[BUG] arm64: Sleeping function called from invalid context in do_debug_exception on PREEMPT_RT

[Yunseong Kim]

Hi Yeoreum,

Thank you for pointing it!

On 8/13/25 3:56 PM, Yeoreum Yun wrote:
> Hi Yunseong,
> 
>>
>> | BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
>> | in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 20466, name: syz.0.1689
>> | preempt_count: 1, expected: 0
>> | RCU nest depth: 0, expected: 0
>> | Preemption disabled at:
>> | [<ffff800080241600>] debug_exception_enter arch/arm64/mm/fault.c:978 [inline]
>> | [<ffff800080241600>] do_debug_exception+0x68/0x2fc arch/arm64/mm/fault.c:997
>> | CPU: 0 UID: 0 PID: 20466 Comm: syz.0.1689 Not tainted 6.16.0-rc1-rt1-dirty #12 PREEMPT_RT
>> | Hardware name: QEMU KVM Virtual Machine, BIOS 2025.02-8 05/13/2025
>> | Call trace:
>> |  show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:466 (C)
>> |  __dump_stack+0x30/0x40 lib/dump_stack.c:94
>> |  dump_stack_lvl+0x148/0x1d8 lib/dump_stack.c:120
>> |  dump_stack+0x1c/0x3c lib/dump_stack.c:129
>> |  __might_resched+0x2e4/0x52c kernel/sched/core.c:8800
>> |  __rt_spin_lock kernel/locking/spinlock_rt.c:48 [inline]
>> |  rt_spin_lock+0xa8/0x1bc kernel/locking/spinlock_rt.c:57
>> |  spin_lock include/linux/spinlock_rt.h:44 [inline]
>> |  force_sig_info_to_task+0x6c/0x4a8 kernel/signal.c:1302
>> |  force_sig_fault_to_task kernel/signal.c:1699 [inline]
>> |  force_sig_fault+0xc4/0x110 kernel/signal.c:1704
>> |  arm64_force_sig_fault+0x6c/0x80 arch/arm64/kernel/traps.c:265
>> |  send_user_sigtrap arch/arm64/kernel/debug-monitors.c:237 [inline]
>> |  single_step_handler+0x1f4/0x36c arch/arm64/kernel/debug-monitors.c:257
>> |  do_debug_exception+0x154/0x2fc arch/arm64/mm/fault.c:1002
>> |  el0_dbg+0x44/0x120 arch/arm64/kernel/entry-common.c:756
>> |  el0t_64_sync_handler+0x3c/0x108 arch/arm64/kernel/entry-common.c:832
>> |  el0t_64_sync+0x1ac/0x1b0 arch/arm64/kernel/entry.S:600
>>
>>
>> It seems that commit eaff68b32861 ("arm64: entry: Add entry and exit functions
>> for debug exception") in 6.17-rc1, also present as 6fb44438a5e1 in mainline,
>> removed code that previously avoided sleeping context issues when handling
>> debug exceptions:
>> Link: https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/arch/arm64/mm/fault.c?id=eaff68b3286116d499a3d4e513a36d772faba587
> 
> No. Her patch commit 31575e11ecf7 (arm64: debug: split brk64 exception entry)
> solves your splat since el0_brk64() doesn't call debug_exception_enter()
> by spliting el0/el1 brk64 entry exception entry.
> 
> Formerly, el(0/1)_dbg() are handled in do_debug_exception() together
> and it calls debug_exception_enter() disabling preemption and this makes
> your splat while handling brk excepttion from el0.
> 

Do you think a fix is necessary if this issue also affects the LTS kernel
before 6.17-rc1? As far as I know, most production RT kernels are still
based on the existing LTS versions.

> 
> --
> Sincerely,
> Yeoreum Yun

Thank you,
Yunseong