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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: * Marco Elver [260417 20:26]: > On Sat, 18 Apr 2026 at 01:51, 'Liam R. Howlett' via syzkaller-bugs > wrote: > > > > * syzbot [260417 05:12]: > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: 1d51b370a0f8 Merge tag 'jfs-7.1' of github.com:kleikamp/li.. > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=117dc4ce580000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=7f207c4b1fbf85a3 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=38a879f4a73497f2dfef > > > compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8 > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > ... and you won't. This will work unless we tear aligned unsigned long > > writes/reads. > > > > I'm debating marking these as data_race(). Marking them all as > > READ_ONCE and this one write as WRITE_ONCE. It seems overkill for > > something that won't happen. > > > > Alternatively, I can move the slot store fast path to need an > > allocation, but that's worse. > > The writer: > > > rcu_assign_pointer(slots[offset + 1], wr_mas->entry); > > wr_mas->pivots[offset] = mas->index - 1; // <-- stores pivots[offset] > > The reader races here: > > > if (pivots[offset] >= mas->index) { // <-- load pivots[offset] > > max = pivots[offset]; // <-- load pivots[offset] again > > break; > > } > > The compiler is free to reload them as written. What if there's a > concurrent update between the first and second load? Then the benign race has happened. Looking at [1], we see that care has been taken to limit the slot store code to only !rcu mode, except for a subset of cases. Digging through the information in git will eventually lead you to this note Peng wrote: commit 64891ba3e51fb841b0af70db029038eb93bd5a43 Author: Peng Zhang Date: Wed Jun 28 15:36:57 2023 +0800 maple_tree: add a fast path case in mas_wr_slot_store() When expanding a range in two directions, only partially overwriting the previous and next ranges, the number of entries will not be increased, so we can just update the pivots as a fast path. However, it may introduce potential risks in RCU mode, because it updates two pivots. We only enable it in non-RCU mode. Link: https://lkml.kernel.org/r/20230628073657.75314-5-zhangpeng.00@bytedance.com Signed-off-by: Peng Zhang Reviewed-by: Liam R. Howlett Signed-off-by: Andrew Morton So, you can see that the author of the initial code did look at race conditions. I wanted to read the link for more information but that link isn't working right now (403 error). ----- Or, we can ask an LLM about it: In mas_wr_store_type(), we only allow wr_slot_store in RCU mode for the narrow case where wr_mas->offset_end - mas->offset == 1. That condition means the update touches only one boundary between two adjacent ranges, so the in-place mutation in mas_wr_slot_store() stays limited to a single slot/pivot boundary update and is considered safe for lockless readers. If the span is wider than that, we do not use in-place slot-store under RCU. The broader in-place path in mas_wr_slot_store() is explicitly guarded with WARN_ON_ONCE(mt_in_rcu(...)), and the store-type logic instead falls back to node-replacement paths (wr_node_store/split/rebalance), which preserve RCU reader safety by publishing a new node rather than mutating too much in place. In non-RCU mode (!mt_in_rcu()), we allow the wider in-place cases because readers are expected to be synchronized by locking, so the stricter lockless-reader constraints do not apply. ----- I am sorry, but I don't have time to work through the scenarios as this is not an issue and I no longer have the time budget for mailing lists as I once did. If you can come up with a problem (and ideally a reproducer), then please let me know. [1]. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/maple_tree.c?id=c1f49dea2b8f335813d3b348fd39117fb8efb428#n3696 > > > > Downloadable assets: > > > disk image: https://storage.googleapis.com/syzbot-assets/e08ff8d2b0e5/disk-1d51b370.raw.xz > > > vmlinux: https://storage.googleapis.com/syzbot-assets/c11d4b098bbf/vmlinux-1d51b370.xz > > > kernel image: https://storage.googleapis.com/syzbot-assets/6a4691f32e3d/bzImage-1d51b370.xz > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > Reported-by: syzbot+38a879f4a73497f2dfef@syzkaller.appspotmail.com > > > > > > ================================================================== > > > BUG: KCSAN: data-race in mas_wr_store_entry / mtree_range_walk > > > > > > write to 0xffff888104f71d08 of 8 bytes by task 4757 on cpu 0: > > > mas_wr_slot_store lib/maple_tree.c:3232 [inline] > > > mas_wr_store_entry+0x3405/0x5ad0 lib/maple_tree.c:3528 > > > mas_store_prealloc+0x43e/0x690 lib/maple_tree.c:4936 > > > vma_iter_store_overwrite mm/vma.h:616 [inline] > > > commit_merge+0x6a1/0x720 mm/vma.c:766 > > > vma_expand+0x301/0x460 mm/vma.c:1219 > > > vma_merge_new_range+0x29c/0x320 mm/vma.c:1112 > > > __mmap_region mm/vma.c:2766 [inline] > > > mmap_region+0x1073/0x2110 mm/vma.c:2856 > > > do_mmap+0x9b2/0xbd0 mm/mmap.c:560 > > > vm_mmap_pgoff+0x183/0x2d0 mm/util.c:581 > > > ksys_mmap_pgoff+0xc1/0x310 mm/mmap.c:606 > > > x64_sys_call+0x14df/0x3020 arch/x86/include/generated/asm/syscalls_64.h:10 > > > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] > > > do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94 > > > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > > > > > read to 0xffff888104f71d08 of 8 bytes by task 4759 on cpu 1: > > > mtree_range_walk+0x1a6/0x490 lib/maple_tree.c:2032 > > > mas_state_walk lib/maple_tree.c:2952 [inline] > > > mas_walk+0x1cc/0x370 lib/maple_tree.c:4366 > > > lock_vma_under_rcu+0xc9/0x210 mm/mmap_lock.c:304 > > > do_user_addr_fault+0x232/0x1050 arch/x86/mm/fault.c:1325 > > > handle_page_fault arch/x86/mm/fault.c:1474 [inline] > > > exc_page_fault+0x62/0xa0 arch/x86/mm/fault.c:1527 > > > asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618 > > > > > > value changed: 0x00007f68dc2a5fff -> 0x00007f68dc284fff > > > > > > Reported by Kernel Concurrency Sanitizer on: > > > CPU: 1 UID: 0 PID: 4759 Comm: syz.5.348 Not tainted syzkaller #0 PREEMPT(full) > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026 > > > ================================================================== > > > netlink: 64 bytes leftover after parsing attributes in process `syz.5.348'. > > > > > > > > > --- > > > This report is generated by a bot. 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