[tip: perf/core] uprobes: protected uprobe lifetime with SRCU

tip-bot2 for Andrii Nakryiko
2024-09-05T15:03:29Z

The following commit has been merged into the perf/core branch of tip:

Commit-ID: 8617408f7a01e94ce1f73e40a7704530e5dfb25c
Gitweb: https://git.kernel.org/tip/8617408f7a01e94ce1f73e40a7704530e5dfb25c
Author: Andrii Nakryiko <andrii@kernel.org>
AuthorDate: Tue, 03 Sep 2024 10:45:57 -07:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 05 Sep 2024 16:56:13 +02:00

uprobes: protected uprobe lifetime with SRCU

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@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20240903174603.3554182-3-andrii@kernel.org
---
kernel/events/uprobes.c | 94 ++++++++++++++++++++++------------------
1 file changed, 54 insertions(+), 40 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index cd92e8d..d228d2b 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;
@@ -617,6 +620,13 @@ 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))
@@ -638,7 +648,7 @@ static void put_uprobe(struct uprobe *uprobe)
delayed_uprobe_remove(uprobe, NULL);
mutex_unlock(&delayed_uprobe_lock);

- kfree(uprobe);
+ call_srcu(&uprobes_srcu, &uprobe->rcu, uprobe_free_rcu);
}

static __always_inline
@@ -680,33 +690,25 @@ 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));
-
- return NULL;
-}
+ struct rb_node *node;

-/*
- * 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;
}

/*
@@ -1080,10 +1082,10 @@ 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);
@@ -1884,9 +1886,13 @@ 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);
@@ -1913,11 +1919,7 @@ 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;
@@ -1928,8 +1930,9 @@ 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. */
@@ -1944,9 +1947,14 @@ 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;
@@ -1954,12 +1962,15 @@ 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;
}

/*
@@ -2043,7 +2054,8 @@ 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;
@@ -2056,7 +2068,7 @@ 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)
@@ -2202,13 +2214,15 @@ 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. */
@@ -2224,7 +2238,7 @@ static void handle_swbp(struct pt_regs *regs)
*/
instruction_pointer_set(regs, bp_vaddr);
}
- return;
+ goto out;
}

/* change it in advance for ->handler() and restart */
@@ -2259,12 +2273,12 @@ 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);
}

/*