On Sun, Oct 08, 2023 at 11:52:56PM +0200, Alejandro Colomar wrote: > Hi Axel, > > On Tue, Oct 03, 2023 at 12:45:45PM -0700, Axel Rasmussen wrote: > > The old FIXME noted that the zeroing was done to differentiate the two > > EINVAL cases. It's possible something like this was true historically, > > but in current Linux we zero it in *both* EINVAL cases, so this is at > > least no longer true. > > > > After reading the code, I can't determine any clear reason why we zero > > it in some cases but not in others. So, some simple advice we can give > > userspace is: if an error occurs, treat the contents of the structure as > > unspecified. Just re-initialize it before retrying UFFDIO_API again. > > > > Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> > > --- > > man2/ioctl_userfaultfd.2 | 16 ++++++++-------- > > 1 file changed, 8 insertions(+), 8 deletions(-) > > > > diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2 > > index 28dd2fcdd..2ee6a0532 100644 > > --- a/man2/ioctl_userfaultfd.2 > > +++ b/man2/ioctl_userfaultfd.2 > > @@ -248,6 +248,14 @@ operation returns 0 on success. > > On error, \-1 is returned and > > .I errno > > is set to indicate the error. > > +If an error occurs, > > +the kernel may zero the provided > > +.I uffdio_api > > +structure. > > +The caller should treat its contents as unspecified, > > +and reinitialize it before re-attempting another > > +.B UFFDIO_API > > +call. > > Possible errors include: > > .TP > > .B EFAULT > > @@ -281,14 +289,6 @@ feature was enabled, > > but the calling process doesn't have the > > .B CAP_SYS_PTRACE > > capability. > > -.\" FIXME In the above error case, the returned 'uffdio_api' structure is > > -.\" zeroed out. Why is this done? This should be explained in the manual page. > > -.\" > > -.\" Mike Rapoport: > > -.\" In my understanding the uffdio_api > > -.\" structure is zeroed to allow the caller > > -.\" to distinguish the reasons for -EINVAL. > > -.\" > > I've added Mike to the thread in case he wants to comment. Thanks, Alex! It looks like I reviewed v1 of the patchset though :) > Thanks, > Alex > > > .SS UFFDIO_REGISTER > > (Since Linux 4.3.) > > Register a memory address range with the userfaultfd object. > > -- > > 2.42.0.609.gbb76f46606-goog > > > > -- > <https://www.alejandro-colomar.es/> -- Sincerely yours, Mike.
