Updating description of permissions for port-mapped I/O set per-thread
and not per-process. Mentioning iopl can not disable interrupts since
5.5 anymore and is in general deprecated and only provided for legacy X
servers.
See https://bugzilla.kernel.org/show_bug.cgi?id=205317
Reported-by: victorm007@xxxxxxxxx
Signed-off-by: Thomas Piekarski <t.piekarski@xxxxxxxxxxxxxx>
---
man2/iopl.2 | 34 ++++++++++++++--------------------
1 file changed, 14 insertions(+), 20 deletions(-)
diff --git a/man2/iopl.2 b/man2/iopl.2
index e5b216a14..be9acfd1e 100644
--- a/man2/iopl.2
+++ b/man2/iopl.2
@@ -39,29 +39,17 @@ 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,
as specified by the two least significant bits in
.IR level .
.PP
-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
-.BR ioperm (2)
-call is not sufficient.
+The I/O privilege level for a normal thread is 0.
+Permissions are inherited from parents to children.
.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.
-This will probably crash the system, and is not recommended.
-.PP
-Permissions are not inherited by the child process created by
-.BR fork (2)
-and are not preserved across
-.BR execve (2)
-(but see NOTES).
-.PP
-The I/O privilege level for a normal process is 0.
-.PP
-This call is mostly for the i386 architecture.
+This call is deprecated, significantly slower than
+.BR ioperm(2)
+and is only provided for older X servers which require
+access to all 65536 I/O ports. It is mostly for the i386 architecture.
On many other architectures it does not exist or will always
return an error.
.SH RETURN VALUE
@@ -79,7 +67,7 @@ is greater than 3.
This call is unimplemented.
.TP
.B EPERM
-The calling process has insufficient privilege to call
+The calling thread has insufficient privilege to call
.BR iopl ();
the
.B CAP_SYS_RAWIO
@@ -99,6 +87,12 @@ and in
.IR <sys/perm.h> .
Avoid the latter, it is available on i386 only.
.PP
+Prior to Linux 5.5
+.BR iopl ()
+allowed the thread to disable interrupts while running
+at a higher I/O privilege level. This will probably crash
+the system, and is not recommended.
+.PP
Prior to Linux 3.7,
on some architectures (such as i386), permissions
.I were
--
2.20.1
On 24.06.20 11:53 AM, Michael Kerrisk (man-pages) wrote:
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@xxxxxxxxxxxxxx> wrote:
On 28.05.20 3:22 PM, Thomas Gleixner wrote:
"Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> 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?
