Re: [dpdk-dev] [v2] telemetry: fix json output buffer size

["Power, Ciara" <ciara.power@intel.com>]

Hi Gowrishankar,


>-----Original Message-----
>From: Gowrishankar Muthukrishnan <gmuthukrishn@marvell.com>
>Sent: Tuesday 21 September 2021 12:03
>To: dev@dpdk.org
>Cc: Power, Ciara <ciara.power@intel.com>; Gowrishankar Muthukrishnan
><gmuthukrishn@marvell.com>
>Subject: [v2] telemetry: fix json output buffer size
>
>Fix json output buffer size for a single largest value.
>
>Fixes: 52af6ccb2b39 ("telemetry: add utility functions for creating JSON")
>
>v2:
> - split from series 18768 ("cnxk: enable telemetry endpoints").
>
>Signed-off-by: Gowrishankar Muthukrishnan <gmuthukrishn@marvell.com>
>---
> lib/telemetry/telemetry_json.h | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
>diff --git a/lib/telemetry/telemetry_json.h b/lib/telemetry/telemetry_json.h
>index ad270b9b30..ba2fde34cb 100644
>--- a/lib/telemetry/telemetry_json.h
>+++ b/lib/telemetry/telemetry_json.h
>@@ -9,6 +9,7 @@
> #include <stdarg.h>
> #include <stdio.h>
> #include <rte_common.h>
>+#include <rte_telemetry.h>
>
> /**
>  * @file
>@@ -23,13 +24,15 @@
>  * @internal
>  * Copies a value into a buffer if the buffer has enough available space.
>  * Nothing written to buffer if an overflow ocurs.
>- * This function is not for use for values larger than 1k.
>+ * Size of buffer is (single largest value - 6), where at least 6 chars
>+ * would have been used for creating json dict i.e '{"x": ... }'.
>+ * This function is not for use for values larger than this buffer size.
>  */
> __rte_format_printf(3, 4)
> static inline int
> __json_snprintf(char *buf, const int len, const char *format, ...)  {
>-	char tmp[1024];
>+	char tmp[RTE_TEL_MAX_SINGLE_STRING_LEN - 6];
> 	va_list ap;
> 	int ret;
>
>--
>2.25.1

I am not sure about why we would want this to allow for "RTE_TEL_MAX_SINGLE_STRING_LEN - 6".
The RTE_TEL_MAX_SINGLE_STRING_LEN is used to represent the max size of a singular string value e.g. the response to client being   {"<cmd>" : "<string value here up to max size in length>" }

I wonder could we use the "len" parameter in some way here, that would be the available space to be filled of the "buf" being passed in, allowing the function to copy in the maximum amount to fill the buffer.

Thanks,
Ciara