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Date:   Tue, 18 Jul 2023 21:00:10 +0200
From:   Anders Roxell <anders.roxell@linaro.org>
To:     Ian Kent <raven@themaw.net>
Cc:     Arnd Bergmann <arnd@arndb.de>, Tejun Heo <tj@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Eric Sandeen <sandeen@sandeen.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        elver@google.com, imran.f.khan@oracle.com
Subject: Re: [PATCH 1/2] kernfs: dont take i_lock on inode attr read
Message-ID: <20230718190009.GC411@mutt>
References: <166606025456.13363.3829702374064563472.stgit@donald.themaw.net>
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 <7815c8da-7d5f-c2c5-9dfd-7a77ac37c7f7@themaw.net>
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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] ==================================================================

[...]

[ 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] ==================================================================


Cheers,
Anders