[Qemu-devel] [PATCH v5 11/16] nbd: always query export list in fixed new style protocol

["Daniel P. Berrange" <berrange@redhat.com>]

With the new style protocol, the NBD client will currenetly
send NBD_OPT_EXPORT_NAME as the first (and indeed only)
option it wants. The problem is that the NBD protocol spec
does not allow for returning an error message with the
NBD_OPT_EXPORT_NAME option. So if the server mandates use
of TLS, the client will simply see an immediate connection
close after issuing NBD_OPT_EXPORT_NAME which is not user
friendly.

To improve this situation, if we have the fixed new style
protocol, we can sent NBD_OPT_LIST as the first option
to query the list of server exports. We can check for our
named export in this list and raise an error if it is not
found, instead of going ahead and sending NBD_OPT_EXPORT_NAME
with a name that we know will be rejected.

This improves the error reporting both in the case that the
server required TLS, and in the case that the client requested
export name does not exist on the server.

If the server does not support NBD_OPT_LIST, we just ignore
that and carry on with NBD_OPT_EXPORT_NAME as before.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
---
 nbd/client.c | 195 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 nbd/server.c |   2 +
 2 files changed, 194 insertions(+), 3 deletions(-)

diff --git a/nbd/client.c b/nbd/client.c
index 22d69ff..e91821a 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -70,6 +70,177 @@ static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
 
 */
 
+
+static int nbd_handle_reply_err(uint32_t opt, uint32_t type, Error **errp)
+{
+    if (!(type & (1 << 31))) {
+        return 0;
+    }
+
+    switch (type) {
+    case NBD_REP_ERR_UNSUP:
+        error_setg(errp, "Unsupported option type %x", opt);
+        break;
+
+    case NBD_REP_ERR_INVALID:
+        error_setg(errp, "Invalid data length for option %x", opt);
+        break;
+
+    default:
+        error_setg(errp, "Unknown error code when asking for option %x", opt);
+        break;
+    }
+
+    return -1;
+}
+
+static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
+{
+    uint64_t magic;
+    uint32_t opt;
+    uint32_t type;
+    uint32_t len;
+    uint32_t namelen;
+
+    *name = NULL;
+    if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
+        error_setg(errp, "failed to read list option magic");
+        return -1;
+    }
+    magic = be64_to_cpu(magic);
+    if (magic != NBD_REP_MAGIC) {
+        error_setg(errp, "Unexpected option list magic");
+        return -1;
+    }
+    if (read_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
+        error_setg(errp, "failed to read list option");
+        return -1;
+    }
+    opt = be32_to_cpu(opt);
+    if (opt != NBD_OPT_LIST) {
+        error_setg(errp, "Unexpected option type %x expected %x",
+                   opt, NBD_OPT_LIST);
+        return -1;
+    }
+
+    if (read_sync(ioc, &type, sizeof(type)) != sizeof(type)) {
+        error_setg(errp, "failed to read list option type");
+        return -1;
+    }
+    type = be32_to_cpu(type);
+    if (type == NBD_REP_ERR_UNSUP) {
+        return 0;
+    }
+    if (nbd_handle_reply_err(opt, type, errp) < 0) {
+        return -1;
+    }
+
+    if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {
+        error_setg(errp, "failed to read option length");
+        return -1;
+    }
+    len = be32_to_cpu(len);
+
+    if (type == NBD_REP_ACK) {
+        if (len != 0) {
+            error_setg(errp, "length too long for option end");
+            return -1;
+        }
+    } else if (type == NBD_REP_SERVER) {
+        if (read_sync(ioc, &namelen, sizeof(namelen)) != sizeof(namelen)) {
+            error_setg(errp, "failed to read option name length");
+            return -1;
+        }
+        namelen = be32_to_cpu(namelen);
+        if (len != (namelen + sizeof(namelen))) {
+            error_setg(errp, "incorrect option mame length");
+            return -1;
+        }
+        if (namelen > 255) {
+            error_setg(errp, "export name length too long %d", namelen);
+            return -1;
+        }
+
+        *name = g_new0(char, namelen + 1);
+        if (read_sync(ioc, *name, namelen) != namelen) {
+            error_setg(errp, "failed to read export name");
+            g_free(*name);
+            *name = NULL;
+            return -1;
+        }
+        (*name)[namelen] = '\0';
+    } else {
+        error_setg(errp, "Unexpected reply type %x expected %x",
+                   type, NBD_REP_SERVER);
+        return -1;
+    }
+    return 1;
+}
+
+
+static int nbd_receive_query_exports(QIOChannel *ioc,
+                                     const char *wantname,
+                                     Error **errp)
+{
+    uint64_t magic = cpu_to_be64(NBD_OPTS_MAGIC);
+    uint32_t opt = cpu_to_be32(NBD_OPT_LIST);
+    uint32_t length = 0;
+    bool foundExport = false;
+
+    TRACE("Querying export list");
+    if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
+        error_setg(errp, "Failed to send list option magic");
+        return -1;
+    }
+
+    if (write_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
+        error_setg(errp, "Failed to send list option number");
+        return -1;
+    }
+
+    if (write_sync(ioc, &length, sizeof(length)) != sizeof(length)) {
+        error_setg(errp, "Failed to send list option length");
+        return -1;
+    }
+
+    TRACE("Reading available export names");
+    while (1) {
+        char *name = NULL;
+        int ret = nbd_receive_list(ioc, &name, errp);
+
+        if (ret < 0) {
+            g_free(name);
+            name = NULL;
+            return -1;
+        }
+        if (ret == 0) {
+            /* Server doesn't support export listing, so
+             * we will just assume an export with our
+             * wanted name exists */
+            foundExport = true;
+            break;
+        }
+        if (name == NULL) {
+            TRACE("End of export name list");
+            break;
+        }
+        if (g_str_equal(name, wantname)) {
+            foundExport = true;
+            TRACE("Found desired export name '%s'", name);
+        } else {
+            TRACE("Ignored export name '%s'", name);
+        }
+        g_free(name);
+    }
+
+    if (!foundExport) {
+        error_setg(errp, "No export with name '%s' available", wantname);
+        return -1;
+    }
+
+    return 0;
+}
+
 int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint32_t *flags,
                           off_t *size, Error **errp)
 {
@@ -120,28 +291,44 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint32_t *flags,
         uint32_t namesize;
         uint16_t globalflags;
         uint16_t exportflags;
+        bool fixedNewStyle = false;
 
         if (read_sync(ioc, &globalflags, sizeof(globalflags)) !=
             sizeof(globalflags)) {
             error_setg(errp, "Failed to read server flags");
             goto fail;
         }
-        *flags = be16_to_cpu(globalflags) << 16;
+        globalflags = be16_to_cpu(globalflags);
+        *flags = globalflags << 16;
+        TRACE("Global flags are %x", globalflags);
         if (globalflags & NBD_FLAG_FIXED_NEWSTYLE) {
+            fixedNewStyle = true;
             TRACE("Server supports fixed new style");
             clientflags |= NBD_FLAG_C_FIXED_NEWSTYLE;
         }
         /* client requested flags */
+        clientflags = cpu_to_be32(clientflags);
         if (write_sync(ioc, &clientflags, sizeof(clientflags)) !=
             sizeof(clientflags)) {
             error_setg(errp, "Failed to send clientflags field");
             goto fail;
         }
-        /* write the export name */
         if (!name) {
             error_setg(errp, "Server requires an export name");
             goto fail;
         }
+        if (fixedNewStyle) {
+            /* Check our desired export is present in the
+             * server export list. Since NBD_OPT_EXPORT_NAME
+             * cannot return an error message, running this
+             * query gives us good error reporting if the
+             * server required TLS
+             */
+            if (nbd_receive_query_exports(ioc, name, errp) < 0) {
+                goto fail;
+            }
+        }
+        /* write the export name */
         magic = cpu_to_be64(magic);
         if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
             error_setg(errp, "Failed to send export name magic");
@@ -175,7 +362,9 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint32_t *flags,
             error_setg(errp, "Failed to read export flags");
             goto fail;
         }
-        *flags |= be16_to_cpu(exportflags);
+        exportflags = be16_to_cpu(exportflags);
+        *flags |= exportflags;
+        TRACE("Export flags are %x", exportflags);
     } else if (magic == NBD_CLIENT_MAGIC) {
         if (name) {
             error_setg(errp, "Server does not support export names");
diff --git a/nbd/server.c b/nbd/server.c
index 643d1dc..8bc083e 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -293,6 +293,8 @@ static int nbd_negotiate_handle_export_name(NBDClient *client, uint32_t length)
     }
     name[length] = '\0';
 
+    TRACE("Client requested export '%s'", name);
+
     client->exp = nbd_export_find(name);
     if (!client->exp) {
         LOG("export not found");
-- 
2.5.0