[RFC PATCH 1/3] arm64: remove EL0 exception frame record

[Mark Brown]

From: Mark Rutland <mark.rutland at arm.com>

When entering an exception from EL0, the entry code creates a synthetic
frame record with a NULL PC. This was used by the code introduced in
commit:

  7326749801396105 ("arm64: unwind: reference pt_regs via embedded stack frame")

... to discover exception entries on the stack and dump the associated
pt_regs. Since the NULL PC was undesirable for the stacktrace, we added
a special case to unwind_frame() to prevent the NULL PC from being
logged.

Since commit:

  a25ffd3a6302a678 ("arm64: traps: Don't print stack or raw PC/LR values in backtraces")

... we no longer try to dump the pt_regs as part of a stacktrace, and
hence no longer need the synthetic exception record.

This patch removes the synthetic exception record and the associated
special case in unwind_frame(). Instead, EL0 exceptions set the FP to
NULL, as is the case for other terminal records (e.g. when a kernel
thread starts). The synthetic record for exceptions from EL1 is
retrained as this has useful unwind information for the interrupted
context.

To make the terminal case a bit clearer, an explicit check is added to
the start of unwind_frame(). This would otherwise be caught implicitly
by the on_accessible_stack() checks.

Reported-by: Mark Brown <broonie at kernel.org>
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Signed-off-by: Mark Brown <broonie at kernel.org>
---
 arch/arm64/kernel/entry.S      | 10 +++++-----
 arch/arm64/kernel/stacktrace.c | 13 ++++---------
 2 files changed, 9 insertions(+), 14 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 55af8b504b65..f7220e9c8520 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -202,16 +202,16 @@ alternative_cb_end
 	stp	lr, x21, [sp, #S_LR]
 
 	/*
-	 * In order to be able to dump the contents of struct pt_regs at the
-	 * time the exception was taken (in case we attempt to walk the call
-	 * stack later), chain it together with the stack frames.
+	 * For exceptions from EL0, terminate the callchain here.
+	 * For exceptions from EL1, create a synthetic frame record so the
+	 * interrupted code shows up in the backtrace.
 	 */
 	.if \el == 0
-	stp	xzr, xzr, [sp, #S_STACKFRAME]
+	mov	x29, xzr
 	.else
 	stp	x29, x22, [sp, #S_STACKFRAME]
-	.endif
 	add	x29, sp, #S_STACKFRAME
+	.endif
 
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 alternative_if_not ARM64_HAS_PAN
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index fa56af1a59c3..0fb42129b469 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -44,6 +44,10 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 	unsigned long fp = frame->fp;
 	struct stack_info info;
 
+	/* Terminal record; nothing to unwind */
+	if (!fp)
+		return -EINVAL;
+
 	if (fp & 0xf)
 		return -EINVAL;
 
@@ -104,15 +108,6 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 
 	frame->pc = ptrauth_strip_insn_pac(frame->pc);
 
-	/*
-	 * Frames created upon entry from EL0 have NULL FP and PC values, so
-	 * don't bother reporting these. Frames created by __noreturn functions
-	 * might have a valid FP even if PC is bogus, so only terminate where
-	 * both are NULL.
-	 */
-	if (!frame->fp && !frame->pc)
-		return -EINVAL;
-
 	return 0;
 }
 NOKPROBE_SYMBOL(unwind_frame);
-- 
2.20.1