Hi Christoph, Please try not to send html, many servers block such emails. Also adding the RCU list. On Wed, Aug 30, 2023 at 09:37:23AM -0700, Christoph Paasch wrote: > for the MPTCP upstream project, we are running syzkaller [1] continuously to > qualify our kernel changes. > > We found one issue with kmemleak and its handling of kfree_rcu. > > Specifically, it looks like kmemleak falsely reports memory-leaks when the > object is being freed by kfree_rcu after a certain grace-period. > > For example, https://github.com/multipath-tcp/mptcp_net-next/issues/398# > issuecomment-1584819482 shows how the syzkaller program reliably produces a > kmemleak report, although the object is not leaked (we confirmed that by simply > increasing MSECS_MIN_AGE to something larger than the grace-period). Not sure which RCU variant you are using but most likely the false positives are caused by the original reference to the object being lost and the pointer added to a new location that kmemleak does not track (e.g. bnode->records[] in the tree-based variant). A quick attempt (untested, not even compiled): diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 1449cb69a0e0..681a1eb7560a 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -53,6 +53,7 @@ #include <linux/sysrq.h> #include <linux/kprobes.h> #include <linux/gfp.h> +#include <linux/kmemleak.h> #include <linux/oom.h> #include <linux/smpboot.h> #include <linux/jiffies.h> @@ -2934,6 +2935,7 @@ drain_page_cache(struct kfree_rcu_cpu *krcp) llist_for_each_safe(pos, n, page_list) { free_page((unsigned long)pos); + kmemleak_free(pos); freed++; } @@ -2962,8 +2964,16 @@ kvfree_rcu_bulk(struct kfree_rcu_cpu *krcp, rcu_state.name, bnode->records[i], 0); vfree(bnode->records[i]); + /* avoid false negatives */ + kmemleak_erase(&bnode->records[i]); } } + /* + * Avoid kmemleak false negatives when such pointers are later + * re-allocated. + */ + for (i = 0; i < bnode->nr_records; i++) + kmemleak_erase(&bnode->records[i]); rcu_lock_release(&rcu_callback_map); } @@ -2972,8 +2982,10 @@ kvfree_rcu_bulk(struct kfree_rcu_cpu *krcp, bnode = NULL; raw_spin_unlock_irqrestore(&krcp->lock, flags); - if (bnode) + if (bnode) { free_page((unsigned long) bnode); + kmemleak_free(bnode); + } cond_resched_tasks_rcu_qs(); } @@ -3241,6 +3253,12 @@ static void fill_page_cache_func(struct work_struct *work) free_page((unsigned long) bnode); break; } + + /* + * Scan the bnode->records[] array until the objects are + * actually freed. + */ + kmemleak_alloc(bnode, PAGE_SIZE, 0, GFP_KERNEL); } atomic_set(&krcp->work_in_progress, 0); @@ -3308,6 +3326,11 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp, // scenarios. bnode = (struct kvfree_rcu_bulk_data *) __get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN); + /* + * Scan the bnode->records[] array until the objects are + * actually freed. + */ + kmemleak_alloc(bnode, PAGE_SIZE, 0, GFP_KERNEL); raw_spin_lock_irqsave(&(*krcp)->lock, *flags); } -- Catalin
