On 3/3/26 12:23, Julian Orth wrote: > On Tue, Mar 3, 2026 at 12:17 PM Michel Dänzer > <[email protected]> wrote: >> >> While I don't object to this change, I'd argue that this is really a >> workaround for broken user space, not a fix for a kernel regression. >> >> User space must initialize the full struct to 0, except for the fields it >> wants to have other values. This kind of kernel-side workaround for >> neglecting that won't be possible in all cases. > > When the code in my example was written, the points field did not > exist. Therefore, as far as that code is concerned, the authors > believed that the struct was fully initialized, since they manually > initialized all fields that were contained in the struct at that time. > The code was then later recompiled against a version of drm/drm.h that > contained the new field. > > Or are you saying that userspace must always use memset(&arg, 0, > sizeof(arg)) to initialize ioctl arguments?
Yes, it must. Any approach which results in newly-added fields not being initialized to 0 is broken. To prevent a potential follow-up concern: What if the kernel uses a larger size of the struct than user space? That's fine, user space encodes the size it uses in the parameters to the ioctl system call, and the kernel uses 0 for all fields beyond that size. -- Earthling Michel Dänzer \ GNOME / Xwayland / Mesa developer https://redhat.com \ Libre software enthusiast
