On 12/15/20 3:36 PM, Song Liu wrote:
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@xxxxxx>
---
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++;
Here, info->tid should be info->tid = curr_tid + 1.
For exmaple, suppose initial curr_tid = info->tid = 10, and the
above task_seq_get_next(...) returns NULL with curr_tid = 100
which means tid = 100 has been visited. So we would like
to set info->tid = 101 to avoid future potential redundant work.
Returning NULL here will signal end of iteration but user
space can still call read()...
+ 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;
We may have some issues here if control is returned to user space
in the middle of iterations. For example,
- seq_ops->next() sets info->vma properly (say corresponds to vma1
of tid1)
- control returns to user space
- control backs to kernel and this is not a new task since
tid is the same
- but we skipped this vma for show().
I think the above skipping should be guarded. If the function
is called from seq_ops->next(), yes it can be skipped.
If the function is called from seq_ops->start(), it should not
be skipped.
Could you double check such a scenario with a smaller buffer
size for read() in user space?
+ }
+ 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;
+ }
+}
+
[...]
