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usb: Fix build with newer gcc

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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>
This commit is contained in:
Eric Blake 2017-07-17 10:13:34 -05:00 committed by Gerd Hoffmann
parent d4e59218ab
commit 121829cb21
1 changed files with 4 additions and 2 deletions
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6
hw/usb/bus.c
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@ -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);