[PATCHv3] Introduced new tracing mode KCOV_MODE_UNIQUE.

* [PATCHv3] Introduced new tracing mode KCOV_MODE_UNIQUE.
@ 2021-03-26 20:51 Alexander Lochmann
  2021-03-27 10:37 ` Greg Kroah-Hartman
  2021-03-27 14:56 ` Andrey Konovalov
  0 siblings, 2 replies; 6+ messages in thread
From: Alexander Lochmann @ 2021-03-26 20:51 UTC (permalink / raw)
  Cc: Alexander Lochmann, Dmitry Vyukov, Andrey Konovalov,
	Jonathan Corbet, Randy Dunlap, Andrew Klychkov, Miguel Ojeda,
	Greg Kroah-Hartman, Andrew Morton, Jakub Kicinski,
	Aleksandr Nogikh, Wei Yongjun, Maciej Grochowski, kasan-dev,
	linux-doc, linux-kernel

It simply stores the executed PCs.
The execution order is discarded.
Each bit in the shared buffer represents every fourth
byte of the text segment.
Since a call instruction on every supported
architecture is at least four bytes, it is safe
to just store every fourth byte of the text segment.
In contrast to KCOV_MODE_TRACE_PC, the shared buffer
cannot overflow. Thus, all executed PCs are recorded.

Signed-off-by: Alexander Lochmann <info@alexander-lochmann.de>
---
 Documentation/dev-tools/kcov.rst | 79 +++++++++++++++++++++++++
 include/linux/kcov.h             | 12 ++--
 include/uapi/linux/kcov.h        | 10 ++++
 kernel/kcov.c                    | 98 ++++++++++++++++++++++++--------
 4 files changed, 172 insertions(+), 27 deletions(-)

diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
index d2c4c27e1702..9b0df2f8474c 100644
--- a/Documentation/dev-tools/kcov.rst
+++ b/Documentation/dev-tools/kcov.rst
@@ -127,6 +127,85 @@ That is, a parent process opens /sys/kernel/debug/kcov, enables trace mode,
 mmaps coverage buffer and then forks child processes in a loop. Child processes
 only need to enable coverage (disable happens automatically on thread end).
 
+If someone is interested in a set of executed PCs, and does not care about
+execution order, he or she can use KCOV_INIT_UNIQUE to do so:
+
+.. code-block:: c
+
+    #include <stdio.h>
+    #include <stddef.h>
+    #include <stdint.h>
+    #include <stdlib.h>
+    #include <sys/types.h>
+    #include <sys/stat.h>
+    #include <sys/ioctl.h>
+    #include <sys/mman.h>
+    #include <unistd.h>
+    #include <fcntl.h>
+
+    #define KCOV_INIT_UNIQUE                _IOR('c', 2, unsigned long)
+    #define KCOV_ENABLE			_IO('c', 100)
+    #define KCOV_DISABLE			_IO('c', 101)
+
+    #define BITS_PER_LONG 64
+    #define KCOV_TRACE_PC  0
+    #define KCOV_TRACE_CMP 1
+    #define KCOV_UNIQUE_PC 2
+    /*
+     * Determine start of text segment via 'nm vmlinux | grep _stext | cut -d " " -f1',
+     * and fill in.
+     */
+    #define STEXT_START 0xffffffff81000000
+
+
+
+    int main(int argc, char **argv)
+    {
+	int fd;
+	unsigned long *cover, n, i;
+
+	/* A single fd descriptor allows coverage collection on a single
+	 * thread.
+	 */
+	fd = open("/sys/kernel/debug/kcov", O_RDWR);
+	if (fd == -1)
+		perror("open"), exit(1);
+	/* Setup trace mode and trace size. */
+	if ((n = ioctl(fd, KCOV_INIT_UNIQUE, 0)) < 0)
+		perror("ioctl"), exit(1);
+	/* Mmap buffer shared between kernel- and user-space. */
+	cover = (unsigned long*)mmap(NULL, n,
+				     PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if ((void*)cover == MAP_FAILED)
+		perror("mmap"), exit(1);
+	/* Enable coverage collection on the current thread. */
+	if (ioctl(fd, KCOV_ENABLE, KCOV_UNIQUE_PC))
+		perror("ioctl"), exit(1);
+	/* That's the target syscal call. */
+	read(-1, NULL, 0);
+	/* Disable coverage collection for the current thread. After this call
+	 * coverage can be enabled for a different thread.
+	 */
+	if (ioctl(fd, KCOV_DISABLE, 0))
+		perror("ioctl"), exit(1);
+        /* Convert byte size into element size */
+        n /= sizeof(unsigned long);
+        /* Print executed PCs in sorted order */
+        for (i = 0; i < n; i++) {
+            for (int j = 0; j < BITS_PER_LONG; j++) {
+                if (cover[i] & (1L << j)) {
+                    printf("0x%jx\n", (uintmax_t)(STEXT_START + (i * BITS_PER_LONG + j) * 4));
+                }
+            }
+        }
+	/* Free resources. */
+	if (munmap(cover, n * sizeof(unsigned long)))
+		perror("munmap"), exit(1);
+	if (close(fd))
+		perror("close"), exit(1);
+	return 0;
+    }
+
 Comparison operands collection
 ------------------------------
 
diff --git a/include/linux/kcov.h b/include/linux/kcov.h
index 4e3037dc1204..99c309b3a53b 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -12,17 +12,21 @@ enum kcov_mode {
 	/* Coverage collection is not enabled yet. */
 	KCOV_MODE_DISABLED = 0,
 	/* KCOV was initialized, but tracing mode hasn't been chosen yet. */
-	KCOV_MODE_INIT = 1,
+	KCOV_MODE_INIT_TRACE = 1,
+	/* KCOV was initialized, but recording of unique PCs hasn't been chosen yet. */
+	KCOV_MODE_INIT_UNIQUE = 2,
 	/*
 	 * Tracing coverage collection mode.
 	 * Covered PCs are collected in a per-task buffer.
 	 */
-	KCOV_MODE_TRACE_PC = 2,
+	KCOV_MODE_TRACE_PC = 4,
 	/* Collecting comparison operands mode. */
-	KCOV_MODE_TRACE_CMP = 3,
+	KCOV_MODE_TRACE_CMP = 8,
+	/* Collecting unique covered PCs. Execution order is not saved. */
+	KCOV_MODE_UNIQUE_PC = 16,
 };
 
-#define KCOV_IN_CTXSW	(1 << 30)
+#define KCOV_IN_CTXSW	(1 << 31)
 
 void kcov_task_init(struct task_struct *t);
 void kcov_task_exit(struct task_struct *t);
diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
index 1d0350e44ae3..5b99b6d1a1ac 100644
--- a/include/uapi/linux/kcov.h
+++ b/include/uapi/linux/kcov.h
@@ -19,6 +19,7 @@ struct kcov_remote_arg {
 #define KCOV_REMOTE_MAX_HANDLES		0x100
 
 #define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
+#define KCOV_INIT_UNIQUE		_IOR('c', 2, unsigned long)
 #define KCOV_ENABLE			_IO('c', 100)
 #define KCOV_DISABLE			_IO('c', 101)
 #define KCOV_REMOTE_ENABLE		_IOW('c', 102, struct kcov_remote_arg)
@@ -35,6 +36,15 @@ enum {
 	KCOV_TRACE_PC = 0,
 	/* Collecting comparison operands mode. */
 	KCOV_TRACE_CMP = 1,
+	/*
+	 * Unique coverage collection mode.
+	 * Unique covered PCs are collected in a per-task buffer.
+	 * De-duplicates the collected PCs. Execution order is *not* saved.
+	 * Each bit in the buffer represents every fourth byte of the text segment.
+	 * Since a call instruction is at least four bytes on every supported
+	 * architecture, storing just every fourth byte is sufficient.
+	 */
+	KCOV_UNIQUE_PC = 2,
 };
 
 /*
diff --git a/kernel/kcov.c b/kernel/kcov.c
index 80bfe71bbe13..9d64d672a5dc 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -24,6 +24,7 @@
 #include <linux/refcount.h>
 #include <linux/log2.h>
 #include <asm/setup.h>
+#include <asm/sections.h>
 
 #define kcov_debug(fmt, ...) pr_debug("%s: " fmt, __func__, ##__VA_ARGS__)
 
@@ -151,10 +152,10 @@ static void kcov_remote_area_put(struct kcov_remote_area *area,
 	list_add(&area->list, &kcov_remote_areas);
 }
 
-static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
+static __always_inline notrace bool check_kcov_mode(enum kcov_mode needed_mode,
+						    struct task_struct *t,
+						    unsigned int *mode)
 {
-	unsigned int mode;
-
 	/*
 	 * We are interested in code coverage as a function of a syscall inputs,
 	 * so we ignore code executed in interrupts, unless we are in a remote
@@ -162,7 +163,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_stru
 	 */
 	if (!in_task() && !(in_serving_softirq() && t->kcov_softirq))
 		return false;
-	mode = READ_ONCE(t->kcov_mode);
+	*mode = READ_ONCE(t->kcov_mode);
 	/*
 	 * There is some code that runs in interrupts but for which
 	 * in_interrupt() returns false (e.g. preempt_schedule_irq()).
@@ -171,7 +172,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_stru
 	 * kcov_start().
 	 */
 	barrier();
-	return mode == needed_mode;
+	return ((int)(*mode & (KCOV_IN_CTXSW | needed_mode))) > 0;
 }
 
 static notrace unsigned long canonicalize_ip(unsigned long ip)
@@ -191,18 +192,27 @@ void notrace __sanitizer_cov_trace_pc(void)
 	struct task_struct *t;
 	unsigned long *area;
 	unsigned long ip = canonicalize_ip(_RET_IP_);
-	unsigned long pos;
+	unsigned long pos, idx;
+	unsigned int mode;
 
 	t = current;
-	if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
+	if (!check_kcov_mode(KCOV_MODE_TRACE_PC | KCOV_MODE_UNIQUE_PC, t, &mode))
 		return;
 
 	area = t->kcov_area;
-	/* The first 64-bit word is the number of subsequent PCs. */
-	pos = READ_ONCE(area[0]) + 1;
-	if (likely(pos < t->kcov_size)) {
-		area[pos] = ip;
-		WRITE_ONCE(area[0], pos);
+	if (likely(mode == KCOV_MODE_TRACE_PC)) {
+		/* The first 64-bit word is the number of subsequent PCs. */
+		pos = READ_ONCE(area[0]) + 1;
+		if (likely(pos < t->kcov_size)) {
+			area[pos] = ip;
+			WRITE_ONCE(area[0], pos);
+		}
+	} else {
+		idx = (ip - canonicalize_ip((unsigned long)&_stext)) / 4;
+		pos = idx % BITS_PER_LONG;
+		idx /= BITS_PER_LONG;
+		if (likely(idx < t->kcov_size))
+			WRITE_ONCE(area[idx], READ_ONCE(area[idx]) | 1L << pos);
 	}
 }
 EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
@@ -213,9 +223,10 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
 	struct task_struct *t;
 	u64 *area;
 	u64 count, start_index, end_pos, max_pos;
+	unsigned int mode;
 
 	t = current;
-	if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t))
+	if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t, &mode))
 		return;
 
 	ip = canonicalize_ip(ip);
@@ -362,7 +373,7 @@ void kcov_task_init(struct task_struct *t)
 static void kcov_reset(struct kcov *kcov)
 {
 	kcov->t = NULL;
-	kcov->mode = KCOV_MODE_INIT;
+	kcov->mode = KCOV_MODE_INIT_TRACE;
 	kcov->remote = false;
 	kcov->remote_size = 0;
 	kcov->sequence++;
@@ -468,12 +479,13 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
 
 	spin_lock_irqsave(&kcov->lock, flags);
 	size = kcov->size * sizeof(unsigned long);
-	if (kcov->mode != KCOV_MODE_INIT || vma->vm_pgoff != 0 ||
+	if (kcov->mode & ~(KCOV_MODE_INIT_TRACE | KCOV_MODE_INIT_UNIQUE) || vma->vm_pgoff != 0 ||
 	    vma->vm_end - vma->vm_start != size) {
 		res = -EINVAL;
 		goto exit;
 	}
 	if (!kcov->area) {
+		kcov_debug("mmap(): Allocating 0x%lx bytes\n", size);
 		kcov->area = area;
 		vma->vm_flags |= VM_DONTEXPAND;
 		spin_unlock_irqrestore(&kcov->lock, flags);
@@ -515,6 +527,8 @@ static int kcov_get_mode(unsigned long arg)
 {
 	if (arg == KCOV_TRACE_PC)
 		return KCOV_MODE_TRACE_PC;
+	else if (arg == KCOV_UNIQUE_PC)
+		return KCOV_MODE_UNIQUE_PC;
 	else if (arg == KCOV_TRACE_CMP)
 #ifdef CONFIG_KCOV_ENABLE_COMPARISONS
 		return KCOV_MODE_TRACE_CMP;
@@ -562,12 +576,14 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 {
 	struct task_struct *t;
 	unsigned long size, unused;
-	int mode, i;
+	int mode, i, text_size, ret = 0;
 	struct kcov_remote_arg *remote_arg;
 	struct kcov_remote *remote;
 	unsigned long flags;
 
 	switch (cmd) {
+	case KCOV_INIT_UNIQUE:
+		fallthrough;
 	case KCOV_INIT_TRACE:
 		/*
 		 * Enable kcov in trace mode and setup buffer size.
@@ -581,11 +597,42 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 		 * that must not overflow.
 		 */
 		size = arg;
-		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
-			return -EINVAL;
-		kcov->size = size;
-		kcov->mode = KCOV_MODE_INIT;
-		return 0;
+		if (cmd == KCOV_INIT_UNIQUE) {
+			if (size != 0)
+				return -EINVAL;
+			text_size = (canonicalize_ip((unsigned long)&_etext)
+				     - canonicalize_ip((unsigned long)&_stext));
+			/**
+			 * A call instr is at least four bytes on every supported architecture.
+			 * Hence, just every fourth instruction can potentially be a call.
+			 */
+			text_size = roundup(text_size, 4);
+			text_size /= 4;
+			/*
+			 * Round up size of text segment to multiple of BITS_PER_LONG.
+			 * Otherwise, we cannot track
+			 * the last (text_size % BITS_PER_LONG) addresses.
+			 */
+			text_size = roundup(text_size, BITS_PER_LONG);
+			/* Get the amount of bytes needed */
+			text_size = text_size / 8;
+			/* mmap() requires size to be a multiple of PAGE_SIZE */
+			text_size = roundup(text_size, PAGE_SIZE);
+			/* Get the cover size (= amount of bytes stored) */
+			ret = text_size;
+			kcov->size = text_size / sizeof(unsigned long);
+			kcov_debug("text size = 0x%lx, roundup = 0x%x, kcov->size = 0x%x\n",
+					((unsigned long)&_etext) - ((unsigned long)&_stext),
+					text_size,
+					kcov->size);
+			kcov->mode = KCOV_MODE_INIT_UNIQUE;
+		} else {
+			if (size < 2 || size > INT_MAX / sizeof(unsigned long))
+				return -EINVAL;
+			kcov->size = size;
+			kcov->mode = KCOV_MODE_INIT_TRACE;
+		}
+		return ret;
 	case KCOV_ENABLE:
 		/*
 		 * Enable coverage for the current task.
@@ -594,7 +641,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 		 * at task exit or voluntary by KCOV_DISABLE. After that it can
 		 * be enabled for another task.
 		 */
-		if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
+		if (!kcov->area)
 			return -EINVAL;
 		t = current;
 		if (kcov->t != NULL || t->kcov != NULL)
@@ -602,6 +649,11 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 		mode = kcov_get_mode(arg);
 		if (mode < 0)
 			return mode;
+		if (kcov->mode == KCOV_MODE_INIT_TRACE && mode == KCOV_MODE_UNIQUE_PC)
+			return -EINVAL;
+		if (kcov->mode == KCOV_MODE_INIT_UNIQUE &&
+		    (mode & (KCOV_MODE_TRACE_PC | KCOV_MODE_TRACE_CMP)))
+			return -EINVAL;
 		kcov_fault_in_area(kcov);
 		kcov->mode = mode;
 		kcov_start(t, kcov, kcov->size, kcov->area, kcov->mode,
@@ -622,7 +674,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 		kcov_put(kcov);
 		return 0;
 	case KCOV_REMOTE_ENABLE:
-		if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
+		if (kcov->mode != KCOV_MODE_INIT_TRACE || !kcov->area)
 			return -EINVAL;
 		t = current;
 		if (kcov->t != NULL || t->kcov != NULL)
-- 
2.30.2


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