent
2024-05-26 15:04 ` Jinliang Zheng
2024-05-26 17:21 ` Paul E. McKenney [this message]
2024-05-26 23:51 ` Ian Kent
2024-05-27 0:18 ` Al Viro
2024-05-28 15:51 ` Brian Foster
2024-05-27 9:41 ` Dave Chinner
2024-05-27 13:56 ` Jinliang Zheng
2024-05-28 2:10 ` Dave Chinner
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