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Date:   Fri, 8 May 2020 11:52:08 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next v3 05/21] bpf: implement bpf_seq_read() for bpf iterator

On Wed, May 6, 2020 at 10:39 PM Yonghong Song <yhs@...com> wrote:
>
> bpf iterator uses seq_file to provide a lossless
> way to transfer data to user space. But we want to call
> bpf program after all objects have been traversed, and
> bpf program may write additional data to the
> seq_file buffer. The current seq_read() does not work
> for this use case.
>
> Besides allowing stop() function to write to the buffer,
> the bpf_seq_read() also fixed the buffer size to one page.
> If any single call of show() or stop() will emit data
> more than one page to cause overflow, -E2BIG error code
> will be returned to user space.
>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---

This loop is much simpler and more streamlined now, thanks a lot! I
think it's correct, see below about one confusing (but apparently
correct) bit, though. Either way:

Acked-by: Andrii Nakryiko <andriin@...com>

>  kernel/bpf/bpf_iter.c | 118 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 118 insertions(+)
>
> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
> index 0542a243b78c..f198597b0ea4 100644
> --- a/kernel/bpf/bpf_iter.c
> +++ b/kernel/bpf/bpf_iter.c
> @@ -26,6 +26,124 @@ static DEFINE_MUTEX(targets_mutex);
>  /* protect bpf_iter_link changes */
>  static DEFINE_MUTEX(link_mutex);
>
> +/* bpf_seq_read, a customized and simpler version for bpf iterator.
> + * no_llseek is assumed for this file.
> + * The following are differences from seq_read():
> + *  . fixed buffer size (PAGE_SIZE)
> + *  . assuming no_llseek
> + *  . stop() may call bpf program, handling potential overflow there
> + */
> +static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
> +                           loff_t *ppos)
> +{
> +       struct seq_file *seq = file->private_data;
> +       size_t n, offs, copied = 0;
> +       int err = 0;
> +       void *p;
> +
> +       mutex_lock(&seq->lock);
> +
> +       if (!seq->buf) {
> +               seq->size = PAGE_SIZE;
> +               seq->buf = kmalloc(seq->size, GFP_KERNEL);
> +               if (!seq->buf) {
> +                       err = -ENOMEM;
> +                       goto done;

oh, thank you for converting to all lower-case label names! :)

> +               }
> +       }
> +
> +       if (seq->count) {
> +               n = min(seq->count, size);
> +               err = copy_to_user(buf, seq->buf + seq->from, n);
> +               if (err) {
> +                       err = -EFAULT;
> +                       goto done;
> +               }
> +               seq->count -= n;
> +               seq->from += n;
> +               copied = n;
> +               goto done;
> +       }
> +
> +       seq->from = 0;
> +       p = seq->op->start(seq, &seq->index);
> +       if (IS_ERR_OR_NULL(p))
> +               goto stop;

if start() returns IS_ERR(p), stop(p) below won't produce any output
(because BPF program is called only for p == NULL), so we'll just
return 0 with no error, do I interpret the code correctly? I think
seq_file's read actually returns PTR_ERR(p) as a result in this case.

so I think you need err = PTR_ERR(p); before goto stop here?

> +
> +       err = seq->op->show(seq, p);
> +       if (err > 0) {
> +               seq->count = 0;
> +       } else if (err < 0 || seq_has_overflowed(seq)) {
> +               if (!err)
> +                       err = -E2BIG;
> +               seq->count = 0;
> +               seq->op->stop(seq, p);
> +               goto done;
> +       }
> +
> +       while (1) {
> +               loff_t pos = seq->index;
> +
> +               offs = seq->count;
> +               p = seq->op->next(seq, p, &seq->index);
> +               if (pos == seq->index) {
> +                       pr_info_ratelimited("buggy seq_file .next function %ps "
> +                               "did not updated position index\n",
> +                               seq->op->next);
> +                       seq->index++;
> +               }
> +
> +               if (IS_ERR_OR_NULL(p)) {
> +                       err = PTR_ERR(p);
> +                       break;
> +               }
> +               if (seq->count >= size)
> +                       break;
> +
> +               err = seq->op->show(seq, p);
> +               if (err > 0) {
> +                       seq->count = offs;
> +               } else if (err < 0 || seq_has_overflowed(seq)) {
> +                       seq->count = offs;
> +                       if (!err)
> +                               err = -E2BIG;

nit: this -E2BIG is set unconditionally even for 2nd+ show(). This
will work, because it will get ignored on next iteration, but I think
it will be much more obvious if written as:

if (!err && offs = 0)
    err = -E2BIG;

It took me few re-readings of the code I'm pretty familiar with
already to realize that this is ok.

I had to write the below piece to realize that this is fine :) Just
leaving here just in case you find it useful:

else if (err < 0 || seq_has_overflowed(seq)) {
    if (!err && offs == 0) /* overflow in first show() output */
        err = -E2BIG;
    if (err) {             /* overflow in first show() or real error happened */
        seq->count = 0; /* not strictly necessary, but shows that we
are truncating output */
        seq->op->stop(seq, p);
        goto done; /* done will return err */
    }
    /* no error and overflow for 2nd+ show(), roll back output and stop */
    seq->count = offs;
    break;
}

> +                       if (offs == 0) {
> +                               seq->op->stop(seq, p);
> +                               goto done;
> +                       }
> +                       break;
> +               }
> +       }
> +stop:
> +       offs = seq->count;
> +       /* bpf program called if !p */
> +       seq->op->stop(seq, p);
> +       if (!p && seq_has_overflowed(seq)) {
> +               seq->count = offs;
> +               if (offs == 0) {
> +                       err = -E2BIG;
> +                       goto done;
> +               }
> +       }
> +
> +       n = min(seq->count, size);
> +       err = copy_to_user(buf, seq->buf, n);
> +       if (err) {
> +               err = -EFAULT;
> +               goto done;
> +       }
> +       copied = n;
> +       seq->count -= n;
> +       seq->from = n;
> +done:
> +       if (!copied)
> +               copied = err;
> +       else
> +               *ppos += copied;
> +       mutex_unlock(&seq->lock);
> +       return copied;
> +}
> +
>  int bpf_iter_reg_target(struct bpf_iter_reg *reg_info)
>  {
>         struct bpf_iter_target_info *tinfo;
> --
> 2.24.1
>

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