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arm64: remove EL0 exception frame record

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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:

  7326749801 ("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:

  a25ffd3a63 ("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>
This commit is contained in:
Mark Rutland 2021-01-13 17:31:55 +00:00 committed by Will Deacon
parent 19c329f680
commit 6106e1112c
2 changed files with 9 additions and 14 deletions
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10
arch/arm64/kernel/entry.S
View File

@ -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

13
arch/arm64/kernel/stacktrace.c
View File

@ -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);