Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read

[Ian Kent <raven@themaw.net>]

On 19/7/23 03:00, Anders Roxell wrote:
> On 2023-01-23 11:11, Ian Kent wrote:
>> On 29/12/22 21:07, Ian Kent wrote:
>>> On 29/12/22 17:20, Arnd Bergmann wrote:
>>>> On Fri, Dec 23, 2022, at 00:11, Ian Kent wrote:
>>>>> On 21/12/22 21:34, Anders Roxell wrote:
>>>>>> On 2022-10-31 12:30, Tejun Heo wrote:
>>>>>>> On Tue, Oct 18, 2022 at 10:32:42AM +0800, Ian Kent wrote:
>>>>>>>> The kernfs write lock is held when the kernfs node inode attributes
>>>>>>>> are updated. Therefore, when either kernfs_iop_getattr() or
>>>>>>>> kernfs_iop_permission() are called the kernfs node inode attributes
>>>>>>>> won't change.
>>>>>>>>
>>>>>>>> Consequently concurrent kernfs_refresh_inode() calls always copy the
>>>>>>>> same values from the kernfs node.
>>>>>>>>
>>>>>>>> So there's no need to take the inode i_lock to get consistent values
>>>>>>>> for generic_fillattr() and generic_permission(), the
>>>>>>>> kernfs read lock
>>>>>>>> is sufficient.
>>>>>>>>
>>>>>>>> Signed-off-by: Ian Kent <raven@themaw.net>
>>>>>>> Acked-by: Tejun Heo <tj@kernel.org>
>>>>>> Hi,
>>>>>>
>>>>>> Building an allmodconfig arm64 kernel on yesterdays next-20221220 and
>>>>>> booting that in qemu I see the following "BUG: KCSAN: data-race in
>>>>>> set_nlink / set_nlink".
>>>>> I'll check if I missed any places where set_link() could be
>>>>> called where the link count could be different.
>>>>>
>>>>>
>>>>> If there aren't any the question will then be can writing the
>>>>> same value to this location in multiple concurrent threads
>>>>> corrupt it?
>>>> I think the race that is getting reported for set_nlink()
>>>> is about this bit getting called simulatenously on multiple
>>>> CPUs with only the read lock held for the inode:
>>>>
>>>>        /* Yes, some filesystems do change nlink from zero to one */
>>>>        if (inode->i_nlink == 0)
>>>> atomic_long_dec(&inode->i_sb->s_remove_count);
>>>>        inode->__i_nlink = nlink;
>>>>
>>>> Since i_nlink and __i_nlink refer to the same memory location,
>>>> the 'inode->i_nlink == 0' check can be true for all of them
>>>> before the nonzero nlink value gets set, and this results in
>>>> s_remove_count being decremented more than once.
>>>
>>> Thanks for the comment Arnd.
>>
>> Hello all,
>>
>>
>> I've been looking at this and after consulting Miklos and his pointing
>>
>> out that it looks like a false positive the urgency dropped off a bit. So
>>
>> apologies for taking so long to report back.
>>
>>
>> Anyway it needs some description of conclusions reached so far.
>>
>>
>> I'm still looking around but in short, kernfs will set directories to <# of
>>
>> directory entries> + 2 unconditionally for directories. I can't yet find
>>
>> any other places where i_nlink is set or changed and if there are none
>>
>> then i_nlink will never be set to zero so the race should not occur.
>>
>>
>> Consequently my claim is this is a real false positive.
>>
>>
>> There are the file system operations that may be passed at mount time
>>
>> but given the way kernfs sets i_nlink it pretty much dictates those
>> operations
>>
>> (if there were any that modify it and there don't appear to be any) leave it
>>
>> alone.
>>
>>
>> So it just doesn't make sense for users of kernfs to fiddle with i_nlink ...
> On todays next tag, next-20230718 this KCSAN BUG poped up again. When I
> built an allmodconfig arm64 kernel and booted it in QEMU. Full log can
> be found http://ix.io/4AUd
>
> [ 1694.987789][  T137] BUG: KCSAN: data-race in inode_permission / kernfs_refresh_inode
> [ 1694.992912][  T137]
> [ 1694.994532][  T137] write to 0xffff00000bab6070 of 2 bytes by task 104 on cpu 0:
> [ 1694.999269][ T137] kernfs_refresh_inode (/home/anders/src/kernel/next/fs/kernfs/inode.c:171)
> [ 1695.002707][ T137] kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:289)
> [ 1695.006148][ T137] inode_permission (/home/anders/src/kernel/next/fs/namei.c:461 /home/anders/src/kernel/next/fs/namei.c:528)
> [ 1695.009420][ T137] link_path_walk (/home/anders/src/kernel/next/fs/namei.c:1720 /home/anders/src/kernel/next/fs/namei.c:2267)
> [ 1695.012643][ T137] path_lookupat (/home/anders/src/kernel/next/fs/namei.c:2478 (discriminator 2))
> [ 1695.015781][ T137] filename_lookup (/home/anders/src/kernel/next/fs/namei.c:2508)
> [ 1695.019059][ T137] vfs_statx (/home/anders/src/kernel/next/fs/stat.c:238)
> [ 1695.022024][ T137] vfs_fstatat (/home/anders/src/kernel/next/fs/stat.c:276)
> [ 1695.025067][ T137] __do_sys_newfstatat (/home/anders/src/kernel/next/fs/stat.c:446)
> [ 1695.028497][ T137] __arm64_sys_newfstatat (/home/anders/src/kernel/next/fs/stat.c:440 /home/anders/src/kernel/next/fs/stat.c:440)
> [ 1695.032080][ T137] el0_svc_common.constprop.0 (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> [ 1695.035916][ T137] do_el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> [ 1695.038796][ T137] el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> [ 1695.041468][ T137] el0t_64_sync_handler (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> [ 1695.044889][ T137] el0t_64_sync (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> [ 1695.047904][  T137]
> [ 1695.049511][  T137] 1 lock held by systemd-udevd/104:
> [ 1695.052837][ T137] #0: ffff000006681e08 (&root->kernfs_iattr_rwsem){++++}-{3:3}, at: kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:288)
> [ 1695.060241][  T137] irq event stamp: 82902
> [ 1695.063006][ T137] hardirqs last enabled at (82901): _raw_spin_unlock_irqrestore (/home/anders/src/kernel/next/arch/arm64/include/asm/alternative-macros.h:250 /home/anders/src/kernel/next/arch/arm64/include/asm/irqflags.h:27 /home/anders/src/kernel/next/arch/arm64/include/asm/irqflags.h:140 /home/anders/src/kernel/next/include/linux/spinlock_api_smp.h:151 /home/anders/src/kernel/next/kernel/locking/spinlock.c:194)
> [ 1695.069673][ T137] hardirqs last disabled at (82902): el1_interrupt (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:472 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:488)
> [ 1695.075474][ T137] softirqs last enabled at (82792): fpsimd_restore_current_state (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> [ 1695.082319][ T137] softirqs last disabled at (82790): fpsimd_restore_current_state (/home/anders/src/kernel/next/include/linux/bottom_half.h:20 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> [ 1695.089049][  T137]
> [ 1695.090659][  T137] read to 0xffff00000bab6070 of 2 bytes by task 137 on cpu 0:
> [ 1695.095374][ T137] inode_permission (/home/anders/src/kernel/next/fs/namei.c:532)
> [ 1695.098655][ T137] link_path_walk (/home/anders/src/kernel/next/fs/namei.c:1720 /home/anders/src/kernel/next/fs/namei.c:2267)
> [ 1695.101857][ T137] path_openat (/home/anders/src/kernel/next/fs/namei.c:3789 (discriminator 2))
> [ 1695.104885][ T137] do_filp_open (/home/anders/src/kernel/next/fs/namei.c:3820)
> [ 1695.108006][ T137] do_sys_openat2 (/home/anders/src/kernel/next/fs/open.c:1418)
> [ 1695.111290][ T137] __arm64_sys_openat (/home/anders/src/kernel/next/fs/open.c:1433)
> [ 1695.114825][ T137] el0_svc_common.constprop.0 (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> [ 1695.118662][ T137] do_el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> [ 1695.121555][ T137] el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> [ 1695.124207][ T137] el0t_64_sync_handler (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> [ 1695.127590][ T137] el0t_64_sync (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> [ 1695.130641][  T137]
> [ 1695.132241][  T137] no locks held by systemd-udevd/137.
> [ 1695.135618][  T137] irq event stamp: 3246
> [ 1695.138519][ T137] hardirqs last enabled at (3245): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/include/linux/seqlock.h:105)
> [ 1695.145825][ T137] hardirqs last disabled at (3246): el1_interrupt (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:472 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:488)
> [ 1695.151942][ T137] softirqs last enabled at (3208): fpsimd_restore_current_state (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> [ 1695.158950][ T137] softirqs last disabled at (3206): fpsimd_restore_current_state (/home/anders/src/kernel/next/include/linux/bottom_half.h:20 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> [ 1695.166036][  T137]
> [ 1695.167621][  T137] Reported by Kernel Concurrency Sanitizer on:
> [ 1695.179990][  T137] Hardware name: linux,dummy-virt (DT)
> [ 1695.183687][  T137] ==================================================================


This one is different to the original.


I can't see where the problem is here, can someone help me out

please.


I don't see any shared data values used by the call

devcgroup_inode_permission(inode, mask) in fs/namei.c:inode_permission()

that might be a problem during the read except possibly inode->i_mode.


I'll check on that ... maybe something's been missed when kernfs_rwsem

was changed to a separate lock (kernfs_iattr_rwsem).


>
> [...]
>
> [ 1738.053819][  T104] BUG: KCSAN: data-race in set_nlink / set_nlink
> [ 1738.058223][  T104]
> [ 1738.059865][  T104] read to 0xffff00000bab6918 of 4 bytes by task 108 on cpu 0:
> [ 1738.064916][ T104] set_nlink (/home/anders/src/kernel/next/fs/inode.c:369)
> [ 1738.067845][ T104] kernfs_refresh_inode (/home/anders/src/kernel/next/fs/kernfs/inode.c:180)
> [ 1738.071607][ T104] kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:289)
> [ 1738.075467][ T104] inode_permission (/home/anders/src/kernel/next/fs/namei.c:461 /home/anders/src/kernel/next/fs/namei.c:528)
> [ 1738.078868][ T104] link_path_walk (/home/anders/src/kernel/next/fs/namei.c:1720 /home/anders/src/kernel/next/fs/namei.c:2267)
> [ 1738.082270][ T104] path_lookupat (/home/anders/src/kernel/next/fs/namei.c:2478 (discriminator 2))
> [ 1738.085488][ T104] filename_lookup (/home/anders/src/kernel/next/fs/namei.c:2508)
> [ 1738.089101][ T104] user_path_at_empty (/home/anders/src/kernel/next/fs/namei.c:2907)
> [ 1738.092469][ T104] do_readlinkat (/home/anders/src/kernel/next/fs/stat.c:477)
> [ 1738.095970][ T104] __arm64_sys_readlinkat (/home/anders/src/kernel/next/fs/stat.c:504 /home/anders/src/kernel/next/fs/stat.c:501 /home/anders/src/kernel/next/fs/stat.c:501)
> [ 1738.099529][ T104] el0_svc_common.constprop.0 (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> [ 1738.103696][ T104] do_el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> [ 1738.106560][ T104] el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> [ 1738.109613][ T104] el0t_64_sync_handler (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> [ 1738.113035][ T104] el0t_64_sync (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> [ 1738.116346][  T104]
> [ 1738.117924][  T104] 1 lock held by systemd-udevd/108:
> [ 1738.121580][ T104] #0: ffff000006681e08 (&root->kernfs_iattr_rwsem){++++}-{3:3}, at: kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:288)
> [ 1738.129355][  T104] irq event stamp: 31000
> [ 1738.132088][ T104] hardirqs last enabled at (31000): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/include/linux/seqlock.h:105)
> [ 1738.139417][ T104] hardirqs last disabled at (30999): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/include/linux/seqlock.h:104)
> [ 1738.146781][ T104] softirqs last enabled at (30973): fpsimd_restore_current_state (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> [ 1738.153891][ T104] softirqs last disabled at (30971): fpsimd_restore_current_state (/home/anders/src/kernel/next/include/linux/bottom_half.h:20 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> [ 1738.161012][  T104]
> [ 1738.162663][  T104] write to 0xffff00000bab6918 of 4 bytes by task 104 on cpu 0:
> [ 1738.167730][ T104] set_nlink (/home/anders/src/kernel/next/fs/inode.c:372)
> [ 1738.170559][ T104] kernfs_refresh_inode (/home/anders/src/kernel/next/fs/kernfs/inode.c:180)
> [ 1738.174355][ T104] kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:289)
> [ 1738.177829][ T104] inode_permission (/home/anders/src/kernel/next/fs/namei.c:461 /home/anders/src/kernel/next/fs/namei.c:528)
> [ 1738.181403][ T104] link_path_walk (/home/anders/src/kernel/next/fs/namei.c:1720 /home/anders/src/kernel/next/fs/namei.c:2267)
> [ 1738.184738][ T104] path_lookupat (/home/anders/src/kernel/next/fs/namei.c:2478 (discriminator 2))
> [ 1738.188268][ T104] filename_lookup (/home/anders/src/kernel/next/fs/namei.c:2508)
> [ 1738.191865][ T104] vfs_statx (/home/anders/src/kernel/next/fs/stat.c:238)
> [ 1738.196236][ T104] vfs_fstatat (/home/anders/src/kernel/next/fs/stat.c:276)
> [ 1738.200120][ T104] __do_sys_newfstatat (/home/anders/src/kernel/next/fs/stat.c:446)
> [ 1738.204095][ T104] __arm64_sys_newfstatat (/home/anders/src/kernel/next/fs/stat.c:440 /home/anders/src/kernel/next/fs/stat.c:440)
> [ 1738.207676][ T104] el0_svc_common.constprop.0 (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:38 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:52 /home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:139)
> [ 1738.211820][ T104] do_el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/syscall.c:188)
> [ 1738.214815][ T104] el0_svc (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:133 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:144 /home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:648)
> [ 1738.217709][ T104] el0t_64_sync_handler (/home/anders/src/kernel/next/arch/arm64/kernel/entry-common.c:666)
> [ 1738.221239][ T104] el0t_64_sync (/home/anders/src/kernel/next/arch/arm64/kernel/entry.S:591)
> [ 1738.224502][  T104]
> [ 1738.226090][  T104] 1 lock held by systemd-udevd/104:
> [ 1738.229747][ T104] #0: ffff000006681e08 (&root->kernfs_iattr_rwsem){++++}-{3:3}, at: kernfs_iop_permission (/home/anders/src/kernel/next/fs/kernfs/inode.c:288)
> [ 1738.237504][  T104] irq event stamp: 108353
> [ 1738.240262][ T104] hardirqs last enabled at (108353): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/include/linux/seqlock.h:105)
> [ 1738.247443][ T104] hardirqs last disabled at (108352): seqcount_lockdep_reader_access (/home/anders/src/kernel/next/include/linux/seqlock.h:104)
> [ 1738.254510][ T104] softirqs last enabled at (108326): fpsimd_restore_current_state (/home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:264 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1791)
> [ 1738.262187][ T104] softirqs last disabled at (108324): fpsimd_restore_current_state (/home/anders/src/kernel/next/include/linux/bottom_half.h:20 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:242 /home/anders/src/kernel/next/arch/arm64/kernel/fpsimd.c:1784)
> [ 1738.270239][  T104]
> [ 1738.272140][  T104] Reported by Kernel Concurrency Sanitizer on:
> [ 1738.285185][  T104] Hardware name: linux,dummy-virt (DT)
> [ 1738.288703][  T104] ==================================================================

This looks just like the original except a different lock is being

used now.


The link count can't be less than two if set_nlink() is called.


Maybe I am missing something but the directory count is changed only

while holding the root->kernfs_iattr_rwsem so the value used by

set_nlink() will not change during concurrent calls to refresh_inode().

Still looks like a false positive, I'll check the write operations
again just to be sure.

Ian