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[PULL,1/1] usb: Fix build with newer gcc

Message ID 20170720084522.28616-2-kraxel@redhat.com
State New
Headers show

Commit Message

Gerd Hoffmann July 20, 2017, 8:45 a.m. UTC 
From: Eric Blake <eblake@redhat.com>

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 <dgilbert@redhat.com>.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170717151334.17954-1-eblake@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/usb/bus.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
diff mbox

Patch

diff --git a/hw/usb/bus.c b/hw/usb/bus.c
index 5939b273b9..d910f849e7 100644
--- a/hw/usb/bus.c
+++ b/hw/usb/bus.c
@@ -407,8 +407,10 @@  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);