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Subject[patch] sys_modify_ldt extension (default_ldt)
Date
Hi,

I am trying to integrate binfmt_xout.c into kernel 2.4 as part of the
linux-abi project (formerly known as iBCS). For old Xenix 286 binaries the
lcall7 gate needs to part of the LDT.

In kernels 2.0 sys_modify_ldt(0,...) used to return the default_ldt (with
lcall7 gate) if there were no segments set up. This behaviour changed in
kernels 2.2 . As a result of a discussion with Linus, David Bruce wrote a
patch for binfmt_xout.c tweaking with gdt and current->tss.ldt to get the
address of default_ldt. This patch does not work any more with kernels 2.4
as tss vanished from task_struct.

I do see 4 ways to cope with this problem:

a) extend sys_modify_ldt with a function to retrieve the default_ldt. I did
this for testing (see attached diff for arch/i386/kernel/ldt.c ).

b) do some work an Davids patch but this is kind of magic for me :-)
(see attached default_ldt patch)

c) loose the option to compile binfmt_xout (and the rest of linux-abi) as
module and simply use the symbol default_ldt. I dint't try that.

d) Forget about those old fashioned 286 binaries. This option will make some
linux users feel sad, as they run these progs for their daily business.

Joerg
--
------------------------------------------------------------------------------
Joerg Ahrens _/
Koenigsberger Strasse 32 _/_/
31226 Peine _/ _/
Tel.: 05171/57308 _/ _/_/_/_/
e-mail: joerg@hydrops.han.de _/_/_/_/ _/
------------------------------------------------------------------------------
--- linux-2.4.0/arch/i386/kernel/ldt.c Fri Dec 29 23:07:20 2000
+++ linux-2.4.0.i/arch/i386/kernel/ldt.c Sat Jun 9 22:48:46 2001
@@ -44,7 +44,24 @@
out:
return err;
}
+static int read_default_ldt(void * ptr, unsigned long bytecount)
+{
+ int err;
+ unsigned long size;
+ void *address;
+
+ err = 0;
+ address = &default_ldt[0];
+ size = sizeof(struct desc_struct);
+ if (size > bytecount)
+ size = bytecount;
+
+ err = size;
+ if (copy_to_user(ptr, address, size))
+ err = -EFAULT;

+ return err;
+}
static int write_ldt(void * ptr, unsigned long bytecount, int oldmode)
{
struct mm_struct * mm = current->mm;
@@ -156,6 +173,9 @@
break;
case 1:
ret = write_ldt(ptr, bytecount, 1);
+ break;
+ case 2:
+ ret = read_default_ldt(ptr, bytecount);
break;
case 0x11:
ret = write_ldt(ptr, bytecount, 0);struct desc_struct def_ldt;
unsigned long *lp, *lp2;

asm volatile ( "sgdt __gdt+2" );

lp = (unsigned long *)(__gdt[1] + current->tss.ldt );

lp2 = (unsigned long *)(((*lp >> 16) & 0x0000ffff)
| (*(lp+1) & 0xff000000)
| ((*(lp+1) << 16) & 0x00ff0000));

def_ldt.a = *lp2;
def_ldt.b = *(lp2+1);
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