[RFC PATCH 06/16] bpf/crib: Introduce task_file open-coded iterator kfuncs

[Juntong Deng]

This patch adds the open-coded iterator style process file iterator
kfuncs bpf_iter_task_file_{new,next,destroy} that iterates over all
files opened by the specified process.

In addition, this patch adds bpf_iter_task_file_get_fd() getter to get
the file descriptor corresponding to the file in the current iteration.

The reference to struct file acquired by the previous
bpf_iter_task_file_next() is released in the next
bpf_iter_task_file_next(), and the last reference is released in the
last bpf_iter_task_file_next() that returns NULL.

In the bpf_iter_task_file_destroy(), if the iterator does not iterate to
the end, then the last struct file reference is released at this time.

Signed-off-by: Juntong Deng <juntong.deng@xxxxxxxxxxx>
---
 include/linux/bpf_crib.h         | 10 ++++
 kernel/bpf/crib/bpf_checkpoint.c | 88 ++++++++++++++++++++++++++++++++
 kernel/bpf/crib/bpf_crib.c       |  5 ++
 3 files changed, 103 insertions(+)

diff --git a/include/linux/bpf_crib.h b/include/linux/bpf_crib.h
index f667b740fcc2..468ae87fa1a5 100644
--- a/include/linux/bpf_crib.h
+++ b/include/linux/bpf_crib.h
@@ -13,4 +13,14 @@
 #include <linux/btf_ids.h>
 #include <linux/filter.h>
 
+struct bpf_iter_task_file {
+	__u64 __opaque[3];
+} __aligned(8);
+
+struct bpf_iter_task_file_kern {
+	struct task_struct *task;
+	struct file *file;
+	int fd;
+} __aligned(8);
+
 #endif /* _BPF_CRIB_H */
diff --git a/kernel/bpf/crib/bpf_checkpoint.c b/kernel/bpf/crib/bpf_checkpoint.c
index efaca6bcdfe4..28ad26986053 100644
--- a/kernel/bpf/crib/bpf_checkpoint.c
+++ b/kernel/bpf/crib/bpf_checkpoint.c
@@ -7,7 +7,95 @@
  */
 
 #include <linux/bpf_crib.h>
+#include <linux/fdtable.h>
+
+extern void bpf_file_release(struct file *file);
 
 __bpf_kfunc_start_defs();
 
+/**
+ * bpf_iter_task_file_new() - Initialize a new task file iterator for a task,
+ * used to iterate over all files opened by a specified task
+ *
+ * @it: The new bpf_iter_task_file to be created
+ * @task: A pointer pointing to a task to be iterated over
+ */
+__bpf_kfunc int bpf_iter_task_file_new(struct bpf_iter_task_file *it,
+		struct task_struct *task)
+{
+	struct bpf_iter_task_file_kern *kit = (void *)it;
+
+	BUILD_BUG_ON(sizeof(struct bpf_iter_task_file_kern) > sizeof(struct bpf_iter_task_file));
+	BUILD_BUG_ON(__alignof__(struct bpf_iter_task_file_kern) !=
+		     __alignof__(struct bpf_iter_task_file));
+
+	kit->task = task;
+	kit->fd = -1;
+	kit->file = NULL;
+
+	return 0;
+}
+
+/**
+ * bpf_iter_task_file_next() - Get the next file in bpf_iter_task_file
+ *
+ * bpf_iter_task_file_next() acquires a reference to the returned struct file.
+ *
+ * The reference to struct file acquired by the previous
+ * bpf_iter_task_file_next() is released in the next bpf_iter_task_file_next(),
+ * and the last reference is released in the last bpf_iter_task_file_next()
+ * that returns NULL.
+ *
+ * @it: The bpf_iter_task_file to be checked
+ *
+ * @returns a pointer to the struct file of the next file if further files
+ * are available, otherwise returns NULL.
+ */
+__bpf_kfunc struct file *bpf_iter_task_file_next(struct bpf_iter_task_file *it)
+{
+	struct bpf_iter_task_file_kern *kit = (void *)it;
+
+	if (kit->file)
+		bpf_file_release(kit->file);
+
+	kit->fd++;
+
+	rcu_read_lock();
+	kit->file = task_lookup_next_fdget_rcu(kit->task, &kit->fd);
+	rcu_read_unlock();
+
+	return kit->file;
+}
+
+/**
+ * bpf_iter_task_file_get_fd() - Get the file descriptor
+ * corresponding to the file in the current iteration
+ *
+ * @it: The bpf_iter_task_file to be checked
+ *
+ * @returns the file descriptor
+ */
+__bpf_kfunc int bpf_iter_task_file_get_fd(struct bpf_iter_task_file *it)
+{
+	struct bpf_iter_task_file_kern *kit = (void *)it;
+
+	return kit->fd;
+}
+
+/**
+ * bpf_iter_task_file_destroy() - Destroy a bpf_iter_task_file
+ *
+ * If the iterator does not iterate to the end, then the last
+ * struct file reference is released at this time.
+ *
+ * @it: The bpf_iter_task_file to be destroyed
+ */
+__bpf_kfunc void bpf_iter_task_file_destroy(struct bpf_iter_task_file *it)
+{
+	struct bpf_iter_task_file_kern *kit = (void *)it;
+
+	if (kit->file)
+		bpf_file_release(kit->file);
+}
+
 __bpf_kfunc_end_defs();
diff --git a/kernel/bpf/crib/bpf_crib.c b/kernel/bpf/crib/bpf_crib.c
index 1c1729ddf233..b901d7d60290 100644
--- a/kernel/bpf/crib/bpf_crib.c
+++ b/kernel/bpf/crib/bpf_crib.c
@@ -52,6 +52,11 @@ BTF_KFUNCS_START(bpf_crib_kfuncs)
 BTF_ID_FLAGS(func, bpf_file_from_task_fd, KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_file_release, KF_RELEASE)
 
+BTF_ID_FLAGS(func, bpf_iter_task_file_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_iter_task_file_next, KF_ITER_NEXT | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_iter_task_file_get_fd, KF_ITER_GETTER)
+BTF_ID_FLAGS(func, bpf_iter_task_file_destroy, KF_ITER_DESTROY)
+
 BTF_KFUNCS_END(bpf_crib_kfuncs)
 
 static int bpf_prog_run_crib(struct bpf_prog *prog,
-- 
2.39.2