[PATCH 3/8] uprobes: protected uprobe lifetime with SRCU

[Andrii Nakryiko]

To avoid unnecessarily taking a (brief) refcount on uprobe during
breakpoint handling in handle_swbp for entry uprobes, make find_uprobe()
not take refcount, but protect the lifetime of a uprobe instance with
RCU. This improves scalability, as refcount gets quite expensive due to
cache line bouncing between multiple CPUs.

Specifically, we utilize our own uprobe-specific SRCU instance for this
RCU protection. put_uprobe() will delay actual kfree() using call_srcu().

For now, uretprobe and single-stepping handling will still acquire
refcount as necessary. We'll address these issues in follow up patches
by making them use SRCU with timeout.

Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 kernel/events/uprobes.c | 93 ++++++++++++++++++++++++-----------------
 1 file changed, 55 insertions(+), 38 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 23dde3ec5b09..6d5c3f4b210f 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -41,6 +41,8 @@ static struct rb_root uprobes_tree = RB_ROOT;
 
 static DEFINE_RWLOCK(uprobes_treelock);	/* serialize rbtree access */
 
+DEFINE_STATIC_SRCU(uprobes_srcu);
+
 #define UPROBES_HASH_SZ	13
 /* serialize uprobe->pending_list */
 static struct mutex uprobes_mmap_mutex[UPROBES_HASH_SZ];
@@ -59,6 +61,7 @@ struct uprobe {
 	struct list_head	pending_list;
 	struct uprobe_consumer	*consumers;
 	struct inode		*inode;		/* Also hold a ref to inode */
+	struct rcu_head		rcu;
 	loff_t			offset;
 	loff_t			ref_ctr_offset;
 	unsigned long		flags;
@@ -612,6 +615,13 @@ static inline bool uprobe_is_active(struct uprobe *uprobe)
 	return !RB_EMPTY_NODE(&uprobe->rb_node);
 }
 
+static void uprobe_free_rcu(struct rcu_head *rcu)
+{
+	struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
+
+	kfree(uprobe);
+}
+
 static void put_uprobe(struct uprobe *uprobe)
 {
 	if (!refcount_dec_and_test(&uprobe->ref))
@@ -632,6 +642,8 @@ static void put_uprobe(struct uprobe *uprobe)
 	mutex_lock(&delayed_uprobe_lock);
 	delayed_uprobe_remove(uprobe, NULL);
 	mutex_unlock(&delayed_uprobe_lock);
+
+	call_srcu(&uprobes_srcu, &uprobe->rcu, uprobe_free_rcu);
 }
 
 static __always_inline
@@ -673,33 +685,25 @@ static inline int __uprobe_cmp(struct rb_node *a, const struct rb_node *b)
 	return uprobe_cmp(u->inode, u->offset, __node_2_uprobe(b));
 }
 
-static struct uprobe *__find_uprobe(struct inode *inode, loff_t offset)
+/*
+ * Assumes being inside RCU protected region.
+ * No refcount is taken on returned uprobe.
+ */
+static struct uprobe *find_uprobe_rcu(struct inode *inode, loff_t offset)
 {
 	struct __uprobe_key key = {
 		.inode = inode,
 		.offset = offset,
 	};
-	struct rb_node *node = rb_find(&key, &uprobes_tree, __uprobe_cmp_key);
-
-	if (node)
-		return try_get_uprobe(__node_2_uprobe(node));
+	struct rb_node *node;
 
-	return NULL;
-}
-
-/*
- * Find a uprobe corresponding to a given inode:offset
- * Acquires uprobes_treelock
- */
-static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
-{
-	struct uprobe *uprobe;
+	lockdep_assert(srcu_read_lock_held(&uprobes_srcu));
 
 	read_lock(&uprobes_treelock);
-	uprobe = __find_uprobe(inode, offset);
+	node = rb_find(&key, &uprobes_tree, __uprobe_cmp_key);
 	read_unlock(&uprobes_treelock);
 
-	return uprobe;
+	return node ? __node_2_uprobe(node) : NULL;
 }
 
 /*
@@ -1073,10 +1077,10 @@ register_for_each_vma(struct uprobe *uprobe, struct uprobe_consumer *new)
 			goto free;
 		/*
 		 * We take mmap_lock for writing to avoid the race with
-		 * find_active_uprobe() which takes mmap_lock for reading.
+		 * find_active_uprobe_rcu() which takes mmap_lock for reading.
 		 * Thus this install_breakpoint() can not make
-		 * is_trap_at_addr() true right after find_uprobe()
-		 * returns NULL in find_active_uprobe().
+		 * is_trap_at_addr() true right after find_uprobe_rcu()
+		 * returns NULL in find_active_uprobe_rcu().
 		 */
 		mmap_write_lock(mm);
 		vma = find_vma(mm, info->vaddr);
@@ -1885,9 +1889,13 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 		return;
 	}
 
+	/* we need to bump refcount to store uprobe in utask */
+	if (!try_get_uprobe(uprobe))
+		return;
+
 	ri = kmalloc(sizeof(struct return_instance), GFP_KERNEL);
 	if (!ri)
-		return;
+		goto fail;
 
 	trampoline_vaddr = uprobe_get_trampoline_vaddr();
 	orig_ret_vaddr = arch_uretprobe_hijack_return_addr(trampoline_vaddr, regs);
@@ -1914,11 +1922,7 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 		}
 		orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
 	}
-	 /*
-	  * uprobe's refcnt is positive, held by caller, so it's safe to
-	  * unconditionally bump it one more time here
-	  */
-	ri->uprobe = get_uprobe(uprobe);
+	ri->uprobe = uprobe;
 	ri->func = instruction_pointer(regs);
 	ri->stack = user_stack_pointer(regs);
 	ri->orig_ret_vaddr = orig_ret_vaddr;
@@ -1929,8 +1933,9 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 	utask->return_instances = ri;
 
 	return;
- fail:
+fail:
 	kfree(ri);
+	put_uprobe(uprobe);
 }
 
 /* Prepare to single-step probed instruction out of line. */
@@ -1945,9 +1950,14 @@ pre_ssout(struct uprobe *uprobe, struct pt_regs *regs, unsigned long bp_vaddr)
 	if (!utask)
 		return -ENOMEM;
 
+	if (!try_get_uprobe(uprobe))
+		return -EINVAL;
+
 	xol_vaddr = xol_get_insn_slot(uprobe);
-	if (!xol_vaddr)
-		return -ENOMEM;
+	if (!xol_vaddr) {
+		err = -ENOMEM;
+		goto err_out;
+	}
 
 	utask->xol_vaddr = xol_vaddr;
 	utask->vaddr = bp_vaddr;
@@ -1955,12 +1965,15 @@ pre_ssout(struct uprobe *uprobe, struct pt_regs *regs, unsigned long bp_vaddr)
 	err = arch_uprobe_pre_xol(&uprobe->arch, regs);
 	if (unlikely(err)) {
 		xol_free_insn_slot(current);
-		return err;
+		goto err_out;
 	}
 
 	utask->active_uprobe = uprobe;
 	utask->state = UTASK_SSTEP;
 	return 0;
+err_out:
+	put_uprobe(uprobe);
+	return err;
 }
 
 /*
@@ -2044,7 +2057,8 @@ static int is_trap_at_addr(struct mm_struct *mm, unsigned long vaddr)
 	return is_trap_insn(&opcode);
 }
 
-static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
+/* assumes being inside RCU protected region */
+static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swbp)
 {
 	struct mm_struct *mm = current->mm;
 	struct uprobe *uprobe = NULL;
@@ -2057,7 +2071,7 @@ static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
 			struct inode *inode = file_inode(vma->vm_file);
 			loff_t offset = vaddr_to_offset(vma, bp_vaddr);
 
-			uprobe = find_uprobe(inode, offset);
+			uprobe = find_uprobe_rcu(inode, offset);
 		}
 
 		if (!uprobe)
@@ -2201,13 +2215,15 @@ static void handle_swbp(struct pt_regs *regs)
 {
 	struct uprobe *uprobe;
 	unsigned long bp_vaddr;
-	int is_swbp;
+	int is_swbp, srcu_idx;
 
 	bp_vaddr = uprobe_get_swbp_addr(regs);
 	if (bp_vaddr == uprobe_get_trampoline_vaddr())
 		return uprobe_handle_trampoline(regs);
 
-	uprobe = find_active_uprobe(bp_vaddr, &is_swbp);
+	srcu_idx = srcu_read_lock(&uprobes_srcu);
+
+	uprobe = find_active_uprobe_rcu(bp_vaddr, &is_swbp);
 	if (!uprobe) {
 		if (is_swbp > 0) {
 			/* No matching uprobe; signal SIGTRAP. */
@@ -2223,6 +2239,7 @@ static void handle_swbp(struct pt_regs *regs)
 			 */
 			instruction_pointer_set(regs, bp_vaddr);
 		}
+		srcu_read_unlock(&uprobes_srcu, srcu_idx);
 		return;
 	}
 
@@ -2258,12 +2275,12 @@ static void handle_swbp(struct pt_regs *regs)
 	if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
 		goto out;
 
-	if (!pre_ssout(uprobe, regs, bp_vaddr))
-		return;
+	if (pre_ssout(uprobe, regs, bp_vaddr))
+		goto out;
 
-	/* arch_uprobe_skip_sstep() succeeded, or restart if can't singlestep */
 out:
-	put_uprobe(uprobe);
+	/* arch_uprobe_skip_sstep() succeeded, or restart if can't singlestep */
+	srcu_read_unlock(&uprobes_srcu, srcu_idx);
 }
 
 /*
-- 
2.43.0