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@xxxxxxxxxx> > > > > > > Acked-by: Tejun Heo <tj@xxxxxxxxxx> > > > > > 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
