Re: [RFC PATCH v2 1/1] arm64: Implement stack trace termination record

From: Madhavan T. Venkataraman
Date: Wed Apr 14 2021 - 08:09:16 EST


Hi Mark Rutland, Mark Brown,

Could you take a look at this version for proper stack termination and let me know
what you think?

Thanks!

Madhavan

On 4/1/21 10:24 PM, madvenka@xxxxxxxxxxxxxxxxxxx wrote:
> From: "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx>
>
> Reliable stacktracing requires that we identify when a stacktrace is
> terminated early. We can do this by ensuring all tasks have a final
> frame record at a known location on their task stack, and checking
> that this is the final frame record in the chain.
>
> Kernel Tasks
> ============
>
> All tasks except the idle task have a pt_regs structure right after the
> task stack. This is called the task pt_regs. The pt_regs structure has a
> special stackframe field. Make this stackframe field the final frame in the
> task stack. This needs to be done in copy_thread() which initializes a new
> task's pt_regs and initial CPU context.
>
> For the idle task, there is no task pt_regs. For our purpose, we need one.
> So, create a pt_regs just like other kernel tasks and make
> pt_regs->stackframe the final frame in the idle task stack. This needs to be
> done at two places:
>
> - On the primary CPU, the boot task runs. It calls start_kernel()
> and eventually becomes the idle task for the primary CPU. Just
> before start_kernel() is called, set up the final frame.
>
> - On each secondary CPU, a startup task runs that calls
> secondary_startup_kernel() and eventually becomes the idle task
> on the secondary CPU. Just before secondary_start_kernel() is
> called, set up the final frame.
>
> User Tasks
> ==========
>
> User tasks are initially set up like kernel tasks when they are created.
> Then, they return to userland after fork via ret_from_fork(). After that,
> they enter the kernel only on an EL0 exception. (In arm64, system calls are
> also EL0 exceptions). The EL0 exception handler stores state in the task
> pt_regs and calls different functions based on the type of exception. The
> stack trace for an EL0 exception must end at the task pt_regs. So, make
> task pt_regs->stackframe as the final frame in the EL0 exception stack.
>
> In summary, task pt_regs->stackframe is where a successful stack trace ends.
>
> Stack trace termination
> =======================
>
> In the unwinder, terminate the stack trace successfully when
> task_pt_regs(task)->stackframe is reached. For stack traces in the kernel,
> this will correctly terminate the stack trace at the right place.
>
> However, debuggers terminate the stack trace when FP == 0. In the
> pt_regs->stackframe, the PC is 0 as well. So, stack traces taken in the
> debugger may print an extra record 0x0 at the end. While this is not
> pretty, this does not do any harm. This is a small price to pay for
> having reliable stack trace termination in the kernel.
>
> Signed-off-by: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>
> ---
> arch/arm64/kernel/entry.S | 8 +++++---
> arch/arm64/kernel/head.S | 29 +++++++++++++++++++++++------
> arch/arm64/kernel/process.c | 5 +++++
> arch/arm64/kernel/stacktrace.c | 10 +++++-----
> 4 files changed, 38 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index a31a0a713c85..e2dc2e998934 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -261,16 +261,18 @@ alternative_else_nop_endif
> stp lr, x21, [sp, #S_LR]
>
> /*
> - * For exceptions from EL0, terminate the callchain here.
> + * For exceptions from EL0, terminate the callchain here at
> + * task_pt_regs(current)->stackframe.
> + *
> * For exceptions from EL1, create a synthetic frame record so the
> * interrupted code shows up in the backtrace.
> */
> .if \el == 0
> - mov x29, xzr
> + stp xzr, xzr, [sp, #S_STACKFRAME]
> .else
> stp x29, x22, [sp, #S_STACKFRAME]
> - add x29, sp, #S_STACKFRAME
> .endif
> + add x29, sp, #S_STACKFRAME
>
> #ifdef CONFIG_ARM64_SW_TTBR0_PAN
> alternative_if_not ARM64_HAS_PAN
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 840bda1869e9..743c019a42c7 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -393,6 +393,23 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
> ret x28
> SYM_FUNC_END(__create_page_tables)
>
> + /*
> + * The boot task becomes the idle task for the primary CPU. The
> + * CPU startup task on each secondary CPU becomes the idle task
> + * for the secondary CPU.
> + *
> + * The idle task does not require pt_regs. But create a dummy
> + * pt_regs so that task_pt_regs(idle_task)->stackframe can be
> + * set up to be the final frame on the idle task stack just like
> + * all the other kernel tasks. This helps the unwinder to
> + * terminate the stack trace at a well-known stack offset.
> + */
> + .macro setup_final_frame
> + sub sp, sp, #PT_REGS_SIZE
> + stp xzr, xzr, [sp, #S_STACKFRAME]
> + add x29, sp, #S_STACKFRAME
> + .endm
> +
> /*
> * The following fragment of code is executed with the MMU enabled.
> *
> @@ -447,9 +464,9 @@ SYM_FUNC_START_LOCAL(__primary_switched)
> #endif
> bl switch_to_vhe // Prefer VHE if possible
> add sp, sp, #16
> - mov x29, #0
> - mov x30, #0
> - b start_kernel
> + setup_final_frame
> + bl start_kernel
> + nop
> SYM_FUNC_END(__primary_switched)
>
> .pushsection ".rodata", "a"
> @@ -606,14 +623,14 @@ SYM_FUNC_START_LOCAL(__secondary_switched)
> cbz x2, __secondary_too_slow
> msr sp_el0, x2
> scs_load x2, x3
> - mov x29, #0
> - mov x30, #0
> + setup_final_frame
>
> #ifdef CONFIG_ARM64_PTR_AUTH
> ptrauth_keys_init_cpu x2, x3, x4, x5
> #endif
>
> - b secondary_start_kernel
> + bl secondary_start_kernel
> + nop
> SYM_FUNC_END(__secondary_switched)
>
> SYM_FUNC_START_LOCAL(__secondary_too_slow)
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 325c83b1a24d..906baa232a89 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -437,6 +437,11 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
> }
> p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
> p->thread.cpu_context.sp = (unsigned long)childregs;
> + /*
> + * For the benefit of the unwinder, set up childregs->stackframe
> + * as the final frame for the new task.
> + */
> + p->thread.cpu_context.fp = (unsigned long)childregs->stackframe;
>
> ptrace_hw_copy_thread(p);
>
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index ad20981dfda4..72f5af8c69dc 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -44,16 +44,16 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
> unsigned long fp = frame->fp;
> struct stack_info info;
>
> - /* Terminal record; nothing to unwind */
> - if (!fp)
> + if (!tsk)
> + tsk = current;
> +
> + /* Final frame; nothing to unwind */
> + if (fp == (unsigned long) task_pt_regs(tsk)->stackframe)
> return -ENOENT;
>
> if (fp & 0xf)
> return -EINVAL;
>
> - if (!tsk)
> - tsk = current;
> -
> if (!on_accessible_stack(tsk, fp, &info))
> return -EINVAL;
>
>