[PATCH] KCOV: Introduced tracing unique covered PCs
From: Alexander Lochmann
Date: Thu Feb 11 2021 - 03:14:48 EST
Introduced new tracing mode KCOV_MODE_UNIQUE.
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@xxxxxxxxxxxxxxxxxxxxx>
---
Documentation/dev-tools/kcov.rst | 80 ++++++++++++++++++++++++++++++++
include/linux/kcov.h | 4 +-
include/uapi/linux/kcov.h | 10 ++++
kernel/kcov.c | 67 ++++++++++++++++++++------
4 files changed, 147 insertions(+), 14 deletions(-)
diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
index 8548b0b04e43..4712a730a06a 100644
--- a/Documentation/dev-tools/kcov.rst
+++ b/Documentation/dev-tools/kcov.rst
@@ -127,6 +127,86 @@ 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 advise KCOV 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_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 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 a10e84707d82..aa0c8bcf8299 100644
--- a/include/linux/kcov.h
+++ b/include/linux/kcov.h
@@ -19,7 +19,9 @@ enum kcov_mode {
*/
KCOV_MODE_TRACE_PC = 2,
/* Collecting comparison operands mode. */
- KCOV_MODE_TRACE_CMP = 3,
+ KCOV_MODE_TRACE_CMP = 4,
+ /* Collecting unique covered PCs. Execution order is not saved. */
+ KCOV_MODE_UNIQUE_PC = 8,
};
#define KCOV_IN_CTXSW (1 << 30)
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 6b8368be89c8..8f00ba6e672a 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__)
@@ -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 (mode & needed_mode) && !(mode & KCOV_IN_CTXSW);
}
static notrace unsigned long canonicalize_ip(unsigned long ip)
@@ -191,18 +192,26 @@ 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;
t = current;
- if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
+ if (!check_kcov_mode(KCOV_MODE_TRACE_PC | KCOV_MODE_UNIQUE_PC, t))
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(t->kcov_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);
@@ -474,6 +483,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
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 +525,8 @@ 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 +574,13 @@ 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:
case KCOV_INIT_TRACE:
/*
* Enable kcov in trace mode and setup buffer size.
@@ -581,11 +594,39 @@ 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;
+ 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 /= 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 longs 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);
+ } else {
+ if (size < 2 || size > INT_MAX / sizeof(unsigned long))
+ return -EINVAL;
+ kcov->size = size;
+ }
kcov->mode = KCOV_MODE_INIT;
- return 0;
+ return ret;
case KCOV_ENABLE:
/*
* Enable coverage for the current task.
--
2.30.0