From: Marco Elver elver@google.com
[ Upstream commit 71a076f4a61a6c779794ad286f356b39725edc3b ]
Rewrite kcsan_prandom_u32_max() to not depend on code that might be instrumented, removing any dependency on lib/random32.c. The rewrite implements a simple linear congruential generator, that is sufficient for our purposes (for udelay() and skip_watch counter randomness).
The initial motivation for this was to allow enabling KCSAN for kernel/sched (remove KCSAN_SANITIZE := n from kernel/sched/Makefile), with CONFIG_DEBUG_PREEMPT=y. Without this change, we could observe recursion:
check_access() [via instrumentation] kcsan_setup_watchpoint() reset_kcsan_skip() kcsan_prandom_u32_max() get_cpu_var() preempt_disable() preempt_count_add() [in kernel/sched/core.c] check_access() [via instrumentation]
Note, while this currently does not affect an unmodified kernel, it'd be good to keep a KCSAN kernel working when KCSAN_SANITIZE := n is removed from kernel/sched/Makefile to permit testing scheduler code with KCSAN if desired.
Fixes: cd290ec24633 ("kcsan: Use tracing-safe version of prandom") Signed-off-by: Marco Elver elver@google.com Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/kcsan/core.c | 26 +++++++++++++------------- 1 file changed, 13 insertions(+), 13 deletions(-)
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c index 3994a217bde76..3bf98db9c702d 100644 --- a/kernel/kcsan/core.c +++ b/kernel/kcsan/core.c @@ -12,7 +12,6 @@ #include <linux/moduleparam.h> #include <linux/percpu.h> #include <linux/preempt.h> -#include <linux/random.h> #include <linux/sched.h> #include <linux/uaccess.h>
@@ -101,7 +100,7 @@ static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1]; static DEFINE_PER_CPU(long, kcsan_skip);
/* For kcsan_prandom_u32_max(). */ -static DEFINE_PER_CPU(struct rnd_state, kcsan_rand_state); +static DEFINE_PER_CPU(u32, kcsan_rand_state);
static __always_inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size, @@ -275,20 +274,17 @@ should_watch(const volatile void *ptr, size_t size, int type, struct kcsan_ctx * }
/* - * Returns a pseudo-random number in interval [0, ep_ro). See prandom_u32_max() - * for more details. - * - * The open-coded version here is using only safe primitives for all contexts - * where we can have KCSAN instrumentation. In particular, we cannot use - * prandom_u32() directly, as its tracepoint could cause recursion. + * 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) { - struct rnd_state *state = &get_cpu_var(kcsan_rand_state); - const u32 res = prandom_u32_state(state); + u32 state = this_cpu_read(kcsan_rand_state); + + state = 1664525 * state + 1013904223; + this_cpu_write(kcsan_rand_state, state);
- put_cpu_var(kcsan_rand_state); - return (u32)(((u64) res * ep_ro) >> 32); + return state % ep_ro; }
static inline void reset_kcsan_skip(void) @@ -639,10 +635,14 @@ static __always_inline void check_access(const volatile void *ptr, size_t size,
void __init kcsan_init(void) { + int cpu; + BUG_ON(!in_task());
kcsan_debugfs_init(); - prandom_seed_full_state(&kcsan_rand_state); + + 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;