[patch] sys_modify_ldt extension (default_ldt)

Joerg Ahrens (joerg@hydrops.han.de)
Sun, 10 Jun 2001 01:47:22 +0200 (MET DST)


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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

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Joerg Ahrens                                              _/           
Koenigsberger Strasse 32                                _/_/
31226 Peine                                           _/  _/
Tel.: 05171/57308                             _/    _/_/_/_/
e-mail: joerg@hydrops.han.de                _/_/_/_/      _/
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--- 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);

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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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