Re: About the conflict between XFS inode recycle and VFS rcu-walk

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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@xxxxxxxxxx wrote:
> >>>>>>> On Wed, Jan 31, 2024 at 02:35:17PM +0800, Jinliang Zheng wrote:
> >>>>>>>> On Fri, 8 Dec 2023 11:14:32 +1100, david@xxxxxxxxxxxxx wrote:
> >>>>>>>>> On Tue, Dec 05, 2023 at 07:38:33PM +0800,
> >>>>>>>>> alexjlzheng@xxxxxxxxx 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@xxxxxxxxxx/
> >>>>>>>>>> - 
> >>>>>>>>>> https://lore.kernel.org/linux-xfs/20220121142454.1994916-1-bfoster@xxxxxxxxxx/
> >>>>>>>>>> - 
> >>>>>>>>>> https://lore.kernel.org/linux-xfs/164180589176.86426.501271559065590169.stgit@xxxxxxxxxxxxxxxxx/
> >>>>>>>>>>
> >>>>>>>>>> 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.

And, perhaps we can gradually take some optimization measures, such as:
https://lore.kernel.org/linux-xfs/20220217172518.3842951-2-bfoster@xxxxxxxxxx/

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@xxxxxxxxxxxxxxxxxxx/ 
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>> 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@xxxxxxxxxxxxx
> >




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