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AJvYcCV5k5bVj9XMR/t3+XgMxwt84beRjsyEPzJygJj5bdy8zwGAZM3nzY2ChfW7fBwg3Iz2C0uRWmzmKLYjK7YoDtw7llqHKVfR454j6jfyWZT/m9UyDA6kpdnCn8zMvtaIT+wY6vGG33Hqe3IbajweGpqyHCiyW0E6CTscFe4HGQ== X-Gm-Message-State: AOJu0YwWj8m98hhhLK+LCxOesKiwvX3+nOtTtITqWd7dtoBEbpDWU+Z1 x7rQmp4cbyPA8j1cRovQLLFwVq8hZaPEof9Q9hE1nc/M10/LrAjI X-Google-Smtp-Source: AGHT+IF+YaN42BK7qUnunaUbmFHnH55q//WAv3SOCm29U6Cb3eT3kf9wWosy7K2xauftoc2363eG+A== X-Received: by 2002:a05:6a21:7895:b0:1a7:a72c:6f4 with SMTP id adf61e73a8af0-1afde1b0156mr19692412637.41.1715835420669; Wed, 15 May 2024 21:57:00 -0700 (PDT) Received: from localhost.localdomain ([14.22.11.165]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1ef0b9d171esm127932715ad.55.2024.05.15.21.56.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 May 2024 21:57:00 -0700 (PDT) From: Jinliang Zheng X-Google-Original-From: Jinliang Zheng To: alexjlzheng@tencent.com Cc: bfoster@redhat.com, david@fromorbit.com, djwong@kernel.org, linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, raven@themaw.net, rcu@vger.kernel.org Subject: Re: About the conflict between XFS inode recycle and VFS rcu-walk Date: Thu, 16 May 2024 12:56:55 +0800 Message-Id: <20240516045655.40122-1-alexjlzheng@tencent.com> X-Mailer: git-send-email 2.39.3 In-Reply-To: <20240515155441.2788093-1-alexjlzheng@tencent.com> References: <20240515155441.2788093-1-alexjlzheng@tencent.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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.) Thanks, Jinliang Zheng > > And I analyzed the disassembly of trailing_symlink() and confirmed that a NULL > ->get_link() happened here: > > >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> > > 0xffffffff812e4850 : nopl 0x0(%rax,%rax,1) [FTRACE NOP] > 0xffffffff812e4855 : push %rbp > 0xffffffff812e4856 : mov %rsp,%rbp > 0xffffffff812e4859 : push %r15 > 0xffffffff812e485b : push %r14 > 0xffffffff812e485d : push %r13 > 0xffffffff812e485f : push %r12 > 0xffffffff812e4861 : push %rbx > 0xffffffff812e4862 : mov %rdi,%rbx # rbx = &nameidate > 0xffffffff812e4865 : sub $0x8,%rsp > 0xffffffff812e4869 : mov 0x1765845(%rip),%edx # 0xffffffff82a4a0b4 > 0xffffffff812e486f : mov 0x38(%rdi),%eax > 0xffffffff812e4872 : test %edx,%edx > 0xffffffff812e4874 : je 0xffffffff812e48ac > 0xffffffff812e4876 : mov %gs:0x1ad00,%rdx > 0xffffffff812e487f : mov 0xc8(%rdi),%rcx # rcx = nameidata->link_inode > 0xffffffff812e4886 : mov 0xc18(%rdx),%rdx > 0xffffffff812e488d : mov 0x4(%rcx),%ecx # ecx = link_inode->uid > 0xffffffff812e4890 : cmp %ecx,0x1c(%rdx) > 0xffffffff812e4893 : je 0xffffffff812e48ac > 0xffffffff812e4895 : mov 0x30(%rdi),%rsi > 0xffffffff812e4899 : movzwl (%rsi),%edx > 0xffffffff812e489c : and $0x202,%dx > 0xffffffff812e48a1 : cmp $0x202,%dx > 0xffffffff812e48a6 : je 0xffffffff812e495f > 0xffffffff812e48ac : or $0x10,%eax > 0xffffffff812e48af : mov %eax,0x38(%rbx) # nd->flags |= LOOKUP_PARENT > 0xffffffff812e48b2 : mov 0x50(%rbx),%rax # rax = nd->stack > 0xffffffff812e48b6 : movq $0x0,0x20(%rax) # stack[0].name = NULL > 0xffffffff812e48be : mov 0x48(%rbx),%eax # nd->depth > 0xffffffff812e48c1 : mov 0x50(%rbx),%rdx # nd->stack > 0xffffffff812e48c5 : mov 0xc8(%rbx),%r13 # nd->link_inode > 0xffffffff812e48cc : lea (%rax,%rax,2),%rax # rax = depth * 3 > 0xffffffff812e48d0 : shl $0x4,%rax # rax = rax << 4, sizeof(saved):0x30 > 0xffffffff812e48d4 : lea -0x30(%rdx,%rax,1),%r15 # r15 = last > 0xffffffff812e48d9 : mov 0x8(%r15),%r14 # r14 = last->link.dentry > 0xffffffff812e48dd : testb $0x40,0x38(%rbx) > 0xffffffff812e48e1 : je 0xffffffff812e4950 > 0xffffffff812e48e3 : mov %r13,%rsi > 0xffffffff812e48e6 : mov %r15,%rdi > 0xffffffff812e48e9 : callq 0xffffffff812f8a00 > 0xffffffff812e48ee : test %al,%al > 0xffffffff812e48f0 : jne 0xffffffff812e4a56 > 0xffffffff812e48f6 : mov 0x38(%rbx),%edx > 0xffffffff812e48f9 : mov %r13,%rsi > 0xffffffff812e48fc : mov %r14,%rdi > 0xffffffff812e48ff : shr $0x6,%edx > 0xffffffff812e4902 : and $0x1,%edx > 0xffffffff812e4905 : callq 0xffffffff81424310 > 0xffffffff812e490a : movslq %eax,%r12 > 0xffffffff812e490d : test %eax,%eax > 0xffffffff812e490f : jne 0xffffffff812e4939 > 0xffffffff812e4911 : movl $0x4,0x44(%rbx) > 0xffffffff812e4918 : mov 0x248(%r13),%r12 > 0xffffffff812e491f : test %r12,%r12 > 0xffffffff812e4922 : je 0xffffffff812e49e5 > 0xffffffff812e4928 : movzbl (%r12),%eax > 0xffffffff812e492d : cmp $0x2f,%al > 0xffffffff812e492f : je 0xffffffff812e49b7 > 0xffffffff812e4935 : test %al,%al > 0xffffffff812e4937 : je 0xffffffff812e49ae > 0xffffffff812e4939 : test %r12,%r12 > 0xffffffff812e493c : je 0xffffffff812e49ae > 0xffffffff812e493e : add $0x8,%rsp > 0xffffffff812e4942 : mov %r12,%rax > 0xffffffff812e4945 : pop %rbx > 0xffffffff812e4946 : pop %r12 > 0xffffffff812e4948 : pop %r13 > 0xffffffff812e494a : pop %r14 > 0xffffffff812e494c : pop %r15 > 0xffffffff812e494e : pop %rbp > 0xffffffff812e494f : retq > 0xffffffff812e4950 : mov %r15,%rdi > 0xffffffff812e4953 : callq 0xffffffff812f8ae0 > 0xffffffff812e4958 : callq 0xffffffff81a26410 <_cond_resched> > 0xffffffff812e495d : jmp 0xffffffff812e48f6 > 0xffffffff812e495f : mov 0x4(%rsi),%edx > 0xffffffff812e4962 : cmp $0xffffffff,%edx > 0xffffffff812e4965 : je 0xffffffff812e496f > 0xffffffff812e4967 : cmp %edx,%ecx > 0xffffffff812e4969 : je 0xffffffff812e48ac > 0xffffffff812e496f : mov $0xfffffffffffffff6,%r12 > 0xffffffff812e4976 : test $0x40,%al > 0xffffffff812e4978 : jne 0xffffffff812e493e > 0xffffffff812e497a : mov %gs:0x1ad00,%rax > 0xffffffff812e4983 : mov 0xce0(%rax),%rax > 0xffffffff812e498a : test %rax,%rax > 0xffffffff812e498d : je 0xffffffff812e4999 > 0xffffffff812e498f : mov (%rax),%eax > 0xffffffff812e4991 : test %eax,%eax > 0xffffffff812e4993 : je 0xffffffff812e4a6f > 0xffffffff812e4999 : mov $0xffffffff82319b4f,%rdi > 0xffffffff812e49a0 : mov $0xfffffffffffffff3,%r12 > 0xffffffff812e49a7 : callq 0xffffffff81161310 > 0xffffffff812e49ac : jmp 0xffffffff812e493e > 0xffffffff812e49ae : mov $0xffffffff8230164d,%r12 > 0xffffffff812e49b5 : jmp 0xffffffff812e493e > 0xffffffff812e49b7 : cmpq $0x0,0x20(%rbx) > 0xffffffff812e49bc : je 0xffffffff812e4a8a > 0xffffffff812e49c2 : mov %rbx,%rdi > 0xffffffff812e49c5 : callq 0xffffffff812e2da0 > 0xffffffff812e49ca : test %eax,%eax > 0xffffffff812e49cc : jne 0xffffffff812e4a97 > 0xffffffff812e49d2 : add $0x1,%r12 > 0xffffffff812e49d6 : movzbl (%r12),%eax > 0xffffffff812e49db : cmp $0x2f,%al > 0xffffffff812e49dd : jne 0xffffffff812e4935 > 0xffffffff812e49e3 : jmp 0xffffffff812e49d2 > 0xffffffff812e49e5 : mov 0x20(%r13),%rax # inode->i_op > 0xffffffff812e49e9 : add $0x10,%r15 > 0xffffffff812e49ed : mov %r13,%rsi > 0xffffffff812e49f0 : mov %r15,%rdx > 0xffffffff812e49f3 : mov 0x8(%rax),%rcx # inode_operations->get_link > 0xffffffff812e49f7 : testb $0x40,0x38(%rbx) > 0xffffffff812e49fb : jne 0xffffffff812e4a1f > 0xffffffff812e49fd : mov %r14,%rdi # nd->flags & LOOKUP_RCU == 0 > 0xffffffff812e4a00 : callq 0xffffffff81e00f70 <__x86_indirect_thunk_rcx> # jmpq *%rcx > 0xffffffff812e4a05 : mov %rax,%r12 > 0xffffffff812e4a08 : test %r12,%r12 > 0xffffffff812e4a0b : je 0xffffffff812e49ae > 0xffffffff812e4a0d : cmp $0xfffffffffffff000,%r12 > 0xffffffff812e4a14 : jbe 0xffffffff812e4928 > 0xffffffff812e4a1a : jmpq 0xffffffff812e493e > 0xffffffff812e4a1f : xor %edi,%edi # nd->flags & LOOKUP_RCU != 0 > 0xffffffff812e4a21 : mov %rcx,-0x30(%rbp) > 0xffffffff812e4a25 : callq 0xffffffff81e00f70 <__x86_indirect_thunk_rcx> # jmpq *%rcx > 0xffffffff812e4a2a : mov %rax,%r12 > 0xffffffff812e4a2d : cmp $0xfffffffffffffff6,%rax > 0xffffffff812e4a31 : jne 0xffffffff812e4a08 > 0xffffffff812e4a33 : mov %rbx,%rdi > 0xffffffff812e4a36 : callq 0xffffffff812e3840 > 0xffffffff812e4a3b : test %eax,%eax > 0xffffffff812e4a3d : jne 0xffffffff812e4a97 > 0xffffffff812e4a3f : mov %r15,%rdx > 0xffffffff812e4a42 : mov %r13,%rsi > 0xffffffff812e4a45 : mov %r14,%rdi > 0xffffffff812e4a48 : mov -0x30(%rbp),%rcx > 0xffffffff812e4a4c : callq 0xffffffff81e00f70 <__x86_indirect_thunk_rcx> > 0xffffffff812e4a51 : mov %rax,%r12 > 0xffffffff812e4a54 : jmp 0xffffffff812e4a08 > 0xffffffff812e4a56 : mov %rbx,%rdi > 0xffffffff812e4a59 : callq 0xffffffff812e3840 > 0xffffffff812e4a5e : test %eax,%eax > 0xffffffff812e4a60 : jne 0xffffffff812e4a97 > 0xffffffff812e4a62 : mov %r15,%rdi > 0xffffffff812e4a65 : callq 0xffffffff812f8ae0 > 0xffffffff812e4a6a : jmpq 0xffffffff812e48f6 > 0xffffffff812e4a6f : mov 0x50(%rbx),%rax > 0xffffffff812e4a73 : mov 0xb8(%rbx),%rdi > 0xffffffff812e4a7a : xor %edx,%edx > 0xffffffff812e4a7c : mov 0x8(%rax),%rsi > 0xffffffff812e4a80 : callq 0xffffffff811673f0 <__audit_inode> > 0xffffffff812e4a85 : jmpq 0xffffffff812e4999 > 0xffffffff812e4a8a : mov %rbx,%rdi > 0xffffffff812e4a8d : callq 0xffffffff812e4790 > 0xffffffff812e4a92 : jmpq 0xffffffff812e49c2 > 0xffffffff812e4a97 : mov $0xfffffffffffffff6,%r12 > 0xffffffff812e4a9e : jmpq 0xffffffff812e493e > > <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< > > 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 > > >