arm64: stacktrace: restore terminal records

We removed the terminal frame records in commit:

   6106e111 ("arm64: remove EL0 exception frame record")

... on the assumption that as we no longer used them to find the pt_regs
at exception boundaries, they were no longer necessary.

However, Leo reports that as an unintended side-effect, this causes
traces which cross secondary_start_kernel to terminate one entry too
late, with a spurious "0" entry.

There are a few ways we could sovle this, but as we're planning to use
terminal records for RELIABLE_STACKTRACE, let's revert the logic change
for now, keeping the update comments and accounting for the changes in
commit:

  3c026001 ("arm64: stacktrace: Report when we reach the end of the stack")

This is effectively a partial revert of commit:

  6106e111 ("arm64: remove EL0 exception frame record")

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 6106e111 ("arm64: remove EL0 exception frame record")
Reported-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Leo Yan <leo.yan@linaro.org>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Link: https://lore.kernel.org/r/20210429104813.GA33550@C02TD0UTHF1T.local


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -283,16 +283,16 @@ alternative_else_nop_endif
 	stp	lr, x21, [sp, #S_LR]
 
 	/*
-	 * For exceptions from EL0, terminate the callchain here.
+	 * For exceptions from EL0, create a terminal frame record.
 	 * For exceptions from EL1, create a synthetic frame record so the
 	 * interrupted code shows up in the backtrace.
 	 */
 	.if \el == 0
-	mov	x29, xzr
+	stp	xzr, xzr, [sp, #S_STACKFRAME]
 	.else
 	stp	x29, x22, [sp, #S_STACKFRAME]
-	add	x29, sp, #S_STACKFRAME
 	.endif
+	add	x29, sp, #S_STACKFRAME
 
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 alternative_if_not ARM64_HAS_PAN

arch/arm64/kernel/stacktrace.c

@@ -68,10 +68,6 @@ 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 -ENOENT;
-
 	if (fp & 0xf)
 		return -EINVAL;
 
@@ -132,6 +128,12 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 
 	frame->pc = ptrauth_strip_insn_pac(frame->pc);
 
+	/*
+	 * This is a terminal record, so we have finished unwinding.
+	 */
+	if (!frame->fp && !frame->pc)
+		return -ENOENT;
+
 	return 0;
 }
 NOKPROBE_SYMBOL(unwind_frame);