From: Venkatesh Srinivas <me@acm.jhu.edu>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Subject: [9fans] segattach off-by-one?
Date: Thu, 24 Sep 2009 23:33:52 -0400 [thread overview]
Message-ID: <f75780240909242033k11fb23a0o237f7dbedf9a7bee@mail.gmail.com> (raw)
Hi,
This little program:
#include <u.h>
#include <libc.h>
#define SEGBASE ((char *) 0x10001001)
#define SEGSIZE 0x1000
void main(void) {
segattach(0, "shared", SEGBASE, SEGSIZE);
// Works fine (writing to 0x10001fff)
*(char *) (SEGBASE + SEGSIZE - 2) = 'a';
// Suicide! (writing to 0x10002000)
*(char *) (SEGBASE + SEGSIZE - 1) = 'a';
}
However, segattach's manpage claims: "... and va+len is rounded up."
Shouldn't the second page here be mapped?
I propose this patch to /sys/src/9/port/segment.c::
--- segment.c.orig 2009-09-24 22:41:59.000000000 -0400
+++ segment.c 2009-09-24 22:38:25.000000000 -0400
@@ -641,6 +641,11 @@
int sno;
Segment *s, *os;
Physseg *ps;
+ ulong ova;
+
+ ova = va;
+ va = va&~(BY2PG-1);
+ len += (ova - va);
if(va != 0 && va >= USTKTOP)
error(Ebadarg);
This patch also prevents segattaching to the zero page, which I think
was worth doing...
Could people try this? Comments?
Thanks,
-- vs
next reply other threads:[~2009-09-25 3:33 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-09-25 3:33 Venkatesh Srinivas [this message]
[not found] <<f75780240909242033k11fb23a0o237f7dbedf9a7bee@mail.gmail.com>
2009-09-25 4:34 ` erik quanstrom
2009-09-25 18:01 ` Venkatesh Srinivas
2009-09-25 18:18 ` erik quanstrom
2009-09-25 18:32 ` erik quanstrom
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