[qemu-xen stable-4.14] block: Call attention to truncation of long NBD exports

[patchbot@xxxxxxx]

commit 4cc0a28a6ed7b7746ea074c4ef60e39212fe1f11
Author:     Eric Blake <eblake@xxxxxxxxxx>
AuthorDate: Mon Jun 8 13:26:38 2020 -0500
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon Aug 24 18:56:27 2020 -0500

    block: Call attention to truncation of long NBD exports
    
    Commit 93676c88 relaxed our NBD client code to request export names up
    to the NBD protocol maximum of 4096 bytes without NUL terminator, even
    though the block layer can't store anything longer than 4096 bytes
    including NUL terminator for display to the user.  Since this means
    there are some export names where we have to truncate things, we can
    at least try to make the truncation a bit more obvious for the user.
    Note that in spite of the truncated display name, we can still
    communicate with an NBD server using such a long export name; this was
    deemed nicer than refusing to even connect to such a server (since the
    server may not be under our control, and since determining our actual
    length limits gets tricky when nbd://host:port/export and
    nbd+unix:///export?socket=/path are themselves variable-length
    expansions beyond the export name but count towards the block layer
    name length).
    
    Reported-by: Xueqiang Wei <xuwei@xxxxxxxxxx>
    Fixes: https://bugzilla.redhat.com/1843684
    Signed-off-by: Eric Blake <eblake@xxxxxxxxxx>
    Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx>
    Message-Id: <20200610163741.3745251-3-eblake@xxxxxxxxxx>
    (cherry picked from commit 5c86bdf1208916ece0b87e1151c9b48ee54faa3e)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 block.c     |  7 +++++--
 block/nbd.c | 21 +++++++++++++--------
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/block.c b/block.c
index 2e3905c99e..e7e0a92536 100644
--- a/block.c
+++ b/block.c
@@ -6710,8 +6710,11 @@ void bdrv_refresh_filename(BlockDriverState *bs)
         pstrcpy(bs->filename, sizeof(bs->filename), bs->exact_filename);
     } else {
         QString *json = qobject_to_json(QOBJECT(bs->full_open_options));
-        snprintf(bs->filename, sizeof(bs->filename), "json:%s",
-                 qstring_get_str(json));
+        if (snprintf(bs->filename, sizeof(bs->filename), "json:%s",
+                     qstring_get_str(json)) >= sizeof(bs->filename)) {
+            /* Give user a hint if we truncated things. */
+            strcpy(bs->filename + sizeof(bs->filename) - 4, "...");
+        }
         qobject_unref(json);
     }
 }
diff --git a/block/nbd.c b/block/nbd.c
index 2160859f64..bfc0be6af6 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1986,6 +1986,7 @@ static void nbd_refresh_filename(BlockDriverState *bs)
 {
     BDRVNBDState *s = bs->opaque;
     const char *host = NULL, *port = NULL, *path = NULL;
+    size_t len = 0;
 
     if (s->saddr->type == SOCKET_ADDRESS_TYPE_INET) {
         const InetSocketAddress *inet = &s->saddr->u.inet;
@@ -1998,17 +1999,21 @@ static void nbd_refresh_filename(BlockDriverState *bs)
     } /* else can't represent as pseudo-filename */
 
     if (path && s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd+unix:///%s?socket=%s", s->export, path);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd+unix:///%s?socket=%s", s->export, path);
     } else if (path && !s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd+unix://?socket=%s", path);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd+unix://?socket=%s", path);
     } else if (host && s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd://%s:%s/%s", host, port, s->export);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd://%s:%s/%s", host, port, s->export);
     } else if (host && !s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd://%s:%s", host, port);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd://%s:%s", host, port);
+    }
+    if (len > sizeof(bs->exact_filename)) {
+        /* Name is too long to represent exactly, so leave it empty. */
+        bs->exact_filename[0] = '\0';
     }
 }
 
--
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