Re: [PATCH v3 bpf-next 2/7] bpf: add bpf_trace_vprintk helper

On 9/3/21 4:00 AM, Daniel Borkmann wrote:   

On 8/28/21 7:20 AM, Dave Marchevsky wrote:

quoted

This helper is meant to be "bpf_trace_printk, but with proper vararg
support". Follow bpf_snprintf's example and take a u64 pseudo-vararg
array. Write to /sys/kernel/debug/tracing/trace_pipe using the same
mechanism as bpf_trace_printk.

Signed-off-by: Dave Marchevsky <redacted>

lgtm, minor comments below:

quoted

---
  include/linux/bpf.h            |  1 +
  include/uapi/linux/bpf.h       |  9 ++++++
  kernel/bpf/core.c              |  5 ++++
  kernel/bpf/helpers.c           |  2 ++
  kernel/trace/bpf_trace.c       | 52 +++++++++++++++++++++++++++++++++-
  tools/include/uapi/linux/bpf.h |  9 ++++++
  6 files changed, 77 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index be8d57e6e78a..b6c45a6cbbba 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1088,6 +1088,7 @@ bool bpf_prog_array_compatible(struct bpf_array *array, const struct bpf_prog *f

  int bpf_prog_calc_tag(struct bpf_prog *fp);
    const struct bpf_func_proto *bpf_get_trace_printk_proto(void);
+const struct bpf_func_proto *bpf_get_trace_vprintk_proto(void);
    typedef unsigned long (*bpf_ctx_copy_t)(void *dst, const void *src,
                      unsigned long off, unsigned long len);

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 791f31dd0abe..f171d4d33136 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -4877,6 +4877,14 @@ union bpf_attr {

   *        Get the struct pt_regs associated with **task**.
   *    Return
   *        A pointer to struct pt_regs.
+ *
+ * u64 bpf_trace_vprintk(const char *fmt, u32 fmt_size, const void *data, u32 data_len)

s/u64/long/

quoted

+ *    Description
+ *        Behaves like **bpf_trace_printk**\ () helper, but takes an array of u64

nit: maybe for users it's more clear from description if you instead mention that data_len
needs to be multiple of 8 bytes? Or somehow mention the relation with data more clearly
resp. which shortcoming it addresses compared to bpf_trace_printk(), so developers can more
easily parse it.

In a previous review pass, Andrii preferred having bpf_trace_vprintk's reference other helpers
instead of copy/pasting. So in v5 (patch 9) of this patchset I've added "multiple of 8 bytes"
to helper comments for bpf_seq_printf and bpf_snprintf. Added a sentence mentioning benefits
of vprintk over printk in v5 (patch 3).

quoted

+ *        to format. Arguments are to be used as in **bpf_seq_printf**\ () helper.
+ *    Return
+ *        The number of bytes written to the buffer, or a negative error
+ *        in case of failure.
   */

[...]

quoted

      default:
          return NULL;
      }

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 10672ebc63b7..ea8358b0c748 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -398,7 +398,7 @@ static const struct bpf_func_proto bpf_trace_printk_proto = {

      .arg2_type    = ARG_CONST_SIZE,
  };
  -const struct bpf_func_proto *bpf_get_trace_printk_proto(void)
+static void __set_printk_clr_event(void)
  {
      /*
       * This program might be calling bpf_trace_printk,

@@ -410,10 +410,58 @@ const struct bpf_func_proto *bpf_get_trace_printk_proto(void)

       */
      if (trace_set_clr_event("bpf_trace", "bpf_trace_printk", 1))
          pr_warn_ratelimited("could not enable bpf_trace_printk events");
+}
  +const struct bpf_func_proto *bpf_get_trace_printk_proto(void)
+{
+    __set_printk_clr_event();
      return &bpf_trace_printk_proto;
  }
  +BPF_CALL_4(bpf_trace_vprintk, char *, fmt, u32, fmt_size, const void *, data,
+       u32, data_len)
+{
+    static char buf[BPF_TRACE_PRINTK_SIZE];
+    unsigned long flags;
+    int ret, num_args;
+    u32 *bin_args;
+
+    if (data_len & 7 || data_len > MAX_BPRINTF_VARARGS * 8 ||
+        (data_len && !data))
+        return -EINVAL;
+    num_args = data_len / 8;
+
+    ret = bpf_bprintf_prepare(fmt, fmt_size, data, &bin_args, num_args);
+    if (ret < 0)
+        return ret;

Given you have ARG_PTR_TO_MEM_OR_NULL for data, does this gracefully handle the
case where you pass in fmt string containing e.g. %ps but data being NULL? From
reading bpf_bprintf_prepare() looks like it does just fine, but might be nice
to explicitly add a tiny selftest case for it while you're at it.

quoted

+    raw_spin_lock_irqsave(&trace_printk_lock, flags);
+    ret = bstr_printf(buf, sizeof(buf), fmt, bin_args);
+
+    trace_bpf_trace_printk(buf);
+    raw_spin_unlock_irqrestore(&trace_printk_lock, flags);
+
+    bpf_bprintf_cleanup();
+
+    return ret;
+}

Thanks,
Daniel