[qemu-xen staging-4.9] usb: Fix build with newer gcc

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[qemu-xen staging-4.9] usb: Fix build with newer gcc

commit c089583e504df752f98008319a6a48aee16b2059
Author:     Eric Blake <[hidden email]>
AuthorDate: Mon Jul 17 10:13:34 2017 -0500
Commit:     Anthony PERARD <[hidden email]>
CommitDate: Mon Jul 9 12:37:56 2018 +0100

    usb: Fix build with newer gcc
    gcc 7 is pickier about our sources:
    hw/usb/bus.c: In function ‘usb_port_location’:
    hw/usb/bus.c:410:66: error: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size between 0 and 15 [-Werror=format-truncation=]
             snprintf(downstream->path, sizeof(downstream->path), "%s.%d",
    hw/usb/bus.c:410:9: note: ‘snprintf’ output between 3 and 28 bytes into a destination of size 16
             snprintf(downstream->path, sizeof(downstream->path), "%s.%d",
                      upstream->path, portnr);
    But we know that there are at most 5 levels of USB hubs, with at
    most two digits per level; that plus the separating dots means we
    use at most 15 bytes (including trailing NUL) of our 16-byte field.
    Adding an assertion to show gcc that we checked for truncation is
    enough to shut up the false-positive warning.
    Inspired by an idea by Dr. David Alan Gilbert <[hidden email]>.
    Signed-off-by: Eric Blake <[hidden email]>
    Reviewed-by: Philippe Mathieu-Daudé <[hidden email]>
    Message-id: [hidden email]
    Signed-off-by: Gerd Hoffmann <[hidden email]>
    (cherry picked from commit 121829cb2160e9cd82482c1542699fa589688106)
 hw/usb/bus.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/hw/usb/bus.c b/hw/usb/bus.c
index 25913ad488..9c4e5cbcf8 100644
--- a/hw/usb/bus.c
+++ b/hw/usb/bus.c
@@ -406,8 +406,10 @@ void usb_register_companion(const char *masterbus, USBPort *ports[],
 void usb_port_location(USBPort *downstream, USBPort *upstream, int portnr)
     if (upstream) {
-        snprintf(downstream->path, sizeof(downstream->path), "%s.%d",
-                 upstream->path, portnr);
+        int l = snprintf(downstream->path, sizeof(downstream->path), "%s.%d",
+                         upstream->path, portnr);
+        /* Max string is nn.nn.nn.nn.nn, which fits in 16 bytes */
+        assert(l < sizeof(downstream->path));
         downstream->hubcount = upstream->hubcount + 1;
     } else {
         snprintf(downstream->path, sizeof(downstream->path), "%d", portnr);
generated by git-patchbot for /home/xen/git/qemu-xen.git#staging-4.9

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