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@xxxxxxx>
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@xxxxxxx>
---
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
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