Commit 9756f64c authored by Marco Elver's avatar Marco Elver Committed by Paul E. McKenney
Browse files

kcsan: Avoid checking scoped accesses from nested contexts 



Avoid checking scoped accesses from nested contexts (such as nested
interrupts or in scheduler code) which share the same kcsan_ctx.

This is to avoid detecting false positive races of accesses in the same
thread with currently scoped accesses: consider setting up a watchpoint
for a non-scoped (normal) access that also "conflicts" with a current
scoped access. In a nested interrupt (or in the scheduler), which shares
the same kcsan_ctx, we cannot check scoped accesses set up in the parent
context -- simply ignore them in this case.

With the introduction of kcsan_ctx::disable_scoped, we can also clean up
kcsan_check_scoped_accesses()'s recursion guard, and do not need to
modify the list's prev pointer.

Signed-off-by: default avatarMarco Elver <elver@google.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 71f8de70

include/linux/kcsan.h

+1 −0
Original line number Diff line number Diff line
@@ -21,6 +21,7 @@ 
 */

struct kcsan_ctx {

	int disable_count; /* disable counter */

	int disable_scoped; /* disable scoped access counter */

	int atomic_next; /* number of following atomic ops */



	/*

kernel/kcsan/core.c

+15 −3
Original line number Diff line number Diff line
@@ -204,15 +204,17 @@ check_access(const volatile void *ptr, size_t size, int type, unsigned long ip); 
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. */

	if (ctx->disable_scoped)

		return;



	ctx->disable_scoped++;

	list_for_each_entry(scoped_access, &ctx->scoped_accesses, list) {

		check_access(scoped_access->ptr, scoped_access->size,

			     scoped_access->type, scoped_access->ip);

	}

	ctx->scoped_accesses.prev = prev_save;

	ctx->disable_scoped--;

}



/* Rules for generic atomic accesses. Called from fast-path. */
@@ -465,6 +467,15 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned 
		goto out;

	}



	/*

	 * Avoid races of scoped accesses from nested interrupts (or scheduler).

	 * Assume setting up a watchpoint for a non-scoped (normal) access that

	 * also conflicts with a current scoped access. In a nested interrupt,

	 * which shares the context, it would check a conflicting scoped access.

	 * To avoid, disable scoped access checking.

	 */

	ctx->disable_scoped++;



	/*

	 * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's

	 * runtime is entered for every memory access, and potentially useful
@@ -578,6 +589,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned 
	if (!kcsan_interrupt_watcher)

		local_irq_restore(irq_flags);

	kcsan_restore_irqtrace(current);

	ctx->disable_scoped--;

out:

	user_access_restore(ua_flags);

}