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Subject[RFC PATCH v1 0/1] arm64: Unwinder enhancements for reliable stack trace
From: "Madhavan T. Venkataraman" <>

I have made an attempt to add some enhancements to the stack trace code
so it is a few steps closer to what is required for livepatch.

Unwinder changes


Currently, the unwinder terminates when both the FP (frame pointer)
and the PC (return address) of a frame are 0. But a frame could get
corrupted and zeroed. There needs to be a better check.

The following special terminating frame and function have been
defined for this purpose:

const u64 arm64_last_frame[2] __attribute__ ((aligned (16)));

void arm64_last_func(void)

In this patch, the FP is set to arm64_last_frame and the PC is set
to arm64_last_func in the bottom most frame.

Exception/Interrupt detection

An EL1 exception renders the stack trace unreliable as it can happen
anywhere including the frame pointer prolog and epilog. The
unwinder needs to be able to detect the exception on the stack.

Currently, the EL1 exception handler sets up pt_regs on the stack.
pt_regs contains a stack frame field that can hold an FP and a PC.
The exception handler chains this stack frame field along with other
frames on the stack. In other words, the EL1 handler creates a
synthetic exception frame. Currently, the unwinder does not know
where this exception frame is in the stack trace.

In this patch, the LSB of the exception frame FP is set. This is
similar to what is done on X86. When the unwinder detects the frame
with the LSB set, it needs to make sure that it is really an
exception frame and not the result of any stack corruption.

It can do this if the FP and PC are also recorded elsewhere in the
pt_regs for comparison. Currently, the FP is also stored in
regs->regs[29]. The PC is stored in regs->pc. However, regs->pc can
be changed by lower level functions. So, the PC needs to be stored
somewhere else as well.

This patch defines a new field, pt_regs->orig_pc, and records the
PC there. With this, the unwinder can validate the exception frame
and set a flag so that the caller of the unwinder can know when
an exception frame is encountered.

Unwinder return value

Currently, the unwinder returns -EINVAL for stack trace termination
as well as stack trace error. In this patch, the unwinder returns
-ENOENT for stack trace termination and -EINVAL for error. This idea
has been plagiarized from Mark Brown.

Reliable stack trace function

arch_stack_walk_reliable() is implemented in this patch. It walks the
stack like the existing stack trace functions with a couple of additional

Return address check

For each frame, the return address is checked to see if it is
a proper kernel text address. If not, the stack walk fails.

Exception frame check

Each frame is checked to see if it is an EL1 exception frame.
If it is, the stack walk fails.

Signed-off-by: Madhavan T. Venkataraman <>

Madhavan T. Venkataraman (1):
arm64: Unwinder enhancements for reliable stack trace

arch/arm64/include/asm/processor.h | 2 +
arch/arm64/include/asm/ptrace.h | 7 ++
arch/arm64/include/asm/stacktrace.h | 5 ++
arch/arm64/kernel/asm-offsets.c | 1 +
arch/arm64/kernel/entry.S | 14 +++-
arch/arm64/kernel/head.S | 8 +--
arch/arm64/kernel/process.c | 12 ++++
arch/arm64/kernel/stacktrace.c | 103 +++++++++++++++++++++++++---
8 files changed, 137 insertions(+), 15 deletions(-)

base-commit: e0756cfc7d7cd08c98a53b6009c091a3f6a50be6

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