[PATCH] arm64: stacktrace: Clarify documentation of EL0 entry frame skipping

[Mark Rutland]

Hi Mark,

On Wed, Oct 07, 2020 at 06:11:38PM +0100, Mark Brown wrote:
> The unwinder suppresses reporting of frames created on entry from EL0 where
> both FP and PC are NULL in order to avoid having the final NULL PC
> cluttering traces and potentially creating confusion. There is a comment
> explaining this which doesn't explicitly call out the case where we have
> a valid PC but NULL FP, do so in order to ensure that things are as
> clear as possible to readers unfamiliar with the code.
> 
> Signed-off-by: Mark Brown <broonie at kernel.org>
> ---
>  arch/arm64/kernel/stacktrace.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index fa56af1a59c3..0c474d81d4e9 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -107,8 +107,9 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>  	/*
>  	 * 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.
> +	 * might have a valid FP even if PC is bogus and the last frame in a
> +	 * normal stack will have a valid PC but NULL FP, so only terminate
> +	 * early where both are NULL.
>  	 */

I reckon we should remove the special case entirely rather than trying
to shore up the comment here -- the related comment in entry.S is stale
and misleading, and this will be a mystery to unfamiliar readers anyhow.

Does the below patch look ok to you? I suspect the explicit check for the
terminal case may help with the reliable stacktrace stuff? e.g. we could
make that return -ENOENT to distinguish a terminal case from a broken
frame record.

Thanks,
Mark.

---->8----
>From 61cb79bd9d926744190a43a5e1ec9aefda7ba052 Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland at arm.com>
Date: Thu, 8 Oct 2020 10:47:59 +0100
Subject: [PATCH] arm64: remove EL0 exception frame record

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.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Reported-by: Mark Brown <broonie at kernel.org>
Cc: Ard Biesheuvel <ardb at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will 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 f30007dff35f7..8af811b6b2259 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -228,16 +228,16 @@ alternative_else_nop_endif
 	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 fa56af1a59c39..0fb42129b4691 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);
-- 
2.11.0