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



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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