The existing trace mode stores PCs in execution order. This could lead to a buffer overflow if sufficient amonut of kernel code is executed. Thus, a user might not see all executed PCs. KCOV_MODE_UNIQUE favors completeness over execution order. While ignoring the execution order, it marks a PC as exectued by setting a bit representing that PC. 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. Prior to above changes, a comparison of the current kcov mode with KCOV_IN_CTXSW set would evaluate to false, and tracing does not take place. Since the kcov mode is now a bit mask, simply setting KCOV_IN_CTXSW isn't sufficient anymore for the comparison to turn false. With KCOV_IN_CTXSW being the MSB, the following comparison will be less than zero: ((int)(*mode & (KCOV_IN_CTXSW | needed_mode))) > 0 This restores the previous semantics: When KCOV_IN_CTXSW is set, tracing does not take place. We have used this mode so far to perform coverage analysis on the Linux Test Project: http://dx.doi.org/10.18420/fgbs2020h-01. Since LTP's testsuites are considerably large, we experienced buffer overflows with KCOVE_MODE_TRACE_PC. Signed-off-by: Alexander Lochmann <info@xxxxxxxxxxxxxxxxxxxxx> --- Documentation/dev-tools/kcov.rst | 79 +++++++++++++++++++++++++++ include/linux/kcov.h | 12 +++-- include/uapi/linux/kcov.h | 10 ++++ kernel/kcov.c | 93 +++++++++++++++++++++++++------- 4 files changed, 171 insertions(+), 23 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 55dc338f6bcd..e2c0f7cc16a1 100644 --- a/include/linux/kcov.h +++ b/include/linux/kcov.h @@ -13,17 +13,21 @@ 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..578f07f28428 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 { + pos = (ip - canonicalize_ip((unsigned long)&_stext)) / 4; + idx = pos % BITS_PER_LONG; + pos /= BITS_PER_LONG; + if (likely(pos < t->kcov_size)) + WRITE_ONCE(area[pos], READ_ONCE(area[pos]) | 1L << idx); } } 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 +576,48 @@ 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; struct kcov_remote_arg *remote_arg; struct kcov_remote *remote; unsigned long flags; switch (cmd) { + case KCOV_INIT_UNIQUE: + /* + * Enable kcov in trace mode and setup buffer size. + * Must happen before anything else. + */ + if (kcov->mode != KCOV_MODE_DISABLED) + return -EBUSY; + size = arg; + 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) */ + 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; + return text_size; case KCOV_INIT_TRACE: /* * Enable kcov in trace mode and setup buffer size. @@ -584,7 +634,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd, if (size < 2 || size > INT_MAX / sizeof(unsigned long)) return -EINVAL; kcov->size = size; - kcov->mode = KCOV_MODE_INIT; + kcov->mode = KCOV_MODE_INIT_TRACE; return 0; case KCOV_ENABLE: /* @@ -594,7 +644,7 @@ 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 +652,11 @@ 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 +677,7 @@ 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) -- 2.33.0
