Re: [PATCH 1/2] sched_getattr.2: update to include changed size semantics

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

On 9/30/20 12:35 AM, 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. We
> also document the original error code correctly (it was EFBIG not
> E2BIG) in the BUGS section.

Thanks, Aleksa. Patch applied.

Cheers,

Michael


> 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 | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/man2/sched_setattr.2 b/man2/sched_setattr.2
> index 2592a761a086..da0972a6a86d 100644
> --- a/man2/sched_setattr.2
> +++ b/man2/sched_setattr.2
> @@ -286,10 +286,8 @@ 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.
> @@ -416,6 +414,16 @@ failed with the error
>  instead of
>  .BR E2BIG
>  for the case described in ERRORS.
> +.PP
> +In Linux versions up to 5.3,
> +.BR sched_getattr ()
> +failed with the error
> +.BR EFBIG
> +if the in-kernel
> +.IR sched_attr
> +structure was larger than the
> +.IR size
> +passed by user space.
>  .\" In Linux versions up to up 3.15,
>  .\" FIXME . patch from Peter Zijlstra pending
>  .\" .BR sched_setattr ()
> 


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