Re: [PATCH bpf-next v4 1/6] bpf: Factorize bpf_trace_printk and bpf_seq_printf

[Andrii Nakryiko]

On Wed, Apr 14, 2021 at 11:54 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> Two helpers (trace_printk and seq_printf) have very similar
> implementations of format string parsing and a third one is coming
> (snprintf). To avoid code duplication and make the code easier to
> maintain, this moves the operations associated with format string
> parsing (validation and argument sanitization) into one generic
> function.
>
> The implementation of the two existing helpers already drifted quite a
> bit so unifying them entailed a lot of changes:
>
> - bpf_trace_printk always expected fmt[fmt_size] to be the terminating
>   NULL character, this is no longer true, the first 0 is terminating.
> - bpf_trace_printk now supports %% (which produces the percentage char).
> - bpf_trace_printk now skips width formating fields.
> - bpf_trace_printk now supports the X modifier (capital hexadecimal).
> - bpf_trace_printk now supports %pK, %px, %pB, %pi4, %pI4, %pi6 and %pI6
> - argument casting on 32 bit has been simplified into one macro and
>   using an enum instead of obscure int increments.
>
> - bpf_seq_printf now uses bpf_trace_copy_string instead of
>   strncpy_from_kernel_nofault and handles the %pks %pus specifiers.
> - bpf_seq_printf now prints longs correctly on 32 bit architectures.
>
> - both were changed to use a global per-cpu tmp buffer instead of one
>   stack buffer for trace_printk and 6 small buffers for seq_printf.
> - to avoid per-cpu buffer usage conflict, these helpers disable
>   preemption while the per-cpu buffer is in use.
> - both helpers now support the %ps and %pS specifiers to print symbols.
>
> The implementation is also moved from bpf_trace.c to helpers.c because
> the upcoming bpf_snprintf helper will be made available to all BPF
> programs and will need it.
>
> Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> ---
>  include/linux/bpf.h      |  20 +++
>  kernel/bpf/helpers.c     | 254 +++++++++++++++++++++++++++
>  kernel/trace/bpf_trace.c | 371 ++++-----------------------------------
>  3 files changed, 311 insertions(+), 334 deletions(-)
>

[...]

> +static int try_get_fmt_tmp_buf(char **tmp_buf)
> +{
> +       struct bpf_printf_buf *bufs;
> +       int used;
> +
> +       if (*tmp_buf)
> +               return 0;
> +
> +       preempt_disable();
> +       used = this_cpu_inc_return(bpf_printf_buf_used);
> +       if (WARN_ON_ONCE(used > 1)) {
> +               this_cpu_dec(bpf_printf_buf_used);

this makes me uncomfortable. If used > 1, you won't preempt_enable()
here, but you'll decrease count. Then later bpf_printf_cleanup() will
be called (inside bpf_printf_prepare()) and will further decrease
count (which it didn't increase, so it's a mess now).

> +               return -EBUSY;
> +       }
> +       bufs = this_cpu_ptr(&bpf_printf_buf);
> +       *tmp_buf = bufs->tmp_buf;
> +
> +       return 0;
> +}
> +

[...]

> +                       i += 2;
> +                       if (!final_args)
> +                               goto fmt_next;
> +
> +                       if (try_get_fmt_tmp_buf(&tmp_buf)) {
> +                               err = -EBUSY;
> +                               goto out;

this probably should bypass doing bpf_printf_cleanup() and
try_get_fmt_tmp_buf() should enable preemption internally on error.

> +                       }
> +
> +                       copy_size = (fmt[i + 2] == '4') ? 4 : 16;
> +                       if (tmp_buf_len < copy_size) {
> +                               err = -ENOSPC;
> +                               goto out;
> +                       }
> +

[...]

> -static __printf(1, 0) int bpf_do_trace_printk(const char *fmt, ...)
> +BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
> +          u64, arg2, u64, arg3)
>  {
> +       u64 args[MAX_TRACE_PRINTK_VARARGS] = { arg1, arg2, arg3 };
> +       enum bpf_printf_mod_type mod[MAX_TRACE_PRINTK_VARARGS];
>         static char buf[BPF_TRACE_PRINTK_SIZE];
>         unsigned long flags;
> -       va_list ap;
>         int ret;
>
> -       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 */
> +       ret = bpf_printf_prepare(fmt, fmt_size, args, args, mod,
> +                                MAX_TRACE_PRINTK_VARARGS);
> +       if (ret < 0)
> +               return ret;
> +
> +       ret = snprintf(buf, sizeof(buf), fmt, BPF_CAST_FMT_ARG(0, args, mod),
> +               BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod));
> +       /* snprintf() will not append null for zero-length strings */
>         if (ret == 0)
>                 buf[0] = '\0';
> +
> +       raw_spin_lock_irqsave(&trace_printk_lock, flags);
>         trace_bpf_trace_printk(buf);
>         raw_spin_unlock_irqrestore(&trace_printk_lock, flags);
>
> -       return ret;

see here, no + 1 :(

> -}
> -
> -/*
> - * Only limited trace_printk() conversion specifiers allowed:
> - * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %pB %pks %pus %s
> - */

[...]