[moderation] [fs?] KCSAN: data-race in alloc_fd / do_select (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: ccd1cdca5cd4 Merge tag 'nfsd-6.19-1' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=140e509a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b319ff1b6a2797ca
dashboard link: https://syzkaller.appspot.com/bug?extid=f4d7fa978975628a932f
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [bra...@kernel.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/12fc5a9ed91b/disk-ccd1cdca.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b6208e3d3a67/vmlinux-ccd1cdca.xz
kernel image: https://storage.googleapis.com/syzbot-assets/785c07926b9a/bzImage-ccd1cdca.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+f4d7fa...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in alloc_fd / do_select

write to 0xffff88810d25a6c0 of 8 bytes by task 4377 on cpu 0:
instrument_write include/linux/instrumented.h:41 [inline]
___set_bit include/asm-generic/bitops/instrumented-non-atomic.h:28 [inline]
__set_open_fd fs/file.c:335 [inline]
alloc_fd+0x275/0x3c0 fs/file.c:606
__get_unused_fd_flags fs/file.c:617 [inline]
get_unused_fd_flags+0x46/0x50 fs/file.c:622
sock_map_fd net/socket.c:507 [inline]
__sys_socket+0xf9/0x180 net/socket.c:1756
__do_sys_socket net/socket.c:1761 [inline]
__se_sys_socket net/socket.c:1759 [inline]
__x64_sys_socket+0x3f/0x50 net/socket.c:1759
x64_sys_call+0x127b/0x3000 arch/x86/include/generated/asm/syscalls_64.h:42
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88810d25a6c0 of 8 bytes by task 4378 on cpu 1:
max_select_fd fs/select.c:444 [inline]
do_select+0x295/0xf40 fs/select.c:494
core_sys_select+0x3dc/0x6e0 fs/select.c:677
do_pselect fs/select.c:759 [inline]
__do_sys_pselect6 fs/select.c:798 [inline]
__se_sys_pselect6+0x213/0x280 fs/select.c:789
__x64_sys_pselect6+0x78/0x90 fs/select.c:789
x64_sys_call+0x2e98/0x3000 arch/x86/include/generated/asm/syscalls_64.h:271
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x000000000000007f -> 0x00000000000001ff

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 4378 Comm: syz.6.207 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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