Re: [PATCH] arm: Add unwinding annotations for 64bit division functions

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Sep 22, 2011 at 01:13:01PM +0100, Jon Medhurst (Tixy) wrote:
> On Thu, 2011-09-22 at 12:57 +0100, Catalin Marinas wrote:
> > On Thu, Sep 22, 2011 at 12:06:46PM +0100, Jon Medhurst (Tixy) wrote:
> > > On Thu, 2011-09-22 at 10:48 +0100, Catalin Marinas wrote:
> > > > We could improve things a bit in the unwinder and assume
> > > > that if the fault address is the same as the .fnstart address, the
> > > > return value is always in LR and the SP not affected (that's unwinding
> > > > bytecode 0xb0). For a few instructions into the function prologue we
> > > > can't reliably get the unwinding information.
> > > 
> > > That would help make it possible to unwind out of kprobes handlers to
> > > the probed function. The kprobes code itself would need work as well,
> > > and possibly the undef handler. Do we think it is worthwhile to do
> > > this? 
> > 
> > Does kprobes need to trace beyond the probed function? If not, you get
> > the address of the probed function via pt_regs anyway, so no need for
> > unwinding beyond that.
> 
> To be honest, I'm not very sure how kprobes get used in the real world.
> Though, if stack unwinding from their handlers currently doesn't work
> and people had a usecase for it, we would expect them to complain.

The unwinding fix should be simple (I haven't tested it yet):

8<-----------------------------
ARM: Ignore the unwinding information for the first instruction in a function

From: Catalin Marinas <catalin.marinas@arm.com>

When backtracing from the first instruction of a function, the prologue
has not been executed and the unwinding information is not valid. This
patch checks for this case and just assumes that the return address is
in LR.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
---
 arch/arm/kernel/unwind.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
index d2cb0b3..946face 100644
--- a/arch/arm/kernel/unwind.c
+++ b/arch/arm/kernel/unwind.c
@@ -293,6 +293,16 @@ int unwind_frame(struct stackframe *frame)
 		return -URC_FAILURE;
 	}
 
+	/*
+	 * Check for backtrace on the first instruction of a function. The
+	 * prologue has not been executed yet and the unwinding information is
+	 * not valid. Assume that the return address is in LR.
+	 */
+	if (idx.addr == frame->pc) {
+		frame->pc = frame->lr;
+		return URC_OK;
+	}
+
 	ctrl.vrs[FP] = frame->fp;
 	ctrl.vrs[SP] = frame->sp;
 	ctrl.vrs[LR] = frame->lr;

-- 
Catalin