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@xxxxxxxxxx > 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) > . >
