Commit 6106e111 authored Jan 13, 2021 by Mark Rutland Committed by Will Deacon Jan 20, 2021

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:

  73267498 ("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:

  a25ffd3a ("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@kernel.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210113173155.43063-1-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>

parent 19c329f6

arch/arm64/kernel/entry.S

+5 −5

Original line number	Diff line number	Diff line
		@@ -261,16 +261,16 @@ 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

arch/arm64/kernel/stacktrace.c

+4 −9

Original line number	Diff line number	Diff line
		@@ -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);