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linux/kernel/kcsan/core.c
Mark Rutland 7bbe6dc0ad kcsan: Report observed value changes 
When a thread detects that a memory location was modified without its
watchpoint being hit, the report notes that a change was detected, but
does not provide concrete values for the change. Knowing the concrete
values can be very helpful in tracking down any racy writers (e.g. as
specific values may only be written in some portions of code, or under
certain conditions).

When we detect a modification, let's report the concrete old/new values,
along with the access's mask of relevant bits (and which relevant bits
were modified). This can make it easier to identify potential racy
writers. As the snapshots are at most 8 bytes, we can only report values
for acceses up to this size, but this appears to cater for the common
case.

When we detect a race via a watchpoint, we may or may not have concrete
values for the modification. To be helpful, let's attempt to log them
when we do as they can be ignored where irrelevant.

The resulting reports appears as follows, with values zero-padded to the
access width:

| ==================================================================
| BUG: KCSAN: data-race in el0_svc_common+0x34/0x25c arch/arm64/kernel/syscall.c:96
|
| race at unknown origin, with read to 0xffff00007ae6aa00 of 8 bytes by task 223 on cpu 1:
|  el0_svc_common+0x34/0x25c arch/arm64/kernel/syscall.c:96
|  do_el0_svc+0x48/0xec arch/arm64/kernel/syscall.c:178
|  el0_svc arch/arm64/kernel/entry-common.c:226 [inline]
|  el0_sync_handler+0x1a4/0x390 arch/arm64/kernel/entry-common.c:236
|  el0_sync+0x140/0x180 arch/arm64/kernel/entry.S:674
|
| value changed: 0x0000000000000000 -> 0x0000000000000002
|
| Reported by Kernel Concurrency Sanitizer on:
| CPU: 1 PID: 223 Comm: syz-executor.1 Not tainted 5.8.0-rc3-00094-ga73f923ecc8e-dirty #3
| Hardware name: linux,dummy-virt (DT)
| ==================================================================

If an access mask is set, it is shown underneath the "value changed"
line as "bits changed: 0x<bits changed> with mask 0x<non-zero mask>".

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
[ elver@google.com: align "value changed" and "bits changed" lines,
  which required massaging the message; do not print bits+mask if no
  mask set. ]
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2021-05-18 10:58:15 -07:00

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// SPDX-License-Identifier: GPL-2.0
/*
* KCSAN core runtime.
*
* Copyright (C) 2019, Google LLC.
*/
#define pr_fmt(fmt) "kcsan: " fmt
#include <linux/atomic.h>
#include <linux/bug.h>
#include <linux/delay.h>
#include <linux/export.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/moduleparam.h>
#include <linux/percpu.h>
#include <linux/preempt.h>
#include <linux/sched.h>
#include <linux/uaccess.h>
#include "atomic.h"
#include "encoding.h"
#include "kcsan.h"
static bool kcsan_early_enable = IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE);
unsigned int kcsan_udelay_task = CONFIG_KCSAN_UDELAY_TASK;
unsigned int kcsan_udelay_interrupt = CONFIG_KCSAN_UDELAY_INTERRUPT;
static long kcsan_skip_watch = CONFIG_KCSAN_SKIP_WATCH;
static bool kcsan_interrupt_watcher = IS_ENABLED(CONFIG_KCSAN_INTERRUPT_WATCHER);
#ifdef MODULE_PARAM_PREFIX
#undef MODULE_PARAM_PREFIX
#endif
#define MODULE_PARAM_PREFIX "kcsan."
module_param_named(early_enable, kcsan_early_enable, bool, 0);
module_param_named(udelay_task, kcsan_udelay_task, uint, 0644);
module_param_named(udelay_interrupt, kcsan_udelay_interrupt, uint, 0644);
module_param_named(skip_watch, kcsan_skip_watch, long, 0644);
module_param_named(interrupt_watcher, kcsan_interrupt_watcher, bool, 0444);
bool kcsan_enabled;
/* Per-CPU kcsan_ctx for interrupts */
static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
.disable_count = 0,
.atomic_next = 0,
.atomic_nest_count = 0,
.in_flat_atomic = false,
.access_mask = 0,
.scoped_accesses = {LIST_POISON1, NULL},
};
/*
* Helper macros to index into adjacent slots, starting from address slot
* itself, followed by the right and left slots.
*
* The purpose is 2-fold:
*
* 1. if during insertion the address slot is already occupied, check if
* any adjacent slots are free;
* 2. accesses that straddle a slot boundary due to size that exceeds a
* slot's range may check adjacent slots if any watchpoint matches.
*
* Note that accesses with very large size may still miss a watchpoint; however,
* given this should be rare, this is a reasonable trade-off to make, since this
* will avoid:
*
* 1. excessive contention between watchpoint checks and setup;
* 2. larger number of simultaneous watchpoints without sacrificing
* performance.
*
* Example: SLOT_IDX values for KCSAN_CHECK_ADJACENT=1, where i is [0, 1, 2]:
*
* slot=0: [ 1, 2, 0]
* slot=9: [10, 11, 9]
* slot=63: [64, 65, 63]
*/
#define SLOT_IDX(slot, i) (slot + ((i + KCSAN_CHECK_ADJACENT) % NUM_SLOTS))
/*
* SLOT_IDX_FAST is used in the fast-path. Not first checking the address's primary
* slot (middle) is fine if we assume that races occur rarely. The set of
* indices {SLOT_IDX(slot, i) | i in [0, NUM_SLOTS)} is equivalent to
* {SLOT_IDX_FAST(slot, i) | i in [0, NUM_SLOTS)}.
*/
#define SLOT_IDX_FAST(slot, i) (slot + i)
/*
* Watchpoints, with each entry encoded as defined in encoding.h: in order to be
* able to safely update and access a watchpoint without introducing locking
* overhead, we encode each watchpoint as a single atomic long. The initial
* zero-initialized state matches INVALID_WATCHPOINT.
*
* Add NUM_SLOTS-1 entries to account for overflow; this helps avoid having to
* use more complicated SLOT_IDX_FAST calculation with modulo in the fast-path.
*/
static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1];
/*
* Instructions to skip watching counter, used in should_watch(). We use a
* per-CPU counter to avoid excessive contention.
*/
static DEFINE_PER_CPU(long, kcsan_skip);
/* For kcsan_prandom_u32_max(). */
static DEFINE_PER_CPU(u32, kcsan_rand_state);
static __always_inline atomic_long_t *find_watchpoint(unsigned long addr,
size_t size,
bool expect_write,
long *encoded_watchpoint)
{
const int slot = watchpoint_slot(addr);
const unsigned long addr_masked = addr & WATCHPOINT_ADDR_MASK;
atomic_long_t *watchpoint;
unsigned long wp_addr_masked;
size_t wp_size;
bool is_write;
int i;
BUILD_BUG_ON(CONFIG_KCSAN_NUM_WATCHPOINTS < NUM_SLOTS);
for (i = 0; i < NUM_SLOTS; ++i) {
watchpoint = &watchpoints[SLOT_IDX_FAST(slot, i)];
*encoded_watchpoint = atomic_long_read(watchpoint);
if (!decode_watchpoint(*encoded_watchpoint, &wp_addr_masked,
&wp_size, &is_write))
continue;
if (expect_write && !is_write)
continue;
/* Check if the watchpoint matches the access. */
if (matching_access(wp_addr_masked, wp_size, addr_masked, size))
return watchpoint;
}
return NULL;
}
static inline atomic_long_t *
insert_watchpoint(unsigned long addr, size_t size, bool is_write)
{
const int slot = watchpoint_slot(addr);
const long encoded_watchpoint = encode_watchpoint(addr, size, is_write);
atomic_long_t *watchpoint;
int i;
/* Check slot index logic, ensuring we stay within array bounds. */
BUILD_BUG_ON(SLOT_IDX(0, 0) != KCSAN_CHECK_ADJACENT);
BUILD_BUG_ON(SLOT_IDX(0, KCSAN_CHECK_ADJACENT+1) != 0);
BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT) != ARRAY_SIZE(watchpoints)-1);
BUILD_BUG_ON(SLOT_IDX(CONFIG_KCSAN_NUM_WATCHPOINTS-1, KCSAN_CHECK_ADJACENT+1) != ARRAY_SIZE(watchpoints) - NUM_SLOTS);
for (i = 0; i < NUM_SLOTS; ++i) {
long expect_val = INVALID_WATCHPOINT;
/* Try to acquire this slot. */
watchpoint = &watchpoints[SLOT_IDX(slot, i)];
if (atomic_long_try_cmpxchg_relaxed(watchpoint, &expect_val, encoded_watchpoint))
return watchpoint;
}
return NULL;
}
/*
* Return true if watchpoint was successfully consumed, false otherwise.
*
* This may return false if:
*
* 1. another thread already consumed the watchpoint;
* 2. the thread that set up the watchpoint already removed it;
* 3. the watchpoint was removed and then re-used.
*/
static __always_inline bool
try_consume_watchpoint(atomic_long_t *watchpoint, long encoded_watchpoint)
{
return atomic_long_try_cmpxchg_relaxed(watchpoint, &encoded_watchpoint, CONSUMED_WATCHPOINT);
}
/* Return true if watchpoint was not touched, false if already consumed. */
static inline bool consume_watchpoint(atomic_long_t *watchpoint)
{
return atomic_long_xchg_relaxed(watchpoint, CONSUMED_WATCHPOINT) != CONSUMED_WATCHPOINT;
}
/* Remove the watchpoint -- its slot may be reused after. */
static inline void remove_watchpoint(atomic_long_t *watchpoint)
{
atomic_long_set(watchpoint, INVALID_WATCHPOINT);
}
static __always_inline struct kcsan_ctx *get_ctx(void)
{
/*
* In interrupts, use raw_cpu_ptr to avoid unnecessary checks, that would
* also result in calls that generate warnings in uaccess regions.
*/
return in_task() ? &current->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx);
}
/* Check scoped accesses; never inline because this is a slow-path! */
static noinline void kcsan_check_scoped_accesses(void)
{
struct kcsan_ctx *ctx = get_ctx();
struct list_head *prev_save = ctx->scoped_accesses.prev;
struct kcsan_scoped_access *scoped_access;
ctx->scoped_accesses.prev = NULL; /* Avoid recursion. */
list_for_each_entry(scoped_access, &ctx->scoped_accesses, list)
__kcsan_check_access(scoped_access->ptr, scoped_access->size, scoped_access->type);
ctx->scoped_accesses.prev = prev_save;
}
/* Rules for generic atomic accesses. Called from fast-path. */
static __always_inline bool
is_atomic(const volatile void *ptr, size_t size, int type, struct kcsan_ctx *ctx)
{
if (type & KCSAN_ACCESS_ATOMIC)
return true;
/*
* Unless explicitly declared atomic, never consider an assertion access
* as atomic. This allows using them also in atomic regions, such as
* seqlocks, without implicitly changing their semantics.
*/
if (type & KCSAN_ACCESS_ASSERT)
return false;
if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
(type & KCSAN_ACCESS_WRITE) && size <= sizeof(long) &&
!(type & KCSAN_ACCESS_COMPOUND) && IS_ALIGNED((unsigned long)ptr, size))
return true; /* Assume aligned writes up to word size are atomic. */
if (ctx->atomic_next > 0) {
/*
* Because we do not have separate contexts for nested
* interrupts, in case atomic_next is set, we simply assume that
* the outer interrupt set atomic_next. In the worst case, we
* will conservatively consider operations as atomic. This is a
* reasonable trade-off to make, since this case should be
* extremely rare; however, even if extremely rare, it could
* lead to false positives otherwise.
*/
if ((hardirq_count() >> HARDIRQ_SHIFT) < 2)
--ctx->atomic_next; /* in task, or outer interrupt */
return true;
}
return ctx->atomic_nest_count > 0 || ctx->in_flat_atomic;
}
static __always_inline bool
should_watch(const volatile void *ptr, size_t size, int type, struct kcsan_ctx *ctx)
{
/*
* Never set up watchpoints when memory operations are atomic.
*
* Need to check this first, before kcsan_skip check below: (1) atomics
* should not count towards skipped instructions, and (2) to actually
* decrement kcsan_atomic_next for consecutive instruction stream.
*/
if (is_atomic(ptr, size, type, ctx))
return false;
if (this_cpu_dec_return(kcsan_skip) >= 0)
return false;
/*
* NOTE: If we get here, kcsan_skip must always be reset in slow path
* via reset_kcsan_skip() to avoid underflow.
*/
/* this operation should be watched */
return true;
}
/*
* Returns a pseudo-random number in interval [0, ep_ro). Simple linear
* congruential generator, using constants from "Numerical Recipes".
*/
static u32 kcsan_prandom_u32_max(u32 ep_ro)
{
u32 state = this_cpu_read(kcsan_rand_state);
state = 1664525 * state + 1013904223;
this_cpu_write(kcsan_rand_state, state);
return state % ep_ro;
}
static inline void reset_kcsan_skip(void)
{
long skip_count = kcsan_skip_watch -
(IS_ENABLED(CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE) ?
kcsan_prandom_u32_max(kcsan_skip_watch) :
0);
this_cpu_write(kcsan_skip, skip_count);
}
static __always_inline bool kcsan_is_enabled(void)
{
return READ_ONCE(kcsan_enabled) && get_ctx()->disable_count == 0;
}
/* Introduce delay depending on context and configuration. */
static void delay_access(int type)
{
unsigned int delay = in_task() ? kcsan_udelay_task : kcsan_udelay_interrupt;
/* For certain access types, skew the random delay to be longer. */
unsigned int skew_delay_order =
(type & (KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_ASSERT)) ? 1 : 0;
delay -= IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
kcsan_prandom_u32_max(delay >> skew_delay_order) :
0;
udelay(delay);
}
void kcsan_save_irqtrace(struct task_struct *task)
{
#ifdef CONFIG_TRACE_IRQFLAGS
task->kcsan_save_irqtrace = task->irqtrace;
#endif
}
void kcsan_restore_irqtrace(struct task_struct *task)
{
#ifdef CONFIG_TRACE_IRQFLAGS
task->irqtrace = task->kcsan_save_irqtrace;
#endif
}
/*
* Pull everything together: check_access() below contains the performance
* critical operations; the fast-path (including check_access) functions should
* all be inlinable by the instrumentation functions.
*
* The slow-path (kcsan_found_watchpoint, kcsan_setup_watchpoint) are
* non-inlinable -- note that, we prefix these with "kcsan_" to ensure they can
* be filtered from the stacktrace, as well as give them unique names for the
* UACCESS whitelist of objtool. Each function uses user_access_save/restore(),
* since they do not access any user memory, but instrumentation is still
* emitted in UACCESS regions.
*/
static noinline void kcsan_found_watchpoint(const volatile void *ptr,
size_t size,
int type,
atomic_long_t *watchpoint,
long encoded_watchpoint)
{
unsigned long flags;
bool consumed;
if (!kcsan_is_enabled())
return;
/*
* The access_mask check relies on value-change comparison. To avoid
* reporting a race where e.g. the writer set up the watchpoint, but the
* reader has access_mask!=0, we have to ignore the found watchpoint.
*/
if (get_ctx()->access_mask != 0)
return;
/*
* Consume the watchpoint as soon as possible, to minimize the chances
* of !consumed. Consuming the watchpoint must always be guarded by
* kcsan_is_enabled() check, as otherwise we might erroneously
* triggering reports when disabled.
*/
consumed = try_consume_watchpoint(watchpoint, encoded_watchpoint);
/* keep this after try_consume_watchpoint */
flags = user_access_save();
if (consumed) {
kcsan_save_irqtrace(current);
kcsan_report_set_info(ptr, size, type, watchpoint - watchpoints);
kcsan_restore_irqtrace(current);
} else {
/*
* The other thread may not print any diagnostics, as it has
* already removed the watchpoint, or another thread consumed
* the watchpoint before this thread.
*/
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_REPORT_RACES]);
}
if ((type & KCSAN_ACCESS_ASSERT) != 0)
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
else
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_DATA_RACES]);
user_access_restore(flags);
}
static noinline void
kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
{
const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
atomic_long_t *watchpoint;
u64 old, new, diff;
unsigned long access_mask;
enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
unsigned long ua_flags = user_access_save();
unsigned long irq_flags = 0;
/*
* Always reset kcsan_skip counter in slow-path to avoid underflow; see
* should_watch().
*/
reset_kcsan_skip();
if (!kcsan_is_enabled())
goto out;
/*
* Special atomic rules: unlikely to be true, so we check them here in
* the slow-path, and not in the fast-path in is_atomic(). Call after
* kcsan_is_enabled(), as we may access memory that is not yet
* initialized during early boot.
*/
if (!is_assert && kcsan_is_atomic_special(ptr))
goto out;
if (!check_encodable((unsigned long)ptr, size)) {
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_UNENCODABLE_ACCESSES]);
goto out;
}
/*
* Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
* runtime is entered for every memory access, and potentially useful
* information is lost if dirtied by KCSAN.
*/
kcsan_save_irqtrace(current);
if (!kcsan_interrupt_watcher)
local_irq_save(irq_flags);
watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
if (watchpoint == NULL) {
/*
* Out of capacity: the size of 'watchpoints', and the frequency
* with which should_watch() returns true should be tweaked so
* that this case happens very rarely.
*/
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_NO_CAPACITY]);
goto out_unlock;
}
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_SETUP_WATCHPOINTS]);
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
/*
* Read the current value, to later check and infer a race if the data
* was modified via a non-instrumented access, e.g. from a device.
*/
old = 0;
switch (size) {
case 1:
old = READ_ONCE(*(const u8 *)ptr);
break;
case 2:
old = READ_ONCE(*(const u16 *)ptr);
break;
case 4:
old = READ_ONCE(*(const u32 *)ptr);
break;
case 8:
old = READ_ONCE(*(const u64 *)ptr);
break;
default:
break; /* ignore; we do not diff the values */
}
if (IS_ENABLED(CONFIG_KCSAN_DEBUG)) {
kcsan_disable_current();
pr_err("watching %s, size: %zu, addr: %px [slot: %d, encoded: %lx]\n",
is_write ? "write" : "read", size, ptr,
watchpoint_slot((unsigned long)ptr),
encode_watchpoint((unsigned long)ptr, size, is_write));
kcsan_enable_current();
}
/*
* Delay this thread, to increase probability of observing a racy
* conflicting access.
*/
delay_access(type);
/*
* Re-read value, and check if it is as expected; if not, we infer a
* racy access.
*/
access_mask = get_ctx()->access_mask;
new = 0;
switch (size) {
case 1:
new = READ_ONCE(*(const u8 *)ptr);
break;
case 2:
new = READ_ONCE(*(const u16 *)ptr);
break;
case 4:
new = READ_ONCE(*(const u32 *)ptr);
break;
case 8:
new = READ_ONCE(*(const u64 *)ptr);
break;
default:
break; /* ignore; we do not diff the values */
}
diff = old ^ new;
if (access_mask)
diff &= access_mask;
/* Were we able to observe a value-change? */
if (diff != 0)
value_change = KCSAN_VALUE_CHANGE_TRUE;
/* Check if this access raced with another. */
if (!consume_watchpoint(watchpoint)) {
/*
* Depending on the access type, map a value_change of MAYBE to
* TRUE (always report) or FALSE (never report).
*/
if (value_change == KCSAN_VALUE_CHANGE_MAYBE) {
if (access_mask != 0) {
/*
* For access with access_mask, we require a
* value-change, as it is likely that races on
* ~access_mask bits are expected.
*/
value_change = KCSAN_VALUE_CHANGE_FALSE;
} else if (size > 8 || is_assert) {
/* Always assume a value-change. */
value_change = KCSAN_VALUE_CHANGE_TRUE;
}
}
/*
* No need to increment 'data_races' counter, as the racing
* thread already did.
*
* Count 'assert_failures' for each failed ASSERT access,
* therefore both this thread and the racing thread may
* increment this counter.
*/
if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
kcsan_report_known_origin(ptr, size, type, value_change,
watchpoint - watchpoints,
old, new, access_mask);
} else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
/* Inferring a race, since the value should not have changed. */
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN]);
if (is_assert)
atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert)
kcsan_report_unknown_origin(ptr, size, type, old, new, access_mask);
}
/*
* Remove watchpoint; must be after reporting, since the slot may be
* reused after this point.
*/
remove_watchpoint(watchpoint);
atomic_long_dec(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
out_unlock:
if (!kcsan_interrupt_watcher)
local_irq_restore(irq_flags);
kcsan_restore_irqtrace(current);
out:
user_access_restore(ua_flags);
}
static __always_inline void check_access(const volatile void *ptr, size_t size,
int type)
{
const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
atomic_long_t *watchpoint;
long encoded_watchpoint;
/*
* Do nothing for 0 sized check; this comparison will be optimized out
* for constant sized instrumentation (__tsan_{read,write}N).
*/
if (unlikely(size == 0))
return;
/*
* Avoid user_access_save in fast-path: find_watchpoint is safe without
* user_access_save, as the address that ptr points to is only used to
* check if a watchpoint exists; ptr is never dereferenced.
*/
watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
&encoded_watchpoint);
/*
* It is safe to check kcsan_is_enabled() after find_watchpoint in the
* slow-path, as long as no state changes that cause a race to be
* detected and reported have occurred until kcsan_is_enabled() is
* checked.
*/
if (unlikely(watchpoint != NULL))
kcsan_found_watchpoint(ptr, size, type, watchpoint,
encoded_watchpoint);
else {
struct kcsan_ctx *ctx = get_ctx(); /* Call only once in fast-path. */
if (unlikely(should_watch(ptr, size, type, ctx)))
kcsan_setup_watchpoint(ptr, size, type);
else if (unlikely(ctx->scoped_accesses.prev))
kcsan_check_scoped_accesses();
}
}
/* === Public interface ===================================================== */
void __init kcsan_init(void)
{
int cpu;
BUG_ON(!in_task());
for_each_possible_cpu(cpu)
per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();
/*
* We are in the init task, and no other tasks should be running;
* WRITE_ONCE without memory barrier is sufficient.
*/
if (kcsan_early_enable) {
pr_info("enabled early\n");
WRITE_ONCE(kcsan_enabled, true);
}
}
/* === Exported interface =================================================== */
void kcsan_disable_current(void)
{
++get_ctx()->disable_count;
}
EXPORT_SYMBOL(kcsan_disable_current);
void kcsan_enable_current(void)
{
if (get_ctx()->disable_count-- == 0) {
/*
* Warn if kcsan_enable_current() calls are unbalanced with
* kcsan_disable_current() calls, which causes disable_count to
* become negative and should not happen.
*/
kcsan_disable_current(); /* restore to 0, KCSAN still enabled */
kcsan_disable_current(); /* disable to generate warning */
WARN(1, "Unbalanced %s()", __func__);
kcsan_enable_current();
}
}
EXPORT_SYMBOL(kcsan_enable_current);
void kcsan_enable_current_nowarn(void)
{
if (get_ctx()->disable_count-- == 0)
kcsan_disable_current();
}
EXPORT_SYMBOL(kcsan_enable_current_nowarn);
void kcsan_nestable_atomic_begin(void)
{
/*
* Do *not* check and warn if we are in a flat atomic region: nestable
* and flat atomic regions are independent from each other.
* See include/linux/kcsan.h: struct kcsan_ctx comments for more
* comments.
*/
++get_ctx()->atomic_nest_count;
}
EXPORT_SYMBOL(kcsan_nestable_atomic_begin);
void kcsan_nestable_atomic_end(void)
{
if (get_ctx()->atomic_nest_count-- == 0) {
/*
* Warn if kcsan_nestable_atomic_end() calls are unbalanced with
* kcsan_nestable_atomic_begin() calls, which causes
* atomic_nest_count to become negative and should not happen.
*/
kcsan_nestable_atomic_begin(); /* restore to 0 */
kcsan_disable_current(); /* disable to generate warning */
WARN(1, "Unbalanced %s()", __func__);
kcsan_enable_current();
}
}
EXPORT_SYMBOL(kcsan_nestable_atomic_end);
void kcsan_flat_atomic_begin(void)
{
get_ctx()->in_flat_atomic = true;
}
EXPORT_SYMBOL(kcsan_flat_atomic_begin);
void kcsan_flat_atomic_end(void)
{
get_ctx()->in_flat_atomic = false;
}
EXPORT_SYMBOL(kcsan_flat_atomic_end);
void kcsan_atomic_next(int n)
{
get_ctx()->atomic_next = n;
}
EXPORT_SYMBOL(kcsan_atomic_next);
void kcsan_set_access_mask(unsigned long mask)
{
get_ctx()->access_mask = mask;
}
EXPORT_SYMBOL(kcsan_set_access_mask);
struct kcsan_scoped_access *
kcsan_begin_scoped_access(const volatile void *ptr, size_t size, int type,
struct kcsan_scoped_access *sa)
{
struct kcsan_ctx *ctx = get_ctx();
__kcsan_check_access(ptr, size, type);
ctx->disable_count++; /* Disable KCSAN, in case list debugging is on. */
INIT_LIST_HEAD(&sa->list);
sa->ptr = ptr;
sa->size = size;
sa->type = type;
if (!ctx->scoped_accesses.prev) /* Lazy initialize list head. */
INIT_LIST_HEAD(&ctx->scoped_accesses);
list_add(&sa->list, &ctx->scoped_accesses);
ctx->disable_count--;
return sa;
}
EXPORT_SYMBOL(kcsan_begin_scoped_access);
void kcsan_end_scoped_access(struct kcsan_scoped_access *sa)
{
struct kcsan_ctx *ctx = get_ctx();
if (WARN(!ctx->scoped_accesses.prev, "Unbalanced %s()?", __func__))
return;
ctx->disable_count++; /* Disable KCSAN, in case list debugging is on. */
list_del(&sa->list);
if (list_empty(&ctx->scoped_accesses))
/*
* Ensure we do not enter kcsan_check_scoped_accesses()
* slow-path if unnecessary, and avoids requiring list_empty()
* in the fast-path (to avoid a READ_ONCE() and potential
* uaccess warning).
*/
ctx->scoped_accesses.prev = NULL;
ctx->disable_count--;
__kcsan_check_access(sa->ptr, sa->size, sa->type);
}
EXPORT_SYMBOL(kcsan_end_scoped_access);
void __kcsan_check_access(const volatile void *ptr, size_t size, int type)
{
check_access(ptr, size, type);
}
EXPORT_SYMBOL(__kcsan_check_access);
/*
* KCSAN uses the same instrumentation that is emitted by supported compilers
* for ThreadSanitizer (TSAN).
*
* When enabled, the compiler emits instrumentation calls (the functions
* prefixed with "__tsan" below) for all loads and stores that it generated;
* inline asm is not instrumented.
*
* Note that, not all supported compiler versions distinguish aligned/unaligned
* accesses, but e.g. recent versions of Clang do. We simply alias the unaligned
* version to the generic version, which can handle both.
*/
#define DEFINE_TSAN_READ_WRITE(size) \
void __tsan_read##size(void *ptr); \
void __tsan_read##size(void *ptr) \
{ \
check_access(ptr, size, 0); \
} \
EXPORT_SYMBOL(__tsan_read##size); \
void __tsan_unaligned_read##size(void *ptr) \
__alias(__tsan_read##size); \
EXPORT_SYMBOL(__tsan_unaligned_read##size); \
void __tsan_write##size(void *ptr); \
void __tsan_write##size(void *ptr) \
{ \
check_access(ptr, size, KCSAN_ACCESS_WRITE); \
} \
EXPORT_SYMBOL(__tsan_write##size); \
void __tsan_unaligned_write##size(void *ptr) \
__alias(__tsan_write##size); \
EXPORT_SYMBOL(__tsan_unaligned_write##size); \
void __tsan_read_write##size(void *ptr); \
void __tsan_read_write##size(void *ptr) \
{ \
check_access(ptr, size, \
KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE); \
} \
EXPORT_SYMBOL(__tsan_read_write##size); \
void __tsan_unaligned_read_write##size(void *ptr) \
__alias(__tsan_read_write##size); \
EXPORT_SYMBOL(__tsan_unaligned_read_write##size)
DEFINE_TSAN_READ_WRITE(1);
DEFINE_TSAN_READ_WRITE(2);
DEFINE_TSAN_READ_WRITE(4);
DEFINE_TSAN_READ_WRITE(8);
DEFINE_TSAN_READ_WRITE(16);
void __tsan_read_range(void *ptr, size_t size);
void __tsan_read_range(void *ptr, size_t size)
{
check_access(ptr, size, 0);
}
EXPORT_SYMBOL(__tsan_read_range);
void __tsan_write_range(void *ptr, size_t size);
void __tsan_write_range(void *ptr, size_t size)
{
check_access(ptr, size, KCSAN_ACCESS_WRITE);
}
EXPORT_SYMBOL(__tsan_write_range);
/*
* Use of explicit volatile is generally disallowed [1], however, volatile is
* still used in various concurrent context, whether in low-level
* synchronization primitives or for legacy reasons.
* [1] https://lwn.net/Articles/233479/
*
* We only consider volatile accesses atomic if they are aligned and would pass
* the size-check of compiletime_assert_rwonce_type().
*/
#define DEFINE_TSAN_VOLATILE_READ_WRITE(size) \
void __tsan_volatile_read##size(void *ptr); \
void __tsan_volatile_read##size(void *ptr) \
{ \
const bool is_atomic = size <= sizeof(long long) && \
IS_ALIGNED((unsigned long)ptr, size); \
if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic) \
return; \
check_access(ptr, size, is_atomic ? KCSAN_ACCESS_ATOMIC : 0); \
} \
EXPORT_SYMBOL(__tsan_volatile_read##size); \
void __tsan_unaligned_volatile_read##size(void *ptr) \
__alias(__tsan_volatile_read##size); \
EXPORT_SYMBOL(__tsan_unaligned_volatile_read##size); \
void __tsan_volatile_write##size(void *ptr); \
void __tsan_volatile_write##size(void *ptr) \
{ \
const bool is_atomic = size <= sizeof(long long) && \
IS_ALIGNED((unsigned long)ptr, size); \
if (IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS) && is_atomic) \
return; \
check_access(ptr, size, \
KCSAN_ACCESS_WRITE | \
(is_atomic ? KCSAN_ACCESS_ATOMIC : 0)); \
} \
EXPORT_SYMBOL(__tsan_volatile_write##size); \
void __tsan_unaligned_volatile_write##size(void *ptr) \
__alias(__tsan_volatile_write##size); \
EXPORT_SYMBOL(__tsan_unaligned_volatile_write##size)
DEFINE_TSAN_VOLATILE_READ_WRITE(1);
DEFINE_TSAN_VOLATILE_READ_WRITE(2);
DEFINE_TSAN_VOLATILE_READ_WRITE(4);
DEFINE_TSAN_VOLATILE_READ_WRITE(8);
DEFINE_TSAN_VOLATILE_READ_WRITE(16);
/*
* The below are not required by KCSAN, but can still be emitted by the
* compiler.
*/
void __tsan_func_entry(void *call_pc);
void __tsan_func_entry(void *call_pc)
{
}
EXPORT_SYMBOL(__tsan_func_entry);
void __tsan_func_exit(void);
void __tsan_func_exit(void)
{
}
EXPORT_SYMBOL(__tsan_func_exit);
void __tsan_init(void);
void __tsan_init(void)
{
}
EXPORT_SYMBOL(__tsan_init);
/*
* Instrumentation for atomic builtins (__atomic_*, __sync_*).
*
* Normal kernel code _should not_ be using them directly, but some
* architectures may implement some or all atomics using the compilers'
* builtins.
*
* Note: If an architecture decides to fully implement atomics using the
* builtins, because they are implicitly instrumented by KCSAN (and KASAN,
* etc.), implementing the ARCH_ATOMIC interface (to get instrumentation via
* atomic-instrumented) is no longer necessary.
*
* TSAN instrumentation replaces atomic accesses with calls to any of the below
* functions, whose job is to also execute the operation itself.
*/
#define DEFINE_TSAN_ATOMIC_LOAD_STORE(bits) \
u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder); \
u##bits __tsan_atomic##bits##_load(const u##bits *ptr, int memorder) \
{ \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, KCSAN_ACCESS_ATOMIC); \
} \
return __atomic_load_n(ptr, memorder); \
} \
EXPORT_SYMBOL(__tsan_atomic##bits##_load); \
void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder); \
void __tsan_atomic##bits##_store(u##bits *ptr, u##bits v, int memorder) \
{ \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC); \
} \
__atomic_store_n(ptr, v, memorder); \
} \
EXPORT_SYMBOL(__tsan_atomic##bits##_store)
#define DEFINE_TSAN_ATOMIC_RMW(op, bits, suffix) \
u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder); \
u##bits __tsan_atomic##bits##_##op(u##bits *ptr, u##bits v, int memorder) \
{ \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | \
KCSAN_ACCESS_ATOMIC); \
} \
return __atomic_##op##suffix(ptr, v, memorder); \
} \
EXPORT_SYMBOL(__tsan_atomic##bits##_##op)
/*
* Note: CAS operations are always classified as write, even in case they
* fail. We cannot perform check_access() after a write, as it might lead to
* false positives, in cases such as:
*
* T0: __atomic_compare_exchange_n(&p->flag, &old, 1, ...)
*
* T1: if (__atomic_load_n(&p->flag, ...)) {
* modify *p;
* p->flag = 0;
* }
*
* The only downside is that, if there are 3 threads, with one CAS that
* succeeds, another CAS that fails, and an unmarked racing operation, we may
* point at the wrong CAS as the source of the race. However, if we assume that
* all CAS can succeed in some other execution, the data race is still valid.
*/
#define DEFINE_TSAN_ATOMIC_CMPXCHG(bits, strength, weak) \
int __tsan_atomic##bits##_compare_exchange_##strength(u##bits *ptr, u##bits *exp, \
u##bits val, int mo, int fail_mo); \
int __tsan_atomic##bits##_compare_exchange_##strength(u##bits *ptr, u##bits *exp, \
u##bits val, int mo, int fail_mo) \
{ \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | \
KCSAN_ACCESS_ATOMIC); \
} \
return __atomic_compare_exchange_n(ptr, exp, val, weak, mo, fail_mo); \
} \
EXPORT_SYMBOL(__tsan_atomic##bits##_compare_exchange_##strength)
#define DEFINE_TSAN_ATOMIC_CMPXCHG_VAL(bits) \
u##bits __tsan_atomic##bits##_compare_exchange_val(u##bits *ptr, u##bits exp, u##bits val, \
int mo, int fail_mo); \
u##bits __tsan_atomic##bits##_compare_exchange_val(u##bits *ptr, u##bits exp, u##bits val, \
int mo, int fail_mo) \
{ \
if (!IS_ENABLED(CONFIG_KCSAN_IGNORE_ATOMICS)) { \
check_access(ptr, bits / BITS_PER_BYTE, \
KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE | \
KCSAN_ACCESS_ATOMIC); \
} \
__atomic_compare_exchange_n(ptr, &exp, val, 0, mo, fail_mo); \
return exp; \
} \
EXPORT_SYMBOL(__tsan_atomic##bits##_compare_exchange_val)
#define DEFINE_TSAN_ATOMIC_OPS(bits) \
DEFINE_TSAN_ATOMIC_LOAD_STORE(bits); \
DEFINE_TSAN_ATOMIC_RMW(exchange, bits, _n); \
DEFINE_TSAN_ATOMIC_RMW(fetch_add, bits, ); \
DEFINE_TSAN_ATOMIC_RMW(fetch_sub, bits, ); \
DEFINE_TSAN_ATOMIC_RMW(fetch_and, bits, ); \
DEFINE_TSAN_ATOMIC_RMW(fetch_or, bits, ); \
DEFINE_TSAN_ATOMIC_RMW(fetch_xor, bits, ); \
DEFINE_TSAN_ATOMIC_RMW(fetch_nand, bits, ); \
DEFINE_TSAN_ATOMIC_CMPXCHG(bits, strong, 0); \
DEFINE_TSAN_ATOMIC_CMPXCHG(bits, weak, 1); \
DEFINE_TSAN_ATOMIC_CMPXCHG_VAL(bits)
DEFINE_TSAN_ATOMIC_OPS(8);
DEFINE_TSAN_ATOMIC_OPS(16);
DEFINE_TSAN_ATOMIC_OPS(32);
DEFINE_TSAN_ATOMIC_OPS(64);
void __tsan_atomic_thread_fence(int memorder);
void __tsan_atomic_thread_fence(int memorder)
{
__atomic_thread_fence(memorder);
}
EXPORT_SYMBOL(__tsan_atomic_thread_fence);
void __tsan_atomic_signal_fence(int memorder);
void __tsan_atomic_signal_fence(int memorder) { }
EXPORT_SYMBOL(__tsan_atomic_signal_fence);
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