Re: [PATCH] iopl.2: Changing description of permissions set per-process to per-thread

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

Hi Thomas P,

Might you have an update for this patch?

Thanks,

Michael


On Thu, 28 May 2020 at 16:52, Thomas Piekarski
<t.piekarski@deloquencia.de> wrote:
>
> Hello Thomas,
> Hello Michael,
>
>
> On 28.05.20 3:22 PM, Thomas Gleixner wrote:
> > "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> >
> >> I expect that the small change at Thomas P proposes in this patch is
> >> correct (i.e., iopl(2) operates per-thread, not per-process). I
> >> remember that you touch the relevant kernel source file often. Perhaps
> >> you are able to give a quick Ack?
> >>>
> >>>    man2/iopl.2 | 6 +++---
> >>>    1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/man2/iopl.2 b/man2/iopl.2
> >>> index e5b216a14..329095808 100644
> >>> --- a/man2/iopl.2
> >>> +++ b/man2/iopl.2
> >>> @@ -39,7 +39,7 @@ iopl \- change I/O privilege level
> >>>    .BI "int iopl(int " level );
> >>>    .SH DESCRIPTION
> >>>    .BR iopl ()
> >>> -changes the I/O privilege level of the calling process,
> >>> +changes the I/O privilege level of the calling thread,
> >
> > I'm fine with the s/process/thread/ changes. The permissions are really
> > per thread.
> >
> > Though the manpage should mention that a thread inherits the permissions
> > from the parent, i.e. clone() vs. fork(), exec().
> >
> >>>    as specified by the two least significant bits in
> >>>    .IR level .
> >>>    .PP
> >>> @@ -50,7 +50,7 @@ Since these X servers require access to all 65536 I/O
> >>> ports, the
> >>>    call is not sufficient.
> >>>    .PP
> >>>    In addition to granting unrestricted I/O port access, running at a higher
> >>> -I/O privilege level also allows the process to disable interrupts.
> >>> +I/O privilege level also allows the thread to disable interrupts.
> >
> > This paragraph became outdated as of
> >
> >     a24ca9976843 ("x86/iopl: Remove legacy IOPL option")
> >
> > in v5.5. The kernel no longer allows user space to disable
> > interrupts. It still grants access to _ALL_ 64k ioports.
> >
> > Also:
> >
> >> This call is necessary to allow 8514-compatible X servers to run under
> >> Linux.  Since these X servers require access to all 65536 I/O ports,
> >> the ioperm(2) call is not sufficient.
> >
> > is outdated.
> >
> > ioperm() allows to set all 64k bits, but its significantly slower than
> > iopl(3) because it needs to copy 8k of data on context switch while
> > iopl(3) just maps an 'all bits set' static bitmap.
> >
> > Aside of that only really old x servers rely on iopl(3).
>
>
> Thanks for your feedback. I'll update the patch accordingly.
>
> 1. Rechecking that it says that permissions are inherited from parents
> 2. Stating that since Kernel v5.5 it is not possible anymore to
>     disable interrupts from user space
> 3. Removing the paragraph "This call is necessary..."
>
> Should the man page mention that iopl is deprecated, provided only for
> compatibility to old X-Servers and significantly slower than ioperm?



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