// SPDX-License-Identifier: GPL-2.0
/*
* KCSAN reporting.
*
* Copyright (C) 2019, Google LLC.
*/
#include <linux/debug_locks.h>
#include <linux/delay.h>
#include <linux/jiffies.h>
#include <linux/kallsyms.h>
#include <linux/kernel.h>
#include <linux/lockdep.h>
#include <linux/preempt.h>
#include <linux/printk.h>
#include <linux/sched.h>
#include <linux/spinlock.h>
#include <linux/stacktrace.h>
#include "kcsan.h"
#include "encoding.h"
/*
* Max. number of stack entries to show in the report.
*/
#define NUM_STACK_ENTRIES 64
/* Common access info. */
struct access_info {
const volatile void *ptr;
size_t size;
int access_type;
int task_pid;
int cpu_id;
unsigned long ip;
};
/*
* Other thread info: communicated from other racing thread to thread that set
* up the watchpoint, which then prints the complete report atomically.
*/
struct other_info {
struct access_info ai;
unsigned long stack_entries[NUM_STACK_ENTRIES];
int num_stack_entries;
/*
* Optionally pass @current. Typically we do not need to pass @current
* via @other_info since just @task_pid is sufficient. Passing @current
* has additional overhead.
*
* To safely pass @current, we must either use get_task_struct/
* put_task_struct, or stall the thread that populated @other_info.
*
* We cannot rely on get_task_struct/put_task_struct in case
* release_report() races with a task being released, and would have to
* free it in release_report(). This may result in deadlock if we want
* to use KCSAN on the allocators.
*
* Since we also want to reliably print held locks for
* CONFIG_KCSAN_VERBOSE, the current implementation stalls the thread
* that populated @other_info until it has been consumed.
*/
struct task_struct *task;
};
/*
* To never block any producers of struct other_info, we need as many elements
* as we have watchpoints (upper bound on concurrent races to report).
*/
static struct other_info other_infos[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1];
/*
* Information about reported races; used to rate limit reporting.
*/
struct report_time {
/*
* The last time the race was reported.
*/
unsigned long time;
/*
* The frames of the 2 threads; if only 1 thread is known, one frame
* will be 0.
*/
unsigned long frame1;
unsigned long frame2;
};
/*
* Since we also want to be able to debug allocators with KCSAN, to avoid
* deadlock, report_times cannot be dynamically resized with krealloc in
* rate_limit_report.
*
* Therefore, we use a fixed-size array, which at most will occupy a page. This
* still adequately rate limits reports, assuming that a) number of unique data
* races is not excessive, and b) occurrence of unique races within the
* same time window is limited.
*/
#define REPORT_TIMES_MAX (PAGE_SIZE / sizeof(struct report_time))
#define REPORT_TIMES_SIZE \
(CONFIG_KCSAN_REPORT_ONCE_IN_MS > REPORT_TIMES_MAX ? \
REPORT_TIMES_MAX : \
CONFIG_KCSAN_REPORT_ONCE_IN_MS)
static struct report_time report_times[REPORT_TIMES_SIZE];
/*
* Spinlock serializing report generation, and access to @other_infos. Although
* it could make sense to have a finer-grained locking story for @other_infos,
* report generation needs to be serialized either way, so not much is gained.
*/
static DEFINE_RAW_SPINLOCK(report_lock);
/*
* Checks if the race identified by thread frames frame1 and frame2 has
* been reported since (now - KCSAN_REPORT_ONCE_IN_MS).
*/
static bool rate_limit_report(unsigned long frame1, unsigned long frame2)
{
struct report_time *use_entry = &report_times[0];
unsigned long invalid_before;
int i;
BUILD_BUG_ON(CONFIG_KCSAN_REPORT_ONCE_IN_MS != 0 && REPORT_TIMES_SIZE == 0);
if (CONFIG_KCSAN_REPORT_ONCE_IN_MS == 0)
return false;
invalid_before = jiffies - msecs_to_jiffies(CONFIG_KCSAN_REPORT_ONCE_IN_MS);
/* Check if a matching race report exists. */
for (i = 0; i < REPORT_TIMES_SIZE; ++i) {
struct report_time *rt = &report_times[i];
/*
* Must always select an entry for use to store info as we
* cannot resize report_times; at the end of the scan, use_entry
* will be the oldest entry, which ideally also happened before
* KCSAN_REPORT_ONCE_IN_MS ago.
*/
if (time_before(rt->time, use_entry->time))
use_entry = rt;
/*
* Initially, no need to check any further as this entry as well
* as following entries have never been used.
*/
if (rt->time == 0)
break;
/* Check if entry expired. */
if (time_before(rt->time, invalid_before))
continue; /* before KCSAN_REPORT_ONCE_IN_MS ago */
/* Reported recently, check if race matches. */
if ((rt->frame1 == frame1 && rt->frame2 == frame2) ||
(rt->frame1 == frame2 && rt->frame2 == frame1))
return true;
}
use_entry->time = jiffies;
use_entry->frame1 = frame1;
use_entry->frame2 = frame2;
return false;
}
/*
* Special rules to skip reporting.
*/
static bool
skip_report(enum kcsan_value_change value_change, unsigned long top_frame)
{
/* Should never get here if value_change==FALSE. */
WARN_ON_ONCE(value_change == KCSAN_VALUE_CHANGE_FALSE);
/*
* The first call to skip_report always has value_change==TRUE, since we
* cannot know the value written of an instrumented access. For the 2nd
* call there are 6 cases with CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY:
*
* 1. read watchpoint, conflicting write (value_change==TRUE): report;
* 2. read watchpoint, conflicting write (value_change==MAYBE): skip;
* 3. write watchpoint, conflicting write (value_change==TRUE): report;
* 4. write watchpoint, conflicting write (value_change==MAYBE): skip;
* 5. write watchpoint, conflicting read (value_change==MAYBE): skip;
* 6. write watchpoint, conflicting read (value_change==TRUE): report;
*
* Cases 1-4 are intuitive and expected; case 5 ensures we do not report
* data races where the write may have rewritten the same value; case 6
* is possible either if the size is larger than what we check value
* changes for or the access type is KCSAN_ACCESS_ASSERT.
*/
if (IS_ENABLED(CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY) &&
value_change == KCSAN_VALUE_CHANGE_MAYBE) {
/*
* The access is a write, but the data value did not change.
*
* We opt-out of this filter for certain functions at request of
* maintainers.
*/
char buf[64];
int len = scnprintf(buf, sizeof(buf), "%ps", (void *)top_frame);
if (!strnstr(buf, "rcu_", len) &&
!strnstr(buf, "_rcu", len) &&
!strnstr(buf, "_srcu", len))
return true;
}
return kcsan_skip_report_debugfs(top_frame);
}
static const char *get_access_type(int type)
{
if (type & KCSAN_ACCESS_ASSERT) {
if (type & KCSAN_ACCESS_SCOPED) {
if (type & KCSAN_ACCESS_WRITE)
return "assert no accesses (reordered)";
else
return "assert no writes (reordered)";
} else {
if (type & KCSAN_ACCESS_WRITE)
return "assert no accesses";
else
return "assert no writes";
}
}
switch (type) {
case 0:
return "read";
case KCSAN_ACCESS_ATOMIC:
return "read (marked)";
case KCSAN_ACCESS_WRITE:
return "write";
case KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
return "write (marked)";
case KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE:
return "read-write";
case KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
return "read-write (marked)";
case KCSAN_ACCESS_SCOPED:
return "read (reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_ATOMIC:
return "read (marked, reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE:
return "write (reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
return "write (marked, reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE:
return "read-write (reordered)";
case KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
return "read-write (marked, reordered)";
default:
BUG();
}
}
static const char *get_bug_type(int type)
{
return (type & KCSAN_ACCESS_ASSERT) != 0 ? "assert: race" : "data-race";
}
/* Return thread description: in task or interrupt. */
static const char *get_thread_desc(int task_id)
{
if (task_id != -1) {
static char buf[32]; /* safe: protected by report_lock */
snprintf(buf, sizeof(buf), "task %i", task_id);
return buf;
}
return "interrupt";
}
/* Helper to skip KCSAN-related functions in stack-trace. */
static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries)
{
char buf[64];
char *cur;
int len, skip;
for (skip = 0; skip < num_entries; ++skip) {
len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skip]);
/* Never show tsan_* or {read,write}_once_size. */
if (strnstr(buf, "tsan_", len) ||
strnstr(buf, "_once_size", len))
continue;
cur = strnstr(buf, "kcsan_", len);
if (cur) {
cur += strlen("kcsan_");
if (!str_has_prefix(cur, "test"))
continue; /* KCSAN runtime function. */
/* KCSAN related test. */
}
/*
* No match for runtime functions -- @skip entries to skip to
* get to first frame of interest.
*/
break;
}
return skip;
}
/*
* Skips to the first entry that matches the function of @ip, and then replaces
* that entry with @ip, returning the entries to skip with @replaced containing
* the replaced entry.
*/
static int
replace_stack_entry(unsigned long stack_entries[], int num_entries, unsigned long ip,
unsigned long *replaced)
{
unsigned long symbolsize, offset;
unsigned long target_func;
int skip;
if (kallsyms_lookup_size_offset(ip, &symbolsize, &offset))
target_func = ip - offset;
else
goto fallback;
for (skip = 0; skip < num_entries; ++skip) {
unsigned long func = stack_entries[skip];
if (!kallsyms_lookup_size_offset(func, &symbolsize, &offset))
goto fallback;
func -= offset;
if (func == target_func) {
*replaced = stack_entries[skip];
stack_entries[skip] = ip;
return skip;
}
}
fallback:
/* Should not happen; the resulting stack trace is likely misleading. */
WARN_ONCE(1, "Cannot find frame for %pS in stack trace", (void *)ip);
return get_stack_skipnr(stack_entries, num_entries);
}
static int
sanitize_stack_entries(unsigned long stack_entries[], int num_entries, unsigned long ip,
unsigned long *replaced)
{
return ip ? replace_stack_entry(stack_entries, num_entries, ip, replaced) :
get_stack_skipnr(stack_entries, num_entries);
}
/* Compares symbolized strings of addr1 and addr2. */
static int sym_strcmp(void *addr1, void *addr2)
{
char buf1[64];
char buf2[64];
snprintf(buf1, sizeof(buf1), "%pS", addr1);
snprintf(buf2, sizeof(buf2), "%pS", addr2);
return strncmp(buf1, buf2, sizeof(buf1));
}
static void
print_stack_trace(unsigned long stack_entries[], int num_entries, unsigned long reordered_to)
{
stack_trace_print(stack_entries, num_entries, 0);
if (reordered_to)
pr_err(" |\n +-> reordered to: %pS\n", (void *)reordered_to);
}
static void print_verbose_info(struct task_struct *task)
{
if (!task)
return;
/* Restore IRQ state trace for printing. */
kcsan_restore_irqtrace(task);
pr_err("\n");
debug_show_held_locks(task);
print_irqtrace_events(task);
}
static void print_report(enum kcsan_value_change value_change,
const struct access_info *ai,
struct other_info *other_info,
u64 old, u64 new, u64 mask)
{
unsigned long reordered_to = 0;
unsigned long stack_entries[NUM_STACK_ENTRIES] = { 0 };
int num_stack_entries = stack_trace_save(stack_entries, NUM_STACK_ENTRIES, 1);
int skipnr = sanitize_stack_entries(stack_entries, num_stack_entries, ai->ip, &reordered_to);
unsigned long this_frame = stack_entries[skipnr];
unsigned long other_reordered_to = 0;
unsigned long other_frame = 0;
int other_skipnr = 0; /* silence uninit warnings */
/*
* Must check report filter rules before starting to print.
*/
if (skip_report(KCSAN_VALUE_CHANGE_TRUE, stack_entries[skipnr]))
return;
if (other_info) {
other_skipnr = sanitize_stack_entries(other_info->stack_entries,
other_info->num_stack_entries,
other_info->ai.ip, &other_reordered_to);
other_frame = other_info->stack_entries[other_skipnr];
/* @value_change is only known for the other thread */
if (skip_report(value_change, other_frame))
return;
}
if (rate_limit_report(this_frame, other_frame))
return;
/* Print report header. */
pr_err("==================================================================\n");
if (other_info) {
int cmp;
/*
* Order functions lexographically for consistent bug titles.
* Do not print offset of functions to keep title short.
*/
cmp = sym_strcmp((void *)other_frame, (void *)this_frame);
pr_err("BUG: KCSAN: %s in %ps / %ps\n",
get_bug_type(ai->access_type | other_info->ai.access_type),
(void *)(cmp < 0 ? other_frame : this_frame),
(void *)(cmp < 0 ? this_frame : other_frame));
} else {
pr_err("BUG: KCSAN: %s in %pS\n", get_bug_type(ai->access_type),
(void *)this_frame);
}
pr_err("\n");
/* Print information about the racing accesses. */
if (other_info) {
pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
get_access_type(other_info->ai.access_type), other_info->ai.ptr,
other_info->ai.size, get_thread_desc(other_info->ai.task_pid),
other_info->ai.cpu_id);
/* Print the other thread's stack trace. */
print_stack_trace(other_info->stack_entries + other_skipnr,
other_info->num_stack_entries - other_skipnr,
other_reordered_to);
if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
print_verbose_info(other_info->task);
pr_err("\n");
pr_err("%s to 0x%px of %zu bytes by %s on cpu %i:\n",
get_access_type(ai->access_type), ai->ptr, ai->size,
get_thread_desc(ai->task_pid), ai->cpu_id);
} else {
pr_err("race at unknown origin, with %s to 0x%px of %zu bytes by %s on cpu %i:\n",
get_access_type(ai->access_type), ai->ptr, ai->size,
get_thread_desc(ai->task_pid), ai->cpu_id);
}
/* Print stack trace of this thread. */
print_stack_trace(stack_entries + skipnr, num_stack_entries - skipnr, reordered_to);
if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
print_verbose_info(current);
/* Print observed value change. */
if (ai->size <= 8) {
int hex_len = ai->size * 2;
u64 diff = old ^ new;
if (mask)
diff &= mask;
if (diff) {
pr_err("\n");
pr_err("value changed: 0x%0*llx -> 0x%0*llx\n",
hex_len, old, hex_len, new);
if (mask) {
pr_err(" bits changed: 0x%0*llx with mask 0x%0*llx\n",
hex_len, diff, hex_len, mask);
}
}
}
/* Print report footer. */
pr_err("\n");
pr_err("Reported by Kernel Concurrency Sanitizer on:\n");
dump_stack_print_info(KERN_DEFAULT);
pr_err("==================================================================\n");
check_panic_on_warn("KCSAN");
}
static void release_report(unsigned long *flags, struct other_info *other_info)
{
/*
* Use size to denote valid/invalid, since KCSAN entirely ignores
* 0-sized accesses.
*/
other_info->ai.size = 0;
raw_spin_unlock_irqrestore(&report_lock, *flags);
}
/*
* Sets @other_info->task and awaits consumption of @other_info.
*
* Precondition: report_lock is held.
* Postcondition: report_lock is held.
*/
static void set_other_info_task_blocking(unsigned long *flags,
const struct access_info *ai,
struct other_info *other_info)
{
/*
* We may be instrumenting a code-path where current->state is already
* something other than TASK_RUNNING.
*/
const bool is_running = task_is_running(current);
/*
* To avoid deadlock in case we are in an interrupt here and this is a
* race with a task on the same CPU (KCSAN_INTERRUPT_WATCHER), provide a
* timeout to ensure this works in all contexts.
*
* Await approximately the worst case delay of the reporting thread (if
* we are not interrupted).
*/
int timeout = max(kcsan_udelay_task, kcsan_udelay_interrupt);
other_info->task = current;
do {
if (is_running) {
/*
* Let lockdep know the real task is sleeping, to print
* the held locks (recall we turned lockdep off, so
* locking/unlocking @report_lock won't be recorded).
*/
set_current_state(TASK_UNINTERRUPTIBLE);
}
raw_spin_unlock_irqrestore(&report_lock, *flags);
/*
* We cannot call schedule() since we also cannot reliably
* determine if sleeping here is permitted -- see in_atomic().
*/
udelay(1);
raw_spin_lock_irqsave(&report_lock, *flags);
if (timeout-- < 0) {
/*
* Abort. Reset @other_info->task to NULL, since it
* appears the other thread is still going to consume
* it. It will result in no verbose info printed for
* this task.
*/
other_info->task = NULL;
break;
}
/*
* If invalid, or @ptr nor @current matches, then @other_info
* has been consumed and we may continue. If not, retry.
*/
} while (other_info->ai.size && other_info->ai.ptr == ai->ptr &&
other_info->task == current);
if (is_running)
set_current_state(TASK_RUNNING);
}
/* Populate @other_info; requires that the provided @other_info not in use. */
static void prepare_report_producer(unsigned long *flags,
const struct access_info *ai,
struct other_info *other_info)
{
raw_spin_lock_irqsave(&report_lock, *flags);
/*
* The same @other_infos entry cannot be used concurrently, because
* there is a one-to-one mapping to watchpoint slots (@watchpoints in
* core.c), and a watchpoint is only released for reuse after reporting
* is done by the consumer of @other_info. Therefore, it is impossible
* for another concurrent prepare_report_producer() to set the same
* @other_info, and are guaranteed exclusivity for the @other_infos
* entry pointed to by @other_info.
*
* To check this property holds, size should never be non-zero here,
* because every consumer of struct other_info resets size to 0 in
* release_report().
*/
WARN_ON(other_info->ai.size);
other_info->ai = *ai;
other_info->num_stack_entries = stack_trace_save(other_info->stack_entries, NUM_STACK_ENTRIES, 2);
if (IS_ENABLED(CONFIG_KCSAN_VERBOSE))
set_other_info_task_blocking(flags, ai, other_info);
raw_spin_unlock_irqrestore(&report_lock, *flags);
}
/* Awaits producer to fill @other_info and then returns. */
static bool prepare_report_consumer(unsigned long *flags,
const struct access_info *ai,
struct other_info *other_info)
{
raw_spin_lock_irqsave(&report_lock, *flags);
while (!other_info->ai.size) { /* Await valid @other_info. */
raw_spin_unlock_irqrestore(&report_lock, *flags);
cpu_relax();
raw_spin_lock_irqsave(&report_lock, *flags);
}
/* Should always have a matching access based on watchpoint encoding. */
if (WARN_ON(!matching_access((unsigned long)other_info->ai.ptr & WATCHPOINT_ADDR_MASK, other_info->ai.size,
(unsigned long)ai->ptr & WATCHPOINT_ADDR_MASK, ai->size)))
goto discard;
if (!matching_access((unsigned long)other_info->ai.ptr, other_info->ai.size,
(unsigned long)ai->ptr, ai->size)) {
/*
* If the actual accesses to not match, this was a false
* positive due to watchpoint encoding.
*/
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ENCODING_FALSE_POSITIVES]);
goto discard;
}
return true;
discard:
release_report(flags, other_info);
return false;
}
static struct access_info prepare_access_info(const volatile void *ptr, size_t size,
int access_type, unsigned long ip)
{
return (struct access_info) {
.ptr = ptr,
.size = size,
.access_type = access_type,
.task_pid = in_task() ? task_pid_nr(current) : -1,
.cpu_id = raw_smp_processor_id(),
/* Only replace stack entry with @ip if scoped access. */
.ip = (access_type & KCSAN_ACCESS_SCOPED) ? ip : 0,
};
}
void kcsan_report_set_info(const volatile void *ptr, size_t size, int access_type,
unsigned long ip, int watchpoint_idx)
{
const struct access_info ai = prepare_access_info(ptr, size, access_type, ip);
unsigned long flags;
kcsan_disable_current();
lockdep_off(); /* See kcsan_report_known_origin(). */
prepare_report_producer(&flags, &ai, &other_infos[watchpoint_idx]);
lockdep_on();
kcsan_enable_current();
}
void kcsan_report_known_origin(const volatile void *ptr, size_t size, int access_type,
unsigned long ip, enum kcsan_value_change value_change,
int watchpoint_idx, u64 old, u64 new, u64 mask)
{
const struct access_info ai = prepare_access_info(ptr, size, access_type, ip);
struct other_info *other_info = &other_infos[watchpoint_idx];
unsigned long flags = 0;
kcsan_disable_current();
/*
* Because we may generate reports when we're in scheduler code, the use
* of printk() could deadlock. Until such time that all printing code
* called in print_report() is scheduler-safe, accept the risk, and just
* get our message out. As such, also disable lockdep to hide the
* warning, and avoid disabling lockdep for the rest of the kernel.
*/
lockdep_off();
if (!prepare_report_consumer(&flags, &ai, other_info))
goto out;
/*
* Never report if value_change is FALSE, only when it is
* either TRUE or MAYBE. In case of MAYBE, further filtering may
* be done once we know the full stack trace in print_report().
*/
if (value_change != KCSAN_VALUE_CHANGE_FALSE)
print_report(value_change, &ai, other_info, old, new, mask);
release_report(&flags, other_info);
out:
lockdep_on();
kcsan_enable_current();
}
void kcsan_report_unknown_origin(const volatile void *ptr, size_t size, int access_type,
unsigned long ip, u64 old, u64 new, u64 mask)
{
const struct access_info ai = prepare_access_info(ptr, size, access_type, ip);
unsigned long flags;
kcsan_disable_current();
lockdep_off(); /* See kcsan_report_known_origin(). */
raw_spin_lock_irqsave(&report_lock, flags);
print_report(KCSAN_VALUE_CHANGE_TRUE, &ai, NULL, old, new, mask);
raw_spin_unlock_irqrestore(&report_lock, flags);
lockdep_on();
kcsan_enable_current();
}