New: ioperm is kept on fork
https://bugzilla.kernel.org/show_bug.cgi?id=99911
Bug ID: 99911
Summary: ioperm is kept on fork
Product: Documentation
Version: unspecified
Hardware: All
OS: Linux
Status: NEW
Severity: normal
Priority: P1
Component: man-pages
Assignee: documentation_man-pages-ztI5WcYan/[email protected]
Reporter: [email protected]
Regression: No
Created attachment 179851
--> https://bugzilla.kernel.org/attachment.cgi?id=179851&action=edit
fork-and-beep test program
man ioperm currently states "Permissions are not inherited by the child created by fork(2); following a fork(2) the child must turn on those permissions that it needs." This is not true. I do not know if it was true at some point in the past, but ioperm is preserved across a fork at least as far back as Linux 2.6.26.
A test program to demonstrate this behavior is attached.
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A message that is not in the archive (sent off-list or not yet received): bug-99911-11311-3bo0kxnWaOQUvHkbgXJLS5sdmw4N0Rt+2LY78lusg7I@public.gmane.org/
[Bug 99911] ioperm is kept on fork
https://bugzilla.kernel.org/show_bug.cgi?id=99911
Michael Kerrisk <[email protected]> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |[email protected]
--- Comment #1 from Michael Kerrisk <[email protected]> --- (In reply to Alex Henrie from comment #0)
Quoted 10 lines
Created attachment 179851 [details]
fork-and-beep test program
man iopermcurrently states "Permissions are not inherited by the child created by fork(2); following a fork(2) the child must turn on those permissions that it needs." This is not true. I do not know if it was true at some point in the past, but ioperm is preserved across a fork at least as far back as Linux 2.6.26.A test program to demonstrate this behavior is attached.
So, there's two possibilities here. Either the man page has always been wrong, or once it was right, and then in the distant past things changed. I suspect that the latter is more likely, but I could be wrong. If the latter is the case, then even if we can't easily work out when the change occurred, it would probably be nice to have a note in the page saying that once upon a time the behavior was different.
With that in mind, could I ask you a favor. Could you take a quick look at the Linux 1.0 sources (https://www.kernel.org/pub/linux/kernel/v1.0/ ). If I am reading them correctly, then indeed back then the man page text was true. I am looking at sys_ioperm() in kernel/ioport.c:
asmlinkage int sys_ioperm(unsigned long from, unsigned long num, int turn_on) {
...
set_bitmap((unsigned long *)current->tss.io_bitmap, from, num,
!turn_on);
return 0;
}and sys_fork() in kernel/fork.c:
asmlinkage int sys_fork(struct pt_regs regs)
{
...
p->tss.bitmap = offsetof(struct tss_struct,io_bitmap);
for (i = 0; i < IO_BITMAP_SIZE+1 ; i++) /* IO bitmap is actually SIZE+1
*/
p->tss.io_bitmap[i] = ~0;
...
}
That latter piece code appears to be resetting the map in the child, but perhaps I am misreading.
Thanks,
Michael
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[Bug 99911] ioperm is kept on fork
In reply to a message not in the archivehttps://bugzilla.kernel.org/show_bug.cgi?id=99911
--- Comment #2 from Alex Henrie <[email protected]> --- I used QEMU to test Slackware 8.0,[1] which included both Linux 2.2.19 and Linux 2.4.5. On 2.2.19, ioperm is lost on fork, but on 2.4.5, ioperm is kept on fork. My guess is that the behavior changed in 2.4.0.
[1]
http://slackware.cs.utah.edu/pub/slackware/slackware-8.0-iso/slackware-8.0-livefs-d2.iso
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[Bug 99911] ioperm is kept on fork
In reply to a message not in the archivehttps://bugzilla.kernel.org/show_bug.cgi?id=99911
Michael Kerrisk <[email protected]> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |CODE_FIX--- Comment #3 from Michael Kerrisk <[email protected]> --- Alex, sorry for the long delay in following up.
I have applied the patch below. Closing this bug.
Cheers,
Michael
man2/ioperm.2 +6 -4
@@ -63,11 +63,9 @@ system call had to be used (with a argument of 3). Since Linux 2.6.8, 65,536 I/O ports can be specified. -Permissions are not inherited by the child created by -.BR fork (2); -following a +Permissions are inherited by the child created by .BR fork (2) -the child must turn on those permissions that it needs. +(but see NOTES). Permissions are preserved across .BR execve (2); this is useful for giving port access permissions to unprivileged
@@ -107,6 +105,10 @@ The .I /proc/ioports file shows the I/O ports that are currently allocated on the system. +Before Linux 2.4, +permissions were not inherited by a child created by +.BR fork (2). + Glibc has an .BR ioperm () prototype both in
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