Re: [PATCH v2 1/2] public/io: xs_wire: Document that EINVAL should always be first in xsd_errors

[Juergen Gross <jgross@xxxxxxxx>]

On 27.06.22 14:36, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
>
> Some tools (e.g. xenstored) always expect EINVAL to be first in xsd_errors.
>
> Document it so, one doesn't add a new entry before hand by mistake.
>
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
>
> ----
>
> I have tried to add a BUILD_BUG_ON() but GCC complained that the value
> was not a constant. I couldn't figure out a way to make GCC happy.
>
> Changes in v2:
>      - New patch
> ---
>   xen/include/public/io/xs_wire.h | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/xen/include/public/io/xs_wire.h b/xen/include/public/io/xs_wire.h
> index c1ec7c73e3b1..dd4c9c9b972d 100644
> --- a/xen/include/public/io/xs_wire.h
> +++ b/xen/include/public/io/xs_wire.h
> @@ -76,6 +76,7 @@ static struct xsd_errors xsd_errors[]
>   __attribute__((unused))
>   #endif
>       = {
> +    /* /!\ Some users (e.g. xenstored) expect EINVAL to be the first entry. */
>       XSD_ERROR(EINVAL),
>       XSD_ERROR(EACCES),
>       XSD_ERROR(EEXIST),
What about another approach, like:

--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -746,16 +746,17 @@ unsigned int get_strings(struct buffered_data *data,
 static void send_error(struct connection *conn, int error)
 {
        unsigned int i;
+       char *err = NULL;

        for (i = 0; error != xsd_errors[i].errnum; i++) {
                if (i == ARRAY_SIZE(xsd_errors) - 1) {
                        eprintf("xenstored: error %i untranslatable", error);
-                       i = 0; /* EINVAL */
+                       err = "EINVAL";
                        break;
                }
        }
-       send_reply(conn, XS_ERROR, xsd_errors[i].errstring,
-                         strlen(xsd_errors[i].errstring) + 1);
+       err = err ? : xsd_errors[i].errstring;
+       send_reply(conn, XS_ERROR, err, strlen(err) + 1);
 }

 void send_reply(struct connection *conn, enum xsd_sockmsg_type type,

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