Re: [PATCH v2 tip/perf/core 5/5] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution
From: Oleg Nesterov <oleg@redhat.com>
Date: 2024-10-02 07:25:55
Also in:
bpf, linux-mm, lkml
On 10/01, Andrii Nakryiko wrote:
+static struct uprobe *find_active_uprobe_speculative(unsigned long bp_vaddr)
+{
+ struct mm_struct *mm = current->mm;
+ struct uprobe *uprobe = NULL;
+ struct vm_area_struct *vma;
+ struct file *vm_file;
+ loff_t offset;
+ long seq;
+
+ guard(rcu)();
+
+ if (!mmap_lock_speculation_start(mm, &seq))
+ return NULL;
+
+ vma = vma_lookup(mm, bp_vaddr);
+ if (!vma)
+ return NULL;
+
+ /* vm_file memory can be reused for another instance of struct file,
+ * but can't be freed from under us, so it's safe to read fields from
+ * it, even if the values are some garbage values; ultimately
+ * find_uprobe_rcu() + mmap_lock_speculation_end() check will ensure
+ * that whatever we speculatively found is correct
+ */
+ vm_file = READ_ONCE(vma->vm_file);
+ if (!vm_file)
+ return NULL;
+
+ offset = (loff_t)(vma->vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vma->vm_start);LGTM. But perhaps vma->vm_pgoff and vma->vm_start need READ_ONCE() as well, if nothing else to shut up KCSAN if this code races with, say, __split_vma() ?
+ uprobe = find_uprobe_rcu(vm_file->f_inode, offset);
OK, I guess vm_file->f_inode is fine without READ_ONCE... Oleg.