[PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter

[Song Liu <songliubraving@fb.com>]

Introduce task_vma bpf_iter to print memory information of a process. It
can be used to print customized information similar to /proc/<pid>/maps.

task_vma iterator releases mmap_lock before calling the BPF program.
Therefore, we cannot pass vm_area_struct directly to the BPF program. A
new __vm_area_struct is introduced to keep key information of a vma. On
each iteration, task_vma gathers information in __vm_area_struct and
passes it to the BPF program.

If the vma maps to a file, task_vma also holds a reference to the file
while calling the BPF program.

Signed-off-by: Song Liu <songliubraving@fb.com>
---
 include/linux/bpf.h    |   2 +-
 kernel/bpf/task_iter.c | 205 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 205 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 07cb5d15e7439..49dd1e29c8118 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1325,7 +1325,7 @@ enum bpf_iter_feature {
 	BPF_ITER_RESCHED	= BIT(0),
 };
 
-#define BPF_ITER_CTX_ARG_MAX 2
+#define BPF_ITER_CTX_ARG_MAX 3
 struct bpf_iter_reg {
 	const char *target;
 	bpf_iter_attach_target_t attach_target;
diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 0458a40edf10a..15a066b442f75 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -304,9 +304,183 @@ static const struct seq_operations task_file_seq_ops = {
 	.show	= task_file_seq_show,
 };
 
+/*
+ * Key information from vm_area_struct. We need this because we cannot
+ * assume the vm_area_struct is still valid after each iteration.
+ */
+struct __vm_area_struct {
+	__u64 start;
+	__u64 end;
+	__u64 flags;
+	__u64 pgoff;
+};
+
+struct bpf_iter_seq_task_vma_info {
+	/* The first field must be struct bpf_iter_seq_task_common.
+	 * this is assumed by {init, fini}_seq_pidns() callback functions.
+	 */
+	struct bpf_iter_seq_task_common common;
+	struct task_struct *task;
+	struct __vm_area_struct vma;
+	struct file *file;
+	u32 tid;
+};
+
+static struct __vm_area_struct *
+task_vma_seq_get_next(struct bpf_iter_seq_task_vma_info *info)
+{
+	struct pid_namespace *ns = info->common.ns;
+	struct task_struct *curr_task;
+	struct vm_area_struct *vma;
+	u32 curr_tid = info->tid;
+	bool new_task = false;
+
+	/* If this function returns a non-NULL __vm_area_struct, it held
+	 * a reference to the task_struct. If info->file is non-NULL, it
+	 * also holds a reference to the file. If this function returns
+	 * NULL, it does not hold any reference.
+	 */
+again:
+	if (info->task) {
+		curr_task = info->task;
+	} else {
+		curr_task = task_seq_get_next(ns, &curr_tid, true);
+		if (!curr_task) {
+			info->task = NULL;
+			info->tid++;
+			return NULL;
+		}
+
+		if (curr_tid != info->tid) {
+			info->tid = curr_tid;
+			new_task = true;
+		}
+
+		if (!curr_task->mm)
+			goto next_task;
+		info->task = curr_task;
+	}
+
+	mmap_read_lock(curr_task->mm);
+	if (new_task) {
+		vma = curr_task->mm->mmap;
+	} else {
+		/* We drop the lock between each iteration, so it is
+		 * necessary to use find_vma() to find the next vma. This
+		 * is similar to the mechanism in show_smaps_rollup().
+		 */
+		vma = find_vma(curr_task->mm, info->vma.end - 1);
+		/* same vma as previous iteration, use vma->next */
+		if (vma && (vma->vm_start == info->vma.start))
+			vma = vma->vm_next;
+	}
+	if (!vma) {
+		mmap_read_unlock(curr_task->mm);
+		goto next_task;
+	}
+	info->task = curr_task;
+	info->vma.start = vma->vm_start;
+	info->vma.end = vma->vm_end;
+	info->vma.pgoff = vma->vm_pgoff;
+	info->vma.flags = vma->vm_flags;
+	if (vma->vm_file)
+		info->file = get_file(vma->vm_file);
+	mmap_read_unlock(curr_task->mm);
+	return &info->vma;
+
+next_task:
+	put_task_struct(curr_task);
+	info->task = NULL;
+	curr_tid = ++(info->tid);
+	goto again;
+}
+
+static void *task_vma_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	struct bpf_iter_seq_task_vma_info *info = seq->private;
+	struct __vm_area_struct *vma;
+
+	vma = task_vma_seq_get_next(info);
+	if (vma && *pos == 0)
+		++*pos;
+
+	return vma;
+}
+
+static void *task_vma_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	struct bpf_iter_seq_task_vma_info *info = seq->private;
+
+	++*pos;
+	if (info->file) {
+		fput(info->file);
+		info->file = NULL;
+	}
+	return task_vma_seq_get_next(info);
+}
+
+struct bpf_iter__task_vma {
+	__bpf_md_ptr(struct bpf_iter_meta *, meta);
+	__bpf_md_ptr(struct task_struct *, task);
+	__bpf_md_ptr(struct __vm_area_struct *, vma);
+	__bpf_md_ptr(struct file *, file);
+};
+
+DEFINE_BPF_ITER_FUNC(task_vma, struct bpf_iter_meta *meta,
+		     struct task_struct *task, struct __vm_area_struct *vma,
+		     struct file *file)
+
+static int __task_vma_seq_show(struct seq_file *seq, bool in_stop)
+{
+	struct bpf_iter_seq_task_vma_info *info = seq->private;
+	struct bpf_iter__task_vma ctx;
+	struct bpf_iter_meta meta;
+	struct bpf_prog *prog;
+
+	meta.seq = seq;
+	prog = bpf_iter_get_info(&meta, in_stop);
+	if (!prog)
+		return 0;
+
+	ctx.meta = &meta;
+	ctx.task = info->task;
+	ctx.vma = &info->vma;
+	ctx.file = info->file;
+	return bpf_iter_run_prog(prog, &ctx);
+}
+
+static int task_vma_seq_show(struct seq_file *seq, void *v)
+{
+	return __task_vma_seq_show(seq, false);
+}
+
+static void task_vma_seq_stop(struct seq_file *seq, void *v)
+{
+	struct bpf_iter_seq_task_vma_info *info = seq->private;
+
+	if (!v) {
+		(void)__task_vma_seq_show(seq, true);
+	} else {
+		put_task_struct(info->task);
+		if (info->file) {
+			fput(info->file);
+			info->file = NULL;
+		}
+		info->task = NULL;
+	}
+}
+
+static const struct seq_operations task_vma_seq_ops = {
+	.start	= task_vma_seq_start,
+	.next	= task_vma_seq_next,
+	.stop	= task_vma_seq_stop,
+	.show	= task_vma_seq_show,
+};
+
 BTF_ID_LIST(btf_task_file_ids)
 BTF_ID(struct, task_struct)
 BTF_ID(struct, file)
+BTF_ID(struct, __vm_area_struct)
 
 static const struct bpf_iter_seq_info task_seq_info = {
 	.seq_ops		= &task_seq_ops,
@@ -346,6 +520,28 @@ static struct bpf_iter_reg task_file_reg_info = {
 	.seq_info		= &task_file_seq_info,
 };
 
+static const struct bpf_iter_seq_info task_vma_seq_info = {
+	.seq_ops		= &task_vma_seq_ops,
+	.init_seq_private	= init_seq_pidns,
+	.fini_seq_private	= fini_seq_pidns,
+	.seq_priv_size		= sizeof(struct bpf_iter_seq_task_vma_info),
+};
+
+static struct bpf_iter_reg task_vma_reg_info = {
+	.target			= "task_vma",
+	.feature		= BPF_ITER_RESCHED,
+	.ctx_arg_info_size	= 3,
+	.ctx_arg_info		= {
+		{ offsetof(struct bpf_iter__task_vma, task),
+		  PTR_TO_BTF_ID_OR_NULL },
+		{ offsetof(struct bpf_iter__task_vma, vma),
+		  PTR_TO_BTF_ID_OR_NULL },
+		{ offsetof(struct bpf_iter__task_vma, file),
+		  PTR_TO_BTF_ID_OR_NULL },
+	},
+	.seq_info		= &task_vma_seq_info,
+};
+
 static int __init task_iter_init(void)
 {
 	int ret;
@@ -357,6 +553,13 @@ static int __init task_iter_init(void)
 
 	task_file_reg_info.ctx_arg_info[0].btf_id = btf_task_file_ids[0];
 	task_file_reg_info.ctx_arg_info[1].btf_id = btf_task_file_ids[1];
-	return bpf_iter_reg_target(&task_file_reg_info);
+	ret =  bpf_iter_reg_target(&task_file_reg_info);
+	if (ret)
+		return ret;
+
+	task_vma_reg_info.ctx_arg_info[0].btf_id = btf_task_file_ids[0];
+	task_vma_reg_info.ctx_arg_info[1].btf_id = btf_task_file_ids[2];
+	task_vma_reg_info.ctx_arg_info[2].btf_id = btf_task_file_ids[1];
+	return bpf_iter_reg_target(&task_vma_reg_info);
 }
 late_initcall(task_iter_init);
-- 
2.24.1