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, madve...@linux.microsoft.com wrote: > From: "Madhavan T. Venkataraman" <madve...@linux.microsoft.com> > > 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 <madve...@linux.microsoft.com> > --- > 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; > >
