Re: [PATCH bpf-next v3 1/6] bpf: Factorize bpf_trace_printk and bpf_seq_printf

[Florent Revest <revest@chromium.org>]

On Wed, Apr 14, 2021 at 1:01 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
> On Mon, Apr 12, 2021 at 8:38 AM Florent Revest <revest@chromium.org> wrote:
> > +/* Per-cpu temp buffers which can be used by printf-like helpers for %s or %p
> > + */
> > +#define MAX_PRINTF_BUF_LEN     512
> > +
> > +struct bpf_printf_buf {
> > +       char tmp_buf[MAX_PRINTF_BUF_LEN];
> > +};
> > +static DEFINE_PER_CPU(struct bpf_printf_buf, bpf_printf_buf);
> > +static DEFINE_PER_CPU(int, bpf_printf_buf_used);
> > +
> > +static int try_get_fmt_tmp_buf(char **tmp_buf)
> >  {
> > -       static char buf[BPF_TRACE_PRINTK_SIZE];
> > -       unsigned long flags;
> > -       va_list ap;
> > -       int ret;
> > +       struct bpf_printf_buf *bufs = this_cpu_ptr(&bpf_printf_buf);
>
> why doing this_cpu_ptr() if below (if *tmp_buf case), you will not use
> it. just a waste of CPU, no?

Sure I can move it past the conditions.

> > +       int used;
> >
> > -       raw_spin_lock_irqsave(&trace_printk_lock, flags);
> > -       va_start(ap, fmt);
> > -       ret = vsnprintf(buf, sizeof(buf), fmt, ap);
> > -       va_end(ap);
> > -       /* vsnprintf() will not append null for zero-length strings */
> > -       if (ret == 0)
> > -               buf[0] = '\0';
> > -       trace_bpf_trace_printk(buf);
> > -       raw_spin_unlock_irqrestore(&trace_printk_lock, flags);
> > +       if (*tmp_buf)
> > +               return 0;
> >
> > -       return ret;
> > +       preempt_disable();
> > +       used = this_cpu_inc_return(bpf_printf_buf_used);
> > +       if (WARN_ON_ONCE(used > 1)) {
> > +               this_cpu_dec(bpf_printf_buf_used);
> > +               return -EBUSY;
> > +       }
>
> get bufs pointer here instead?

Okay :)

> > +       *tmp_buf = bufs->tmp_buf;
> > +
> > +       return 0;
> > +}
> > +
> > +static void put_fmt_tmp_buf(void)
> > +{
> > +       if (this_cpu_read(bpf_printf_buf_used)) {
> > +               this_cpu_dec(bpf_printf_buf_used);
> > +               preempt_enable();
> > +       }
> >  }
> >
> >  /*
> > - * Only limited trace_printk() conversion specifiers allowed:
> > - * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %pB %pks %pus %s
> > + * bpf_parse_fmt_str - Generic pass on format strings for printf-like helpers
> > + *
> > + * Returns a negative value if fmt is an invalid format string or 0 otherwise.
> > + *
> > + * This can be used in two ways:
> > + * - Format string verification only: when final_args and mod are NULL
> > + * - Arguments preparation: in addition to the above verification, it writes in
> > + *   final_args a copy of raw_args where pointers from BPF have been sanitized
> > + *   into pointers safe to use by snprintf. This also writes in the mod array
> > + *   the size requirement of each argument, usable by BPF_CAST_FMT_ARG for ex.
> > + *
> > + * In argument preparation mode, if 0 is returned, safe temporary buffers are
> > + * allocated and put_fmt_tmp_buf should be called to free them after use.
> >   */
> > -BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
> > -          u64, arg2, u64, arg3)
> > -{
> > -       int i, mod[3] = {}, fmt_cnt = 0;
> > -       char buf[64], fmt_ptype;
> > -       void *unsafe_ptr = NULL;
> > -       bool str_seen = false;
> > +int bpf_printf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
> > +                       u64 *final_args, enum bpf_printf_mod_type *mod,
> > +                       u32 num_args)
> > +{
> > +       int err, i, curr_specifier = 0, copy_size;
> > +       char *unsafe_ptr = NULL, *tmp_buf = NULL;
> > +       size_t tmp_buf_len = MAX_PRINTF_BUF_LEN;
> > +       enum bpf_printf_mod_type current_mod;
> > +       u64 current_arg;
>
> naming consistency: current_arg vs curr_specifier? maybe just cur_arg
> and cur_spec?

Ahah, you're right again :)

> > +       char fmt_ptype;
> > +
> > +       if ((final_args && !mod) || (mod && !final_args))
>
> nit: same check:
>
> if (!!final_args != !!mod)

Fancy! :)

> > +               return -EINVAL;
> >
> > -       /*
> > -        * bpf_check()->check_func_arg()->check_stack_boundary()
> > -        * guarantees that fmt points to bpf program stack,
> > -        * fmt_size bytes of it were initialized and fmt_size > 0
> > -        */
> > -       if (fmt[--fmt_size] != 0)
> > +       fmt_size = (strnchr(fmt, fmt_size, 0) - fmt);
>
> extra ()

Oops!

> > +       if (!fmt_size)
>
> hm... strnchr() will return NULL if the character is not found, so
> fmt_size will be some non-zero value (due to - fmt), how is this
> supposed to work?

Ugh!

> some negative tests are clearly missing, it seems, if you didn't catch this

Agree

> >                 return -EINVAL;
> >
> > -       /* check format string for allowed specifiers */
> >         for (i = 0; i < fmt_size; i++) {
> > -               if ((!isprint(fmt[i]) && !isspace(fmt[i])) || !isascii(fmt[i]))
> > -                       return -EINVAL;
> > +               if ((!isprint(fmt[i]) && !isspace(fmt[i])) || !isascii(fmt[i])) {
> > +                       err = -EINVAL;
> > +                       goto out;
> > +               }
> >
> >                 if (fmt[i] != '%')
> >                         continue;
> >
> > -               if (fmt_cnt >= 3)
> > -                       return -EINVAL;
> > +               if (fmt[i + 1] == '%') {
> > +                       i++;
> > +                       continue;
> > +               }
> > +
> > +               if (curr_specifier >= num_args) {
> > +                       err = -EINVAL;
> > +                       goto out;
> > +               }
> >
> >                 /* fmt[i] != 0 && fmt[last] == 0, so we can access fmt[i + 1] */
>
> a bit outdated comment, last doesn't exist anymore. I think the
> comment is trying to say that fmt[i + 1] can be read because in the
> worst case it will be a final zero terminator (which we checked
> above).

Yes that's the idea. I will rewrite it as a sentence if "last" is confusing.

> > +       err = 0;
> > +out:
> > +       put_fmt_tmp_buf();
>
> so you are putting tmp_buf unconditionally, even when there was no
> error. That seems wrong? Should this be:
>
> if (err)
>     put_fmt_tmp_buf()
>
> ?

Yeah the naming is unfortunate, as discussed in the other patch, I
will rename that to bpf_pintf_cleanup instead. It's not clear from the
name that it only "puts" if the buffer was already gotten.