Linux-man Archive on lore.kernel.org
 help / color / Atom feed
* [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; 3+ 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] 3+ 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; 3+ 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] 3+ 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
  0 siblings, 0 replies; 3+ 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] 3+ messages in thread

end of thread, back to index

Thread overview: 3+ 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

Linux-man Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-man/0 linux-man/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-man linux-man/ https://lore.kernel.org/linux-man \
		linux-man@vger.kernel.org
	public-inbox-index linux-man

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-man


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git