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* [PATCH] sched_getattr.2: update to include changed size semantics
@ 2019-11-28 12:01 Aleksa Sarai
  2019-11-28 13:08 ` Thadeu Lima de Souza Cascardo
  0 siblings, 1 reply; 5+ messages in thread
From: Aleksa Sarai @ 2019-11-28 12:01 UTC (permalink / raw)
  To: Michael Kerrisk
  Cc: linux-man, linux-api, Ingo Molnar, Arnaldo Carvalho de Melo,
	Thadeu Lima de Souza Cascardo, Jiri Olsa, Patrick Bellasi,
	Peter Zijlstra, Thomas Gleixner, Aleksa Sarai

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


^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] sched_getattr.2: update to include changed size semantics
  2019-11-28 12:01 [PATCH] sched_getattr.2: update to include changed size semantics Aleksa Sarai
@ 2019-11-28 13:08 ` Thadeu Lima de Souza Cascardo
  2019-11-28 13:55   ` Aleksa Sarai
  0 siblings, 1 reply; 5+ messages in thread
From: Thadeu Lima de Souza Cascardo @ 2019-11-28 13:08 UTC (permalink / raw)
  To: Aleksa Sarai
  Cc: Michael Kerrisk, linux-man, linux-api, Ingo Molnar,
	Arnaldo Carvalho de Melo, Jiri Olsa, Patrick Bellasi,
	Peter Zijlstra, Thomas Gleixner

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.

The EINVAL return is already well documented in the manpage.

However, E2BIG is still mentioned below as a possible return value for
sched_getattr. Can you remove that too?

Thanks.
Cascardo.

^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] sched_getattr.2: update to include changed size semantics
  2019-11-28 13:08 ` Thadeu Lima de Souza Cascardo
@ 2019-11-28 13:55   ` Aleksa Sarai
  2020-04-25 19:12     ` Michael Kerrisk (man-pages)
  0 siblings, 1 reply; 5+ messages in thread
From: Aleksa Sarai @ 2019-11-28 13:55 UTC (permalink / raw)
  To: Thadeu Lima de Souza Cascardo
  Cc: Michael Kerrisk, linux-man, linux-api, Ingo Molnar,
	Arnaldo Carvalho de Melo, Jiri Olsa, Patrick Bellasi,
	Peter Zijlstra, Thomas Gleixner


[-- Attachment #1: Type: text/plain, Size: 2831 bytes --]

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/>

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] sched_getattr.2: update to include changed size semantics
  2019-11-28 13:55   ` Aleksa Sarai
@ 2020-04-25 19:12     ` Michael Kerrisk (man-pages)
  2020-05-15 12:10       ` Michael Kerrisk (man-pages)
  0 siblings, 1 reply; 5+ messages in thread
From: Michael Kerrisk (man-pages) @ 2020-04-25 19:12 UTC (permalink / raw)
  To: Aleksa Sarai
  Cc: Thadeu Lima de Souza Cascardo, linux-man, Linux API, Ingo Molnar,
	Arnaldo Carvalho de Melo, Jiri Olsa, Patrick Bellasi,
	Peter Zijlstra, Thomas Gleixner, Michael Kerrisk

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/

^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH] sched_getattr.2: update to include changed size semantics
  2020-04-25 19:12     ` Michael Kerrisk (man-pages)
@ 2020-05-15 12:10       ` Michael Kerrisk (man-pages)
  0 siblings, 0 replies; 5+ messages in thread
From: Michael Kerrisk (man-pages) @ 2020-05-15 12:10 UTC (permalink / raw)
  To: Aleksa Sarai
  Cc: Thadeu Lima de Souza Cascardo, linux-man, Linux API, Ingo Molnar,
	Arnaldo Carvalho de Melo, Jiri Olsa, Patrick Bellasi,
	Peter Zijlstra, Thomas Gleixner, Michael Kerrisk

Hi Aleksa,

A ping also on this :-).

Cheers,

Michael

On Sat, 25 Apr 2020 at 21:12, Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
>
> 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/



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

^ permalink raw reply	[flat|nested] 5+ messages in thread

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Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-11-28 12:01 [PATCH] sched_getattr.2: update to include changed size semantics Aleksa Sarai
2019-11-28 13:08 ` Thadeu Lima de Souza Cascardo
2019-11-28 13:55   ` Aleksa Sarai
2020-04-25 19:12     ` Michael Kerrisk (man-pages)
2020-05-15 12:10       ` Michael Kerrisk (man-pages)

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