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/* SPDX-License-Identifier: GPL-2.0 */
/*
* KCSAN access checks and modifiers. These can be used to explicitly check
* uninstrumented accesses, or change KCSAN checking behaviour of accesses.
*
* Copyright (C) 2019, Google LLC.
*/
#ifndef _LINUX_KCSAN_CHECKS_H
#define _LINUX_KCSAN_CHECKS_H
/* Note: Only include what is already included by compiler.h. */
#include <linux/compiler_attributes.h>
#include <linux/types.h>
/* Access types -- if KCSAN_ACCESS_WRITE is not set, the access is a read. */
#define KCSAN_ACCESS_WRITE (1 << 0) /* Access is a write. */
#define KCSAN_ACCESS_COMPOUND (1 << 1) /* Compounded read-write instrumentation. */
#define KCSAN_ACCESS_ATOMIC (1 << 2) /* Access is atomic. */
/* The following are special, and never due to compiler instrumentation. */
#define KCSAN_ACCESS_ASSERT (1 << 3) /* Access is an assertion. */
#define KCSAN_ACCESS_SCOPED (1 << 4) /* Access is a scoped access. */
/*
* __kcsan_*: Always calls into the runtime when KCSAN is enabled. This may be used
* even in compilation units that selectively disable KCSAN, but must use KCSAN
* to validate access to an address. Never use these in header files!
*/
#ifdef CONFIG_KCSAN
/**
* __kcsan_check_access - check generic access for races
*
* @ptr: address of access
* @size: size of access
* @type: access type modifier
*/
void __kcsan_check_access(const volatile void *ptr, size_t size, int type);
/**
* kcsan_disable_current - disable KCSAN for the current context
*
* Supports nesting.
*/
void kcsan_disable_current(void);
/**
* kcsan_enable_current - re-enable KCSAN for the current context
*
* Supports nesting.
*/
void kcsan_enable_current(void);
void kcsan_enable_current_nowarn(void); /* Safe in uaccess regions. */
/**
* kcsan_nestable_atomic_begin - begin nestable atomic region
*
* Accesses within the atomic region may appear to race with other accesses but
* should be considered atomic.
*/
void kcsan_nestable_atomic_begin(void);
/**
* kcsan_nestable_atomic_end - end nestable atomic region
*/
void kcsan_nestable_atomic_end(void);
/**
* kcsan_flat_atomic_begin - begin flat atomic region
*
* Accesses within the atomic region may appear to race with other accesses but
* should be considered atomic.
*/
void kcsan_flat_atomic_begin(void);
/**
* kcsan_flat_atomic_end - end flat atomic region
*/
void kcsan_flat_atomic_end(void);
/**
* kcsan_atomic_next - consider following accesses as atomic
*
* Force treating the next n memory accesses for the current context as atomic
* operations.
*
* @n: number of following memory accesses to treat as atomic.
*/
void kcsan_atomic_next(int n);
/**
* kcsan_set_access_mask - set access mask
*
* Set the access mask for all accesses for the current context if non-zero.
* Only value changes to bits set in the mask will be reported.
*
* @mask: bitmask
*/
void kcsan_set_access_mask(unsigned long mask);
/* Scoped access information. */
struct kcsan_scoped_access {
struct list_head list;
const volatile void *ptr;
size_t size;
int type;
};
/*
* Automatically call kcsan_end_scoped_access() when kcsan_scoped_access goes
* out of scope; relies on attribute "cleanup", which is supported by all
* compilers that support KCSAN.
*/
#define __kcsan_cleanup_scoped \
__maybe_unused __attribute__((__cleanup__(kcsan_end_scoped_access)))
/**
* kcsan_begin_scoped_access - begin scoped access
*
* Begin scoped access and initialize @sa, which will cause KCSAN to
* continuously check the memory range in the current thread until
* kcsan_end_scoped_access() is called for @sa.
*
* Scoped accesses are implemented by appending @sa to an internal list for the
* current execution context, and then checked on every call into the KCSAN
* runtime.
*
* @ptr: address of access
* @size: size of access
* @type: access type modifier
* @sa: struct kcsan_scoped_access to use for the scope of the access
*/
struct kcsan_scoped_access *
kcsan_begin_scoped_access(const volatile void *ptr, size_t size, int type,
struct kcsan_scoped_access *sa);
/**
* kcsan_end_scoped_access - end scoped access
*
* End a scoped access, which will stop KCSAN checking the memory range.
* Requires that kcsan_begin_scoped_access() was previously called once for @sa.
*
* @sa: a previously initialized struct kcsan_scoped_access
*/
void kcsan_end_scoped_access(struct kcsan_scoped_access *sa);
#else /* CONFIG_KCSAN */
static inline void __kcsan_check_access(const volatile void *ptr, size_t size,
int type) { }
static inline void kcsan_disable_current(void) { }
static inline void kcsan_enable_current(void) { }
static inline void kcsan_enable_current_nowarn(void) { }
static inline void kcsan_nestable_atomic_begin(void) { }
static inline void kcsan_nestable_atomic_end(void) { }
static inline void kcsan_flat_atomic_begin(void) { }
static inline void kcsan_flat_atomic_end(void) { }
static inline void kcsan_atomic_next(int n) { }
static inline void kcsan_set_access_mask(unsigned long mask) { }
struct kcsan_scoped_access { };
#define __kcsan_cleanup_scoped __maybe_unused
static inline struct kcsan_scoped_access *
kcsan_begin_scoped_access(const volatile void *ptr, size_t size, int type,
struct kcsan_scoped_access *sa) { return sa; }
static inline void kcsan_end_scoped_access(struct kcsan_scoped_access *sa) { }
#endif /* CONFIG_KCSAN */
#ifdef __SANITIZE_THREAD__
/*
* Only calls into the runtime when the particular compilation unit has KCSAN
* instrumentation enabled. May be used in header files.
*/
#define kcsan_check_access __kcsan_check_access
/*
* Only use these to disable KCSAN for accesses in the current compilation unit;
* calls into libraries may still perform KCSAN checks.
*/
#define __kcsan_disable_current kcsan_disable_current
#define __kcsan_enable_current kcsan_enable_current_nowarn
#else
static inline void kcsan_check_access(const volatile void *ptr, size_t size,
int type) { }
static inline void __kcsan_enable_current(void) { }
static inline void __kcsan_disable_current(void) { }
#endif
/**
* __kcsan_check_read - check regular read access for races
*
* @ptr: address of access
* @size: size of access
*/
#define __kcsan_check_read(ptr, size) __kcsan_check_access(ptr, size, 0)
/**
* __kcsan_check_write - check regular write access for races
*
* @ptr: address of access
* @size: size of access
*/
#define __kcsan_check_write(ptr, size) \
__kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
/**
* __kcsan_check_read_write - check regular read-write access for races
*
* @ptr: address of access
* @size: size of access
*/
#define __kcsan_check_read_write(ptr, size) \
__kcsan_check_access(ptr, size, KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE)
/**
* kcsan_check_read - check regular read access for races
*
* @ptr: address of access
* @size: size of access
*/
#define kcsan_check_read(ptr, size) kcsan_check_access(ptr, size, 0)
/**
* kcsan_check_write - check regular write access for races
*
* @ptr: address of access
* @size: size of access
*/
#define kcsan_check_write(ptr, size) \
kcsan_check_access(ptr, size, KCSAN_ACCESS_WRITE)
/**
* kcsan_check_read_write - check regular read-write access for races
*
* @ptr: address of access
* @size: size of access
*/
#define kcsan_check_read_write(ptr, size) \
kcsan_check_access(ptr, size, KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_WRITE)
/*
* Check for atomic accesses: if atomic accesses are not ignored, this simply
* aliases to kcsan_check_access(), otherwise becomes a no-op.
*/
#ifdef CONFIG_KCSAN_IGNORE_ATOMICS
#define kcsan_check_atomic_read(...) do { } while (0)
#define kcsan_check_atomic_write(...) do { } while (0)
#define kcsan_check_atomic_read_write(...) do { } while (0)
#else
#define kcsan_check_atomic_read(ptr, size) \
kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC)
#define kcsan_check_atomic_write(ptr, size) \
kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
#define kcsan_check_atomic_read_write(ptr, size) \
kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND)
#endif
/**
* ASSERT_EXCLUSIVE_WRITER - assert no concurrent writes to @var
*
* Assert that there are no concurrent writes to @var; other readers are
* allowed. This assertion can be used to specify properties of concurrent code,
* where violation cannot be detected as a normal data race.
*
* For example, if we only have a single writer, but multiple concurrent
* readers, to avoid data races, all these accesses must be marked; even
* concurrent marked writes racing with the single writer are bugs.
* Unfortunately, due to being marked, they are no longer data races. For cases
* like these, we can use the macro as follows:
*
* .. code-block:: c
*
* void writer(void) {
* spin_lock(&update_foo_lock);
* ASSERT_EXCLUSIVE_WRITER(shared_foo);
* WRITE_ONCE(shared_foo, ...);
* spin_unlock(&update_foo_lock);
* }
* void reader(void) {
* // update_foo_lock does not need to be held!
* ... = READ_ONCE(shared_foo);
* }
*
* Note: ASSERT_EXCLUSIVE_WRITER_SCOPED(), if applicable, performs more thorough
* checking if a clear scope where no concurrent writes are expected exists.
*
* @var: variable to assert on
*/
#define ASSERT_EXCLUSIVE_WRITER(var) \
__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
/*
* Helper macros for implementation of for ASSERT_EXCLUSIVE_*_SCOPED(). @id is
* expected to be unique for the scope in which instances of kcsan_scoped_access
* are declared.
*/
#define __kcsan_scoped_name(c, suffix) __kcsan_scoped_##c##suffix
#define __ASSERT_EXCLUSIVE_SCOPED(var, type, id) \
struct kcsan_scoped_access __kcsan_scoped_name(id, _) \
__kcsan_cleanup_scoped; \
struct kcsan_scoped_access *__kcsan_scoped_name(id, _dummy_p) \
__maybe_unused = kcsan_begin_scoped_access( \
&(var), sizeof(var), KCSAN_ACCESS_SCOPED | (type), \
&__kcsan_scoped_name(id, _))
/**
* ASSERT_EXCLUSIVE_WRITER_SCOPED - assert no concurrent writes to @var in scope
*
* Scoped variant of ASSERT_EXCLUSIVE_WRITER().
*
* Assert that there are no concurrent writes to @var for the duration of the
* scope in which it is introduced. This provides a better way to fully cover
* the enclosing scope, compared to multiple ASSERT_EXCLUSIVE_WRITER(), and
* increases the likelihood for KCSAN to detect racing accesses.
*
* For example, it allows finding race-condition bugs that only occur due to
* state changes within the scope itself:
*
* .. code-block:: c
*
* void writer(void) {
* spin_lock(&update_foo_lock);
* {
* ASSERT_EXCLUSIVE_WRITER_SCOPED(shared_foo);
* WRITE_ONCE(shared_foo, 42);
* ...
* // shared_foo should still be 42 here!
* }
* spin_unlock(&update_foo_lock);
* }
* void buggy(void) {
* if (READ_ONCE(shared_foo) == 42)
* WRITE_ONCE(shared_foo, 1); // bug!
* }
*
* @var: variable to assert on
*/
#define ASSERT_EXCLUSIVE_WRITER_SCOPED(var) \
__ASSERT_EXCLUSIVE_SCOPED(var, KCSAN_ACCESS_ASSERT, __COUNTER__)
/**
* ASSERT_EXCLUSIVE_ACCESS - assert no concurrent accesses to @var
*
* Assert that there are no concurrent accesses to @var (no readers nor
* writers). This assertion can be used to specify properties of concurrent
* code, where violation cannot be detected as a normal data race.
*
* For example, where exclusive access is expected after determining no other
* users of an object are left, but the object is not actually freed. We can
* check that this property actually holds as follows:
*
* .. code-block:: c
*
* if (refcount_dec_and_test(&obj->refcnt)) {
* ASSERT_EXCLUSIVE_ACCESS(*obj);
* do_some_cleanup(obj);
* release_for_reuse(obj);
* }
*
* Note:
*
* 1. ASSERT_EXCLUSIVE_ACCESS_SCOPED(), if applicable, performs more thorough
* checking if a clear scope where no concurrent accesses are expected exists.
*
* 2. For cases where the object is freed, `KASAN <kasan.html>`_ is a better
* fit to detect use-after-free bugs.
*
* @var: variable to assert on
*/
#define ASSERT_EXCLUSIVE_ACCESS(var) \
__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
/**
* ASSERT_EXCLUSIVE_ACCESS_SCOPED - assert no concurrent accesses to @var in scope
*
* Scoped variant of ASSERT_EXCLUSIVE_ACCESS().
*
* Assert that there are no concurrent accesses to @var (no readers nor writers)
* for the entire duration of the scope in which it is introduced. This provides
* a better way to fully cover the enclosing scope, compared to multiple
* ASSERT_EXCLUSIVE_ACCESS(), and increases the likelihood for KCSAN to detect
* racing accesses.
*
* @var: variable to assert on
*/
#define ASSERT_EXCLUSIVE_ACCESS_SCOPED(var) \
__ASSERT_EXCLUSIVE_SCOPED(var, KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT, __COUNTER__)
/**
* ASSERT_EXCLUSIVE_BITS - assert no concurrent writes to subset of bits in @var
*
* Bit-granular variant of ASSERT_EXCLUSIVE_WRITER().
*
* Assert that there are no concurrent writes to a subset of bits in @var;
* concurrent readers are permitted. This assertion captures more detailed
* bit-level properties, compared to the other (word granularity) assertions.
* Only the bits set in @mask are checked for concurrent modifications, while
* ignoring the remaining bits, i.e. concurrent writes (or reads) to ~mask bits
* are ignored.
*
* Use this for variables, where some bits must not be modified concurrently,
* yet other bits are expected to be modified concurrently.
*
* For example, variables where, after initialization, some bits are read-only,
* but other bits may still be modified concurrently. A reader may wish to
* assert that this is true as follows:
*
* .. code-block:: c
*
* ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
* foo = (READ_ONCE(flags) & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
*
* Note: The access that immediately follows ASSERT_EXCLUSIVE_BITS() is assumed
* to access the masked bits only, and KCSAN optimistically assumes it is
* therefore safe, even in the presence of data races, and marking it with
* READ_ONCE() is optional from KCSAN's point-of-view. We caution, however, that
* it may still be advisable to do so, since we cannot reason about all compiler
* optimizations when it comes to bit manipulations (on the reader and writer
* side). If you are sure nothing can go wrong, we can write the above simply
* as:
*
* .. code-block:: c
*
* ASSERT_EXCLUSIVE_BITS(flags, READ_ONLY_MASK);
* foo = (flags & READ_ONLY_MASK) >> READ_ONLY_SHIFT;
*
* Another example, where this may be used, is when certain bits of @var may
* only be modified when holding the appropriate lock, but other bits may still
* be modified concurrently. Writers, where other bits may change concurrently,
* could use the assertion as follows:
*
* .. code-block:: c
*
* spin_lock(&foo_lock);
* ASSERT_EXCLUSIVE_BITS(flags, FOO_MASK);
* old_flags = flags;
* new_flags = (old_flags & ~FOO_MASK) | (new_foo << FOO_SHIFT);
* if (cmpxchg(&flags, old_flags, new_flags) != old_flags) { ... }
* spin_unlock(&foo_lock);
*
* @var: variable to assert on
* @mask: only check for modifications to bits set in @mask
*/
#define ASSERT_EXCLUSIVE_BITS(var, mask) \
do { \
kcsan_set_access_mask(mask); \
__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT);\
kcsan_set_access_mask(0); \
kcsan_atomic_next(1); \
} while (0)
#endif /* _LINUX_KCSAN_CHECKS_H */
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