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

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Date:	Tue, 9 Aug 2016 18:17:41 -0500
From:	Nilay Vaish <nilayvaish@...il.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>, x86 <x86@...nel.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH v2 30/44] x86/unwind: add new unwind interface and implementations

On 4 August 2016 at 17:22, Josh Poimboeuf <jpoimboe@...hat.com> 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.

> +}
> +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?

--
Nilay