Re: [PATCH] sched_getattr.2: update to include changed size semantics

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Aleksa,

I don't think there was ever a follow-up to this patch. Would you be
willing to send one?

Thanks,

Michael

On Thu, 28 Nov 2019 at 14:55, Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> On 2019-11-28, Thadeu Lima de Souza Cascardo <cascardo@canonical.com> wrote:
> > On Thu, Nov 28, 2019 at 11:01:40PM +1100, Aleksa Sarai wrote:
> > > Due to a userspace breakage, commit 1251201c0d34 ("sched/core: Fix
> > > uclamp ABI bug, clean up and robustify sched_read_attr() ABI logic and
> > > code") changed the semantics of sched_getattr(2) when the userspace
> > > struct is smaller than the kernel struct. Now, any trailing non-zero
> > > data in the kernel structure is ignored when copying to userspace.
> > >
> > > Ref: 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
> > >                     robustify sched_read_attr() ABI logic and code")
> > > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > > ---
> > >  man2/sched_setattr.2 | 6 ++----
> > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/man2/sched_setattr.2 b/man2/sched_setattr.2
> > > index 76ffa14eba85..fbb67b8eb98b 100644
> > > --- a/man2/sched_setattr.2
> > > +++ b/man2/sched_setattr.2
> > > @@ -284,10 +284,8 @@ structure,
> > >  the additional bytes in the user-space structure are not touched.
> > >  If the caller-provided structure is smaller than the kernel
> > >  .I sched_attr
> > > -structure and the kernel needs to return values outside the provided space,
> > > -.BR sched_getattr ()
> > > -fails with the error
> > > -.BR E2BIG .
> > > +structure, the kernel will silently not return any values which would be stored
> > > +outside the provided space.
> > >  As with
> > >  .BR sched_setattr (),
> > >  these semantics allow for future extensibility of the interface.
> > > --
> > > 2.24.0
> > >
> >
> > I was thinking about documenting the difference in behavior of older kernels,
> > before uclamp support.
> >
> > However, in practice, for sched_getattr, the kernel never returned E2BIG (the
> > code uses EFBIG incorrectly, in fact). It does, however, return EINVAL for
> > sizes smaller than SCHED_ATTR_SIZE_VER0.
>
> I've been told the EFBIG was actually a typo and it was always meant to
> be E2BIG. But yes, the precise problem with the old semantics was that
> they weren't tested "in the wild" with a proper struct upgrade -- hence
> all of the headaches.
>
> If we ever do implement a copy_struct_to_user() we are almost certainly
> going to implement it with the new sched_getattr() semantics. To be
> honest, I'm not sure I can imagine a case where an old userspace program
> would benefit from being given an error saying that the kernel has some
> properties that it doesn't understand. (sched_getattr() is also weird
> for other reasons, such as the fact it takes a separate size argument.)
>
> > However, E2BIG is still mentioned below as a possible return value for
> > sched_getattr. Can you remove that too?
>
> Will do.
>
> --
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/