quoted

Also, the Function Graph Tracer modifies the return address of a traced
function to a return trampoline to gather tracing data on function return.
Stack traces taken from that trampoline and functions it calls are
unreliable as the original return address may not be available in
that context. Mark the stack trace unreliable accordingly.

Signed-off-by: Madhavan T. Venkataraman <redacted>
---
 arch/arm64/kernel/entry-ftrace.S | 12 +++++++
 arch/arm64/kernel/stacktrace.c   | 61 ++++++++++++++++++++++++++++++++
 2 files changed, 73 insertions(+)

diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
index b3e4f9a088b1..1f0714a50c71 100644
--- a/arch/arm64/kernel/entry-ftrace.S
+++ b/arch/arm64/kernel/entry-ftrace.S

@@ -86,6 +86,18 @@ SYM_CODE_START(ftrace_caller)
 	b	ftrace_common
 SYM_CODE_END(ftrace_caller)
 
+/*
+ * A stack trace taken from anywhere in the FTRACE trampoline code should be
+ * considered unreliable as a tracer function (patched at ftrace_call) could
+ * potentially set pt_regs->pc and redirect execution to a function different
+ * than the traced function. E.g., livepatch.

IIUC the issue here that we have two copies of the pc: one in the regs,
and one in a frame record, and so after the update to the regs, the
frame record is stale.

This is something that we could fix by having
ftrace_instruction_pointer_set() set both.

Yes. I will look at this.

However, as noted elsewhere there are other issues which mean we'd still
need special unwinding code for this.

The only other cases we have discussed are EL1 exceptions in the ftrace code
and the return trampoline for function graph tracing. Is there any other case?

Thanks.

Madhavan