On Wed, 31 Jan 2024 at 11:30:18 -0800, [email protected] wrote:
> On Wed, Jan 31, 2024 at 02:35:17PM +0800, Jinliang Zheng wrote:
> > On Fri, 8 Dec 2023 11:14:32 +1100, [email protected] wrote:
> > > On Tue, Dec 05, 2023 at 07:38:33PM +0800, [email protected] 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/[email protected]/
> > > > -
> > > > https://lore.kernel.org/linux-xfs/[email protected]/
> > > > -
> > > > https://lore.kernel.org/linux-xfs/[email protected]/
> > > >
> > > > 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
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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/[email protected]/
> >
> > 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
> > > [email protected]
> >
