On Aug 10, 2016 2:27 AM, "Josh Poimboeuf" <[email protected]> wrote: > > On Tue, Aug 09, 2016 at 06:17:41PM -0500, Nilay Vaish wrote: > > On 4 August 2016 at 17:22, Josh Poimboeuf <[email protected]> wrote: > > > diff --git a/arch/x86/kernel/unwind_frame.c > > > b/arch/x86/kernel/unwind_frame.c > > > new file mode 100644 > > > index 0000000..f28f1b5 > > > --- /dev/null > > > +++ b/arch/x86/kernel/unwind_frame.c > > > @@ -0,0 +1,84 @@ > > > +#include <linux/sched.h> > > > +#include <asm/ptrace.h> > > > +#include <asm/bitops.h> > > > +#include <asm/stacktrace.h> > > > +#include <asm/unwind.h> > > > + > > > +#define FRAME_HEADER_SIZE (sizeof(long) * 2) > > > + > > > +unsigned long unwind_get_return_address(struct unwind_state *state) > > > +{ > > > + unsigned long *addr_p = unwind_get_return_address_ptr(state); > > > + unsigned long addr; > > > + > > > + if (state->stack_info.type == STACK_TYPE_UNKNOWN) > > > + return 0; > > > + > > > + addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, > > > *addr_p, > > > + addr_p); > > > + > > > + return __kernel_text_address(addr) ? addr : 0; > > > +} > > > +EXPORT_SYMBOL_GPL(unwind_get_return_address); > > > + > > > +static bool update_stack_state(struct unwind_state *state, void *addr, > > > + size_t len) > > > +{ > > > + struct stack_info *info = &state->stack_info; > > > + > > > + if (on_stack(info, addr, len)) > > > + return true; > > > + > > > + if (get_stack_info(info->next_sp, state->task, info, > > > + &state->stack_mask)) > > > + goto unknown; > > > + > > > + if (!on_stack(info, addr, len)) > > > + goto unknown; > > > + > > > + return true; > > > + > > > +unknown: > > > + info->type = STACK_TYPE_UNKNOWN; > > > + return false; > > > +} > > > + > > > +bool unwind_next_frame(struct unwind_state *state) > > > +{ > > > + unsigned long *next_bp; > > > + > > > + if (unwind_done(state)) > > > + return false; > > > + > > > + next_bp = (unsigned long *)*state->bp; > > > + > > > + /* > > > + * Make sure the next frame is on a valid stack and can be > > > accessed > > > + * safely. > > > + */ > > > + if (!update_stack_state(state, next_bp, FRAME_HEADER_SIZE)) > > > + return false; > > > + > > > + /* move to the next frame */ > > > + state->bp = next_bp; > > > + return true; > > > +} > > > +EXPORT_SYMBOL_GPL(unwind_next_frame); > > > + > > > +void __unwind_start(struct unwind_state *state, struct task_struct *task, > > > + struct pt_regs *regs, unsigned long *sp) > > > +{ > > > + memset(state, 0, sizeof(*state)); > > > + > > > + state->task = task; > > > + state->bp = get_frame_pointer(task, regs); > > > + > > > + get_stack_info(state->bp, state->task, &state->stack_info, > > > + &state->stack_mask); > > > + update_stack_state(state, state->bp, FRAME_HEADER_SIZE); > > > + > > > + /* unwind to the first frame after the specified stack pointer */ > > > + while (state->bp < sp && !unwind_done(state)) > > > + unwind_next_frame(state); > > > > Do we unwind all the frames here? It seems strange to me that in a > > function named __unwind_start(), we unwind all the frames. > > It just skips any stack frames before the specified "sp" pointer. > Several callers use this, for example, to start at regs->sp instead of > the current stack frame. I'll try to make the comment clearer. >
Are you checking the right condition? Shouldn't this check that sp is in bounds for the current stack if a stack switch happened? I admit I don't fully understand the use case. If someone wants to start a trace in the middle, shouldn't they just pass regs in?
