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

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Message-ID: <CALCETrWW8R_Z2gh8V-51_CxZO1T=J4jkZ7OA=7tN4c509N-2Aw@mail.gmail.com>
Date:	Wed, 10 Aug 2016 00:25:11 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Kees Cook <keescook@...omium.org>, x86 <x86@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>,
	Nilay Vaish <nilayvaish@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Brian Gerst <brgerst@...il.com>,
	Byungchul Park <byungchul.park@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 30/44] x86/unwind: add new unwind interface and implementations

On Aug 10, 2016 2:27 AM, "Josh Poimboeuf" <jpoimboe@...hat.com> wrote:
>
> On Tue, Aug 09, 2016 at 06:17:41PM -0500, Nilay Vaish wrote:
> > 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.
>
> 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?