| Message ID | 20231003194547.2237424-4-axelrasmussen@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | userfaultfd man page updates | expand |
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@google.com> > --- > 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 >
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@google.com> > > --- > > 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/>
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. -.\" .SS UFFDIO_REGISTER (Since Linux 4.3.) Register a memory address range with the userfaultfd object.
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@google.com> --- man2/ioctl_userfaultfd.2 | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-)