[PATCH 08/10] arm64: stacktrace: report recovered PCs

[Mark Rutland]

When analysing a stacktrace it can be useful to know whether an unwound
PC has been rewritten by fgraph or kretprobes, as in some situations
these may be suspect or be known to be unreliable.

This patch adds flags to track when an unwind entry has recovered the PC
from fgraph and/or kretprobes, and updates dump_backtrace() to log when
this is the case.

The flags recorded are:

 "F" - the PC was recovered from fgraph
 "K" - the PC was recovered from kretprobes

These flags are recorded and logged in addition to the original source
of the unwound PC.

For example, with the ftrace_graph profiler enabled globally, and
kretprobes installed on generic_handle_domain_irq() and
do_interrupt_handler(), a backtrace triggered by magic-sysrq + L
reports:

| Call trace:
|  show_stack+0x20/0x40 (CF)
|  dump_stack_lvl+0x60/0x80 (F)
|  dump_stack+0x18/0x28
|  nmi_cpu_backtrace+0xfc/0x140
|  nmi_trigger_cpumask_backtrace+0x1c8/0x200
|  arch_trigger_cpumask_backtrace+0x20/0x40
|  sysrq_handle_showallcpus+0x24/0x38 (F)
|  __handle_sysrq+0xa8/0x1b0 (F)
|  handle_sysrq+0x38/0x50 (F)
|  pl011_int+0x460/0x5a8 (F)
|  __handle_irq_event_percpu+0x60/0x220 (F)
|  handle_irq_event+0x54/0xc0 (F)
|  handle_fasteoi_irq+0xa8/0x1d0 (F)
|  generic_handle_domain_irq+0x34/0x58 (F)
|  gic_handle_irq+0x54/0x140 (FK)
|  call_on_irq_stack+0x24/0x58 (F)
|  do_interrupt_handler+0x88/0xa0
|  el1_interrupt+0x34/0x68 (FK)
|  el1h_64_irq_handler+0x18/0x28
|  el1h_64_irq+0x64/0x68
|  default_idle_call+0x34/0x180
|  do_idle+0x204/0x268
|  cpu_startup_entry+0x40/0x50 (F)
|  rest_init+0xe4/0xf0
|  start_kernel+0x744/0x750
|  __primary_switched+0x80/0x90

Note that as these flags are reported next to the recovered PC value,
they appear on the callers of instrumented functions, e.g.
gic_handle_irq has a "K" marker because generic_handle_domain_irq was
instrumented with kretprobes and had its return addredd (gic_handle_irq)
rewritten.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Ard Biesheuvel <ardb at kernel.org>
Cc: Josh Poimboeuf <jpoimboe at kernel.org>
Cc: Kalesh Singh <kaleshsingh at google.com>
Cc: Madhavan T. Venkataraman <madvenka at linux.microsoft.com>
Cc: Marc Zyngier <maz at kernel.org>
Cc: Mark Brown <broonie at kernel.org>
Cc: Miroslav Benes <mbenes at suse.cz>
Cc: Puranjay Mohan <puranjay12 at gmail.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/kernel/stacktrace.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 83ab37e13159e..f8e231683dad9 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -28,6 +28,14 @@ enum kunwind_source {
 	KUNWIND_SOURCE_REGS_PC,
 };
 
+union unwind_flags {
+	unsigned long	all;
+	struct {
+		unsigned long	fgraph : 1,
+				kretprobe : 1;
+	};
+};
+
 /*
  * Kernel unwind state
  *
@@ -46,6 +54,7 @@ struct kunwind_state {
 	struct llist_node *kr_cur;
 #endif
 	enum kunwind_source source;
+	union unwind_flags flags;
 };
 
 static __always_inline void
@@ -55,6 +64,7 @@ kunwind_init(struct kunwind_state *state,
 	unwind_init_common(&state->common);
 	state->task = task;
 	state->source = KUNWIND_SOURCE_UNKNOWN;
+	state->flags.all = 0;
 }
 
 /*
@@ -127,6 +137,7 @@ kunwind_recover_return_address(struct kunwind_state *state)
 		if (WARN_ON_ONCE(state->common.pc == orig_pc))
 			return -EINVAL;
 		state->common.pc = orig_pc;
+		state->flags.fgraph = 1;
 	}
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
@@ -137,6 +148,7 @@ kunwind_recover_return_address(struct kunwind_state *state)
 						  (void *)state->common.fp,
 						  &state->kr_cur);
 		state->common.pc = orig_pc;
+		state->flags.kretprobe = 1;
 	}
 #endif /* CONFIG_KRETPROBES */
 
@@ -157,6 +169,8 @@ kunwind_next(struct kunwind_state *state)
 	unsigned long fp = state->common.fp;
 	int err;
 
+	state->flags.all = 0;
+
 	/* Final frame; nothing to unwind */
 	if (fp == (unsigned long)&task_pt_regs(tsk)->stackframe)
 		return -ENOENT;
@@ -331,12 +345,18 @@ static const char *state_source_string(const struct kunwind_state *state)
 static bool dump_backtrace_entry(const struct kunwind_state *state, void *arg)
 {
 	const char *source = state_source_string(state);
+	union unwind_flags flags = state->flags;
+	bool has_info = source || flags.all;
 	char *loglvl = arg;
-	printk("%s %pSb%s%s%s\n", loglvl,
+
+	printk("%s %pSb%s%s%s%s%s\n", loglvl,
 		(void *)state->common.pc,
-		source ? " (" : "",
+		has_info ? " (" : "",
 		source ? source : "",
-		source ? ")" : "");
+		flags.fgraph ? "F" : "",
+		flags.kretprobe ? "K" : "",
+		has_info ? ")" : "");
+
 	return true;
 }
 
-- 
2.30.2