[PATCH] usercopy: delete __noreturn from usercopy_abort
Jiangfeng Xiao
xiaojiangfeng at huawei.com
Tue Mar 5 20:00:04 PST 2024
On 2024/3/6 1:58, Josh Poimboeuf wrote:
>>>> For the usercopy_abort function, whether '__noreturn' is added
>>>> does not affect the internal behavior of the usercopy_abort function.
>>>> Therefore, it is recommended that '__noreturn' be deleted
>>>> so that backtrace can work properly.
>>>
>>> This isn't acceptable. Removing __noreturn this will break
>>> objtool's processing of execution flow for livepatching, IBT, and
>>> KCFI instrumentation. These all depend on an accurate control flow
>>> descriptions, and usercopy_abort is correctly marked __noreturn.
>
> __noreturn also has the benefit of enabling the compiler to produce more
> compact code for callees.
>
>> Thank you for providing this information.
>> I'll go back to further understand how __noreturn is used
>> in features such as KCFI and livepatching.
>
> Adding ARM folks -- see
> https://lkml.kernel.org/lkml/1709516385-7778-1-git-send-email-xiaojiangfeng@huawei.com
> for the original bug report.
>
> This is an off-by-one bug which is common in unwinders, due to the fact
> that the address on the stack points to the return address rather than
> the call address.
>
Thanks for your advice. I think I understand. To solve this problem, I need to
fix the off-by-one bug which is common in unwinders. I'll try to fix it later
by referring to your patch.
> So, for example, when the last instruction of a function is a function
> call (e.g., to a noreturn function), it can cause the unwinder to
> incorrectly try to unwind from the function *after* the callee.
>
> For ORC (x86), we fixed this by decrementing the PC for call frames (but
> not exception frames). I've seen user space unwinders do similar, for
> non-signal frames.
>
> Something like the following might fix your issue (completely untested):
>
> diff --git a/arch/arm/include/asm/stacktrace.h b/arch/arm/include/asm/stacktrace.h
> index 360f0d2406bf..4891e38cdc1f 100644
> --- a/arch/arm/include/asm/stacktrace.h
> +++ b/arch/arm/include/asm/stacktrace.h
> @@ -21,9 +21,7 @@ struct stackframe {
> struct llist_node *kr_cur;
> struct task_struct *tsk;
> #endif
> -#ifdef CONFIG_UNWINDER_FRAME_POINTER
> bool ex_frame;
> -#endif
> };
>
> static __always_inline
> @@ -37,9 +35,8 @@ void arm_get_current_stackframe(struct pt_regs *regs, struct stackframe *frame)
> frame->kr_cur = NULL;
> frame->tsk = current;
> #endif
> -#ifdef CONFIG_UNWINDER_FRAME_POINTER
> - frame->ex_frame = in_entry_text(frame->pc);
> -#endif
> + frame->ex_frame = !!regs;
> +
> }
>
> extern int unwind_frame(struct stackframe *frame);
> diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
> index 620aa82e3bdd..caed7436da09 100644
> --- a/arch/arm/kernel/stacktrace.c
> +++ b/arch/arm/kernel/stacktrace.c
> @@ -154,9 +154,6 @@ static void start_stack_trace(struct stackframe *frame, struct task_struct *task
> frame->kr_cur = NULL;
> frame->tsk = task;
> #endif
> -#ifdef CONFIG_UNWINDER_FRAME_POINTER
> - frame->ex_frame = in_entry_text(frame->pc);
> -#endif
> }
>
> void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
> @@ -167,6 +164,7 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
> if (regs) {
> start_stack_trace(&frame, NULL, regs->ARM_fp, regs->ARM_sp,
> regs->ARM_lr, regs->ARM_pc);
> + frame.ex_frame = true;
> } else if (task != current) {
> #ifdef CONFIG_SMP
> /*
> @@ -180,6 +178,7 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
> thread_saved_sp(task), 0,
> thread_saved_pc(task));
> #endif
> + frame.ex_frame = false;
> } else {
> here:
> start_stack_trace(&frame, task,
> @@ -187,6 +186,7 @@ void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
> current_stack_pointer,
> (unsigned long)__builtin_return_address(0),
> (unsigned long)&&here);
> + frame.ex_frame = false;
> /* skip this function */
> if (unwind_frame(&frame))
> return;
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index 3bad79db5d6e..46a5b1eb3f0a 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -84,10 +84,10 @@ void dump_backtrace_entry(unsigned long where, unsigned long from,
> printk("%sFunction entered at [<%08lx>] from [<%08lx>]\n",
> loglvl, where, from);
> #elif defined CONFIG_BACKTRACE_VERBOSE
> - printk("%s[<%08lx>] (%ps) from [<%08lx>] (%pS)\n",
> + printk("%s[<%08lx>] (%ps) from [<%08lx>] (%pB)\n",
> loglvl, where, (void *)where, from, (void *)from);
> #else
> - printk("%s %ps from %pS\n", loglvl, (void *)where, (void *)from);
> + printk("%s %ps from %pB\n", loglvl, (void *)where, (void *)from);
> #endif
>
> if (in_entry_text(from) && end <= ALIGN(frame, THREAD_SIZE))
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index 9d2192156087..99ded32196af 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -407,7 +407,7 @@ int unwind_frame(struct stackframe *frame)
> {
> const struct unwind_idx *idx;
> struct unwind_ctrl_block ctrl;
> - unsigned long sp_low;
> + unsigned long sp_low, pc;
>
> /* store the highest address on the stack to avoid crossing it*/
> sp_low = frame->sp;
> @@ -417,19 +417,22 @@ int unwind_frame(struct stackframe *frame)
> pr_debug("%s(pc = %08lx lr = %08lx sp = %08lx)\n", __func__,
> frame->pc, frame->lr, frame->sp);
>
> - idx = unwind_find_idx(frame->pc);
> + pc = frame->ex_frame ? frame->pc : frame->pc - 4;
> +
> + idx = unwind_find_idx(pc);
> if (!idx) {
> - if (frame->pc && kernel_text_address(frame->pc)) {
> - if (in_module_plt(frame->pc) && frame->pc != frame->lr) {
> + if (kernel_text_address(pc)) {
> + if (in_module_plt(pc) && frame->pc != frame->lr) {
> /*
> * Quoting Ard: Veneers only set PC using a
> * PC+immediate LDR, and so they don't affect
> * the state of the stack or the register file
> */
> frame->pc = frame->lr;
> + frame->ex_frame = false;
> return URC_OK;
> }
> - pr_warn("unwind: Index not found %08lx\n", frame->pc);
> + pr_warn("unwind: Index not found %08lx\n", pc);
> }
> return -URC_FAILURE;
> }
> @@ -442,7 +445,7 @@ int unwind_frame(struct stackframe *frame)
> if (idx->insn == 1)
> /* can't unwind */
> return -URC_FAILURE;
> - else if (frame->pc == prel31_to_addr(&idx->addr_offset)) {
> + else if (frame->ex_frame && pc == prel31_to_addr(&idx->addr_offset)) {
> /*
> * Unwinding is tricky when we're halfway through the prologue,
> * since the stack frame that the unwinder expects may not be
> @@ -451,9 +454,10 @@ int unwind_frame(struct stackframe *frame)
> * a function, we are still effectively in the stack frame of
> * the caller, and the unwind info has no relevance yet.
> */
> - if (frame->pc == frame->lr)
> + if (pc == frame->lr)
> return -URC_FAILURE;
> frame->pc = frame->lr;
> + frame->ex_frame = false;
> return URC_OK;
> } else if ((idx->insn & 0x80000000) == 0)
> /* prel31 to the unwind table */
> @@ -515,6 +519,7 @@ int unwind_frame(struct stackframe *frame)
> frame->lr = ctrl.vrs[LR];
> frame->pc = ctrl.vrs[PC];
> frame->lr_addr = ctrl.lr_addr;
> + frame->ex_frame = false;
>
> return URC_OK;
> }
> @@ -544,6 +549,7 @@ void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
> */
> here:
> frame.pc = (unsigned long)&&here;
> + frame.ex_frame = false;
> } else {
> /* task blocked in __switch_to */
> frame.fp = thread_saved_fp(tsk);
> @@ -554,11 +560,12 @@ void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
> */
> frame.lr = 0;
> frame.pc = thread_saved_pc(tsk);
> + frame.ex_frame = false;
> }
>
> while (1) {
> int urc;
> - unsigned long where = frame.pc;
> + unsigned long where = frame.ex_frame ? frame.pc : frame.pc - 4;
>
> urc = unwind_frame(&frame);
> if (urc < 0)
> .
>
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