Re: [PATCH v2 30/44] x86/unwind: add new unwind interface and implementations

[Josh Poimboeuf]

On Tue, Aug 09, 2016 at 06:17:41PM -0500, Nilay Vaish wrote:
> On 4 August 2016 at 17:22, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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.

> > +}
> > +EXPORT_SYMBOL_GPL(__unwind_start);
> > diff --git a/arch/x86/kernel/unwind_guess.c b/arch/x86/kernel/unwind_guess.c
> > new file mode 100644
> > index 0000000..e03df5a
> > --- /dev/null
> > +++ b/arch/x86/kernel/unwind_guess.c
> > @@ -0,0 +1,40 @@
> > +#include <linux/sched.h>
> > +#include <linux/ftrace.h>
> > +#include <asm/ptrace.h>
> > +#include <asm/bitops.h>
> > +#include <asm/stacktrace.h>
> > +#include <asm/unwind.h>
> > +
> > +bool unwind_next_frame(struct unwind_state *state)
> > +{
> > +       struct stack_info *info = &state->stack_info;
> > +
> > +       if (info->type == STACK_TYPE_UNKNOWN)
> > +               return false;
> > +
> > +       do {
> > +               for (state->sp++; state->sp < info->end; state->sp++)
> > +                       if (__kernel_text_address(*state->sp))
> > +                               return true;
> > +
> > +               state->sp = info->next_sp;
> > +
> > +       } while (!get_stack_info(state->sp, state->task, info,
> > +                                &state->stack_mask));
> > +
> > +       return false;
> > +}
> > +
> > +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->sp   = sp;
> > +
> > +       get_stack_info(sp, state->task, &state->stack_info, &state->stack_mask);
> > +
> > +       if (!__kernel_text_address(*sp))
> > +               unwind_next_frame(state);
> > +}
> > --
> > 2.7.4
> >
> 
> Why is it that you need to export symbols in unwind_frame.c but not in
> unwind_guess.c.  As per the Makefile, we would be compiling either of
> those two files.  Should not EXPORT_SYMBOL_GPL(__unwind_start) appear
> in both files?

Yeah, good catch.

-- 
Josh