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 > .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/>
Attachment:
signature.asc
Description: PGP signature
