Re: [PATCH] qga: Fix ubsan warning

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

On Tue, Jul 29, 2025 at 08:02:25AM +0200, Thomas Huth wrote:
> On 28/07/2025 19.53, Daniel P. Berrangé wrote:
> > On Mon, Jul 28, 2025 at 07:30:16PM +0200, Thomas Huth wrote:
> > > From: Thomas Huth <thuth@redhat.com>
> > > 
> > > When compiling QEMU with --enable-ubsan there is a undefined behavior
> > > warning when running "make check":
> > > 
> > >   .../qga/commands-linux.c:452:15: runtime error: applying non-zero offset 5 to null pointer
> > >   #0 0x55ea7b89450c in build_guest_fsinfo_for_pci_dev ..../qga/commands-linux.c:452:15
> > > 
> > > Add a check to avoid incrementing the NULL pointer here.
> > > 
> > > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > > ---
> > >   qga/commands-linux.c | 4 +++-
> > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/qga/commands-linux.c b/qga/commands-linux.c
> > > index 9e8a934b9a6..caf7c3ca22b 100644
> > > --- a/qga/commands-linux.c
> > > +++ b/qga/commands-linux.c
> > > @@ -449,7 +449,9 @@ static bool build_guest_fsinfo_for_pci_dev(char const *syspath,
> > >           has_ata = true;
> > >       } else {
> > >           p = strstr(syspath, "/host");
> > > -        q = p + 5;
> > > +        if (p) {
> > > +            q = p + 5;
> > > +        }
> > >       }
> > >       if (p && sscanf(q, "%u", &host) == 1) {
> > 
> > q is always non-NULL if p is non-NULL, so this is safe, but I would be more
> > happy with this changing to 'q && sscanf' to eliminate the indirection.
> 
> If we agree to do a bigger change here, I'd rather drop the "q" pointer
> completely and use a new integer variable instead, something like:
> 
>     int offset;
>     ...
>     p = strstr(syspath, "/ata");
>     if (p) {
>         offset = 4;
>         has_ata = true;
>     } else {
>         offset = 5;
>         p = strstr(syspath, "/host");
>     }
>     if (p && sscanf(p + offset, "%u", &host) == 1) {
>         ...
>     }
> 
> WDYT?

Works for me.


With regards,
Daniel
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