Re: Tracing text poke / kernel self-modifying code (Was: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion)

[hpa]

On September 12, 2019 8:00:39 AM GMT+01:00, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>On 29/08/19 2:46 PM, Peter Zijlstra wrote:
>> On Thu, Aug 29, 2019 at 12:40:56PM +0300, Adrian Hunter wrote:
>>> Can you expand on "and ensure the poke_handler preserves the
>existing
>>> control flow"?  Whatever the INT3-handler does will be traced
>normally so
>>> long as it does not itself execute self-modified code.
>> 
>> My thinking was that the code shouldn't change semantics before
>emitting
>> the RECORD_TEXT_POKE; but you're right in that that doesn't seem to
>> matter much.
>> 
>> Either we run the old code or INT3 does 'something'.  Then we get
>> RECORD_TEXT_POKE and finish the poke.  Which tells that the moment
>INT3
>> stops the new text is in place.
>> 
>> I suppose that works too, and requires less changes.
>
>
>What about this?
>
>
>diff --git a/arch/x86/include/asm/text-patching.h
>b/arch/x86/include/asm/text-patching.h
>index 70c09967a999..00aa9bef2b9d 100644
>--- a/arch/x86/include/asm/text-patching.h
>+++ b/arch/x86/include/asm/text-patching.h
>@@ -30,6 +30,7 @@ struct text_poke_loc {
> 	void *addr;
> 	size_t len;
> 	const char opcode[POKE_MAX_OPCODE_SIZE];
>+	char old_opcode[POKE_MAX_OPCODE_SIZE];
> };
> 
>extern void text_poke_early(void *addr, const void *opcode, size_t
>len);
>diff --git a/arch/x86/kernel/alternative.c
>b/arch/x86/kernel/alternative.c
>index ccd32013c47a..c781bbbbbafd 100644
>--- a/arch/x86/kernel/alternative.c
>+++ b/arch/x86/kernel/alternative.c
>@@ -3,6 +3,7 @@
> 
> #include <linux/module.h>
> #include <linux/sched.h>
>+#include <linux/perf_event.h>
> #include <linux/mutex.h>
> #include <linux/list.h>
> #include <linux/stringify.h>
>@@ -1045,8 +1046,10 @@ void text_poke_bp_batch(struct text_poke_loc
>*tp, unsigned int nr_entries)
> 	/*
> 	 * First step: add a int3 trap to the address that will be patched.
> 	 */
>-	for (i = 0; i < nr_entries; i++)
>+	for (i = 0; i < nr_entries; i++) {
>+		memcpy(tp[i].old_opcode, tp[i].addr, tp[i].len);
> 		text_poke(tp[i].addr, &int3, sizeof(int3));
>+	}
> 
> 	on_each_cpu(do_sync_core, NULL, 1);
> 
>@@ -1071,6 +1074,11 @@ void text_poke_bp_batch(struct text_poke_loc
>*tp, unsigned int nr_entries)
> 		on_each_cpu(do_sync_core, NULL, 1);
> 	}
> 
>+	for (i = 0; i < nr_entries; i++) {
>+		perf_event_text_poke((unsigned long)tp[i].addr,
>+				     tp[i].old_opcode, tp[i].opcode, tp[i].len);
>+	}
>+
> 	/*
> 	 * Third step: replace the first byte (int3) by the first byte of
> 	 * replacing opcode.
>diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>index 61448c19a132..f4c6095d2110 100644
>--- a/include/linux/perf_event.h
>+++ b/include/linux/perf_event.h
>@@ -1183,6 +1183,8 @@ extern void perf_event_exec(void);
> extern void perf_event_comm(struct task_struct *tsk, bool exec);
> extern void perf_event_namespaces(struct task_struct *tsk);
> extern void perf_event_fork(struct task_struct *tsk);
>+extern void perf_event_text_poke(unsigned long addr, const void
>*old_bytes,
>+				 const void *new_bytes, size_t len);
> 
> /* Callchains */
> DECLARE_PER_CPU(struct perf_callchain_entry, perf_callchain_entry);
>@@ -1406,6 +1408,10 @@ static inline void perf_event_exec(void)				{ }
>static inline void perf_event_comm(struct task_struct *tsk, bool
>exec)	{ }
> static inline void perf_event_namespaces(struct task_struct *tsk)	{ }
> static inline void perf_event_fork(struct task_struct *tsk)		{ }
>+static inline void perf_event_text_poke(unsigned long addr,
>+					const void *old_bytes,
>+					const void *new_bytes,
>+					size_t len)			{ }
> static inline void perf_event_init(void)				{ }
>static inline int  perf_swevent_get_recursion_context(void)		{ return
>-1; }
> static inline void perf_swevent_put_recursion_context(int rctx)		{ }
>diff --git a/include/uapi/linux/perf_event.h
>b/include/uapi/linux/perf_event.h
>index bb7b271397a6..6396d4c0d2f9 100644
>--- a/include/uapi/linux/perf_event.h
>+++ b/include/uapi/linux/perf_event.h
>@@ -375,7 +375,8 @@ struct perf_event_attr {
> 				ksymbol        :  1, /* include ksymbol events */
> 				bpf_event      :  1, /* include bpf events */
> 				aux_output     :  1, /* generate AUX records instead of events */
>-				__reserved_1   : 32;
>+				text_poke      :  1, /* include text poke events */
>+				__reserved_1   : 31;
> 
> 	union {
> 		__u32		wakeup_events;	  /* wakeup every n events */
>@@ -1000,6 +1001,22 @@ enum perf_event_type {
> 	 */
> 	PERF_RECORD_BPF_EVENT			= 18,
> 
>+	/*
>+	 * Records changes to kernel text i.e. self-modified code.
>+	 * 'len' is the number of old bytes, which is the same as the number
>+	 * of new bytes. 'bytes' contains the old bytes followed immediately
>+	 * by the new bytes.
>+	 *
>+	 * struct {
>+	 *	struct perf_event_header	header;
>+	 *	u64				addr;
>+	 *	u16				len;
>+	 *	u8				bytes[];
>+	 *	struct sample_id		sample_id;
>+	 * };
>+	 */
>+ 	PERF_RECORD_TEXT_POKE			= 19,
>+
> 	PERF_RECORD_MAX,			/* non-ABI */
> };
> 
>diff --git a/kernel/events/core.c b/kernel/events/core.c
>index 811bb333c986..43c0d3d232dc 100644
>--- a/kernel/events/core.c
>+++ b/kernel/events/core.c
>@@ -386,6 +386,7 @@ static atomic_t nr_freq_events __read_mostly;
> static atomic_t nr_switch_events __read_mostly;
> static atomic_t nr_ksymbol_events __read_mostly;
> static atomic_t nr_bpf_events __read_mostly;
>+static atomic_t nr_text_poke_events __read_mostly;
> 
> static LIST_HEAD(pmus);
> static DEFINE_MUTEX(pmus_lock);
>@@ -4339,7 +4340,7 @@ static bool is_sb_event(struct perf_event *event)
> 	if (attr->mmap || attr->mmap_data || attr->mmap2 ||
> 	    attr->comm || attr->comm_exec ||
> 	    attr->task || attr->ksymbol ||
>-	    attr->context_switch ||
>+	    attr->context_switch || attr->text_poke ||
> 	    attr->bpf_event)
> 		return true;
> 	return false;
>@@ -4413,6 +4414,8 @@ static void unaccount_event(struct perf_event
>*event)
> 		atomic_dec(&nr_ksymbol_events);
> 	if (event->attr.bpf_event)
> 		atomic_dec(&nr_bpf_events);
>+	if (event->attr.text_poke)
>+		atomic_dec(&nr_text_poke_events);
> 
> 	if (dec) {
> 		if (!atomic_add_unless(&perf_sched_count, -1, 1))
>@@ -8045,6 +8048,85 @@ void perf_event_bpf_event(struct bpf_prog *prog,
> 	perf_iterate_sb(perf_event_bpf_output, &bpf_event, NULL);
> }
> 
>+struct perf_text_poke_event {
>+	const void		*old_bytes;
>+	const void		*new_bytes;
>+	size_t			pad;
>+	u16			len;
>+
>+	struct {
>+		struct perf_event_header	header;
>+
>+		u64				addr;
>+	} event_id;
>+};
>+
>+static int perf_event_text_poke_match(struct perf_event *event)
>+{
>+	return event->attr.text_poke;
>+}
>+
>+static void perf_event_text_poke_output(struct perf_event *event, void
>*data)
>+{
>+	struct perf_text_poke_event *text_poke_event = data;
>+	struct perf_output_handle handle;
>+	struct perf_sample_data sample;
>+	u64 padding = 0;
>+	int ret;
>+
>+	if (!perf_event_text_poke_match(event))
>+		return;
>+
>+	perf_event_header__init_id(&text_poke_event->event_id.header,
>&sample, event);
>+
>+	ret = perf_output_begin(&handle, event,
>text_poke_event->event_id.header.size);
>+	if (ret)
>+		return;
>+
>+	perf_output_put(&handle, text_poke_event->event_id);
>+	perf_output_put(&handle, text_poke_event->len);
>+
>+	__output_copy(&handle, text_poke_event->old_bytes,
>text_poke_event->len);
>+	__output_copy(&handle, text_poke_event->new_bytes,
>text_poke_event->len);
>+
>+	if (text_poke_event->pad)
>+		__output_copy(&handle, &padding, text_poke_event->pad);
>+
>+	perf_event__output_id_sample(event, &handle, &sample);
>+
>+	perf_output_end(&handle);
>+}
>+
>+void perf_event_text_poke(unsigned long addr, const void *old_bytes,
>+			  const void *new_bytes, size_t len)
>+{
>+	struct perf_text_poke_event text_poke_event;
>+	size_t tot, pad;
>+
>+	if (!atomic_read(&nr_text_poke_events))
>+		return;
>+
>+	tot  = sizeof(text_poke_event.len) + (len << 1);
>+	pad  = ALIGN(tot, sizeof(u64)) - tot;
>+
>+	text_poke_event = (struct perf_text_poke_event){
>+		.old_bytes    = old_bytes,
>+		.new_bytes    = new_bytes,
>+		.pad          = pad,
>+		.len          = len,
>+		.event_id  = {
>+			.header = {
>+				.type = PERF_RECORD_TEXT_POKE,
>+				.misc = PERF_RECORD_MISC_KERNEL,
>+				.size = sizeof(text_poke_event.event_id) + tot + pad,
>+			},
>+			.addr = addr,
>+		},
>+	};
>+
>+	perf_iterate_sb(perf_event_text_poke_output, &text_poke_event, NULL);
>+}
>+
> void perf_event_itrace_started(struct perf_event *event)
> {
> 	event->attach_state |= PERF_ATTACH_ITRACE;
>@@ -10331,6 +10413,8 @@ static void account_event(struct perf_event
>*event)
> 		atomic_inc(&nr_ksymbol_events);
> 	if (event->attr.bpf_event)
> 		atomic_inc(&nr_bpf_events);
>+	if (event->attr.text_poke)
>+		atomic_inc(&nr_text_poke_events);
> 
> 	if (inc) {
> 		/*

Wasn't there was a long discussion about this a while ago. Holding (or spinning) on INT 3 has a lot of nice properties, e.g. no need to emulate instructions. However, there were concerns about deadlocks. I proposed an algorithm which I *believe* addressed the deadlocks, but obviously when it comes to deadlock avoidance one pair of eyeballs is not enough.

My flight is about to take off so I can't look up the email thread right now, unfortunately.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.