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@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>
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
unsigned long fp = frame->fp;
struct stack_info info;
+ /* Terminal record; nothing to unwind */
+ if (!fp)
+ return -EINVAL;
+
if (fp & 0xf)
return -EINVAL;
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);