Re: About intercepting linux system call

[JinShan Xiong <jinshan.xiong@gmail.com>]

Hi David,

I don't know how to use the stub code, I have tried to copy it to
overload sys_gettimeofday(), like this,

 memcpy(sys_call_table[__NR_gettimeofday - 1024], new_time_stub, 32),

it crashed the kenrel also. 

Please help me!

Thanks,

JinShan


On Thu, 27 Jan 2005 20:29:49 +0800, JinShan Xiong
<jinshan.xiong@gmail.com> wrote:
> I think I was not used the stub code correctly.
> 
> JinShan
> 
> 
> On Thu, 27 Jan 2005 15:17:30 +0800, JinShan Xiong
> <jinshan.xiong@gmail.com> wrote:
> > Hi,
> >
> > Seems to near our target;-).  But the kernel crashed too while I
> > installed the following module.
> >
> > I am downloading ski, thank you, David.
> >
> > JinShan
> >
> > Here is my test file:/* vi: set ts=4 sw=4 expandtab: */
> >
> > #include <linux/config.h>
> > #include <linux/kernel.h>
> > #include <linux/module.h>
> > #include <linux/unistd.h>
> > #include <linux/sched.h>
> > #include <asm/pgtable.h>
> > #include <linux/vmalloc.h>
> > #include <linux/mm.h>
> > #include <asm/uaccess.h>
> >
> > extern unsigned long sys_call_table[];
> >
> > static long (*old_time)(struct timeval *, struct timezone *);
> > extern void new_time_stub(void);
> > //extern unsigned long new_time_stub;
> >
> > asm (
> > "        .proc new_time_stub\n"
> > "new_time_stub:"
> > "       .prologue\n"
> > "       .regstk 2, 3, 2, 0\n"
> > "       .save ar.pfs, loc1\n"
> > "       alloc loc1 = ar.pfs, 2, 3, 2, 0\n"
> > "       movl r2 = @gprel(zero);;\n"
> > "       .save rp, loc0\n"
> > "       mov loc0 = rp\n"
> > "       mov loc2 = gp\n"
> > "       sub gp = r0, r2\n"
> > "       mov out0 = in0\n"
> > "       mov out1 = in1\n"
> > "       br.call.sptk.many rp = new_time\n"
> > "1:      mov rp = loc0\n"
> > "       mov ar.pfs = loc1\n"
> > "       mov gp = loc2\n"
> > "       br.ret.sptk.many rp\n"
> > "       .endp\n"
> > );
> >
> > long new_time(struct timeval *tv, struct timezone *tz)
> > {
> >     if (tv) {
> >         struct timeval ktv;
> >         do_gettimeofday(&ktv);
> >         if (copy_to_user(tv, &ktv, sizeof(ktv)))
> >             return -EFAULT;
> >     }
> >     if (tz) {
> >         extern struct timezone sys_tz;
> >         if (copy_to_user(tz, &sys_tz, sizeof(sys_tz)))
> >             return -EFAULT;
> >     }
> >     return 0;
> > }
> >
> > int init_module(void)
> > {
> >     printk("new_time_stub is %llx\n", new_time_stub);
> >     old_time = sys_call_table[__NR_gettimeofday - 1024];
> >     sys_call_table[__NR_gettimeofday - 1024] = new_time_stub;
> >     return 0;
> > }
> >
> > void cleanup_module()
> > {
> >     /* should restore syscall here! */
> >     sys_call_table[__NR_gettimeofday - 1024] = old_time;
> >     printk("Byebye!\n");
> > }
> >
> > and makefile:
> > all:
> >     gcc -D__KERNEL__ -DMODULE -I/lib/modules/`uname -r`/build/include -c ro.c
> >     ld -r -o mod.o ro.o --defsym zero=0
> >
> > kernel dump msg:
> > - - - - - - - - - - - - Live Console - - - - - - - - - - - -
> > new_time_stub is a000000000318f70
> > klogd[784]: IA-64 Illegal operation fault 0
> > --> .opd [mod] 0x21 <--
> >
> > Pid: 784, comm:                klogd
> > psr : 0000121008026018 ifs : 8000000000000002 ip  :
> > [<a000000000318f71>]    Tainted: P
> > unat: 0000000000000000 pfs : 0000000000000002 rsc : 0000000000000003
> > rnat: 0000000000000000 bsps: 0000000000000000 pr  : 80000000ff600199
> > ldrs: 0000000000000000 ccv : 00000000000001ad fpsr: 0009804c0270033f
> > b0  : e00000000440df00 b6  : e000000004402f60 b7  : e00000000440d990
> > f6  : 1003ecccccccccccccccd f7  : 1003e0000000000000004
> > f8  : 1003e0000000000000064 f9  : 1003ea3d70a3d70a3d70b
> > r1  : e000000004cf5760 r2  : 0000000000000000 r3  : 00000000000000ff
> > r8  : e0000040fc4a7f00 r9  : 20000000002a4fc0 r10 : 0000000000000000
> > r11 : 6000000000009d50 r12 : e0000040fc4a7e60 r13 : e0000040fc4a0000
> > r14 : e000000000000000 r15 : e00000000440df00 r16 : e0000040fc4a7e70
> > r17 : e0000040fc4a7e78 r18 : 00001413085a6010 r19 : 200000000018f4d0
> > r20 : 0000000000000002 r21 : 0000000000255b0a r22 : 00000000005b0a3e
> > r23 : 60000fffffffaf20 r24 : 0a0a0a0a0a2f5100 r25 : 0a0a0a0a0a0a0a0a
> > r26 : 0000000000000048 r27 : 0000000000000000 r28 : 0000000000000018
> > r29 : 0000000000000028 r30 : 0000000000000008 r31 : 0000000000000000
> >
> > Call Trace: [<e000000004414910>] sp=0xe0000040fc4a79c0 bsp=0xe0000040fc4a12c0
> > decoded to show_stack [kernel] 0x50
> > [<e000000004415140>] sp=0xe0000040fc4a7b80 bsp=0xe0000040fc4a1268
> > decoded to show_regs [kernel] 0x7c0
> > [<e00000000442fd90>] sp=0xe0000040fc4a7ba0 bsp=0xe0000040fc4a1240
> > decoded to die [kernel] 0x190
> > [<e00000000442fe60>] sp=0xe0000040fc4a7ba0 bsp=0xe0000040fc4a1218
> > decoded to die_if_kernel [kernel] 0x40
> > [<e000000004430af0>] sp=0xe0000040fc4a7ba0 bsp=0xe0000040fc4a1200
> > decoded to ia64_illegal_op_fault [kernel] 0x50
> > [<e000000004403ed0>] sp=0xe0000040fc4a7cc0 bsp=0xe0000040fc4a1200
> > decoded to dispatch_illegal_op_fault [kernel] 0x2b0
> >  <0>Kernel panic: not continuing
> > bash[1192]: IA-64 Illegal operation fault 0
> > ....
> >
> >
> > On Wed, 26 Jan 2005 21:32:49 -0800, David Mosberger
> > <davidm@napali.hpl.hp.com> wrote:
> > > Hi JinShan,
> > >
> > > >>>>> On Thu, 27 Jan 2005 12:54:40 +0800, JinShan Xiong <jinshan.xiong@gmail.com> said:
> > >
> > >   JinShan> Hi all, i just want to intercept ia64 linux kernel's
> > >   JinShan> syscall entry. I remapped the physical page contained
> > >   JinShan> syscall table to a new read/write page in a vmalloc
> > >   JinShan> region(0xa0000...) since ia64 linux kernel has been linked
> > >   JinShan> the syscall table into a .rodata section, Yes, I can modify
> > >   JinShan> the syscall entry now, but the kernel crashed after the
> > >   JinShan> kernel entered into my own new function.
> > >
> > >   JinShan> I run my test code on a Hp-ia64 machine with redhat AS-2.1e
> > >   JinShan> installed, and the kernel is 2.4.18-e.47smp.
> > >
> > >   JinShan> I am not familiar with ia64 architecture, please help me,
> > >   JinShan> thanks.
> > >
> > > Hi JinShan,
> > >
> > > There is no need to copy the syscall table to a writable area.  On
> > > ia64, the kernel memory is writable (for the kernel) by default.  I
> > > think the problem in your code is due to the gp register not being
> > > setup properly before calling into the module.  Each module gets its
> > > own global-offset-table (GOT) so the gp needs to be loaded up before
> > > calling any of the module's C function.  However, the kernel assumes
> > > that all system calls are implemented in the kernel proper, so it
> > > bypasses the gp-loading that would normally happen when calling
> > > through a function-pointer.
> > >
> > > This can be fixed with a little stub which takes care of saving the
> > > old gp-value, loading the modules gp, calling the real function and,
> > > upon return, restoring the original gp-value.
> > >
> > > I think something like this might work:
> > >
> > >         .proc new_time_stub
> > > new_time_stub:
> > >         .prologue
> > >         .regstk 2, 3, 2, 0
> > >         .save ar.pfs, loc1
> > >         alloc loc1 = ar.pfs, 2, 3, 2, 0
> > >         movl r2 = @gprel(zero);;
> > >         .save rp, loc0
> > >         mov loc0 = rp
> > >         mov loc2 = gp
> > >         sub gp = r0, r2
> > >         mov out0 = in0
> > >         mov out1 = in1
> > >         br.call.sptk.many rp = new_time
> > > 1:      mov rp = loc0
> > >         mov ar.pfs = loc1
> > >         mov gp = loc2
> > >         br.ret.sptk.many rp
> > >         .endp
> > >
> > > Here, "zero" needs to be a symbol that the linker resolves to 0.  You
> > > can define "zero" either via a linker script or by passing the linker
> > > the option "--defsym zero=0".  It may not be the most elegant way to
> > > get the GP value, but it ought to work both on 2.4 and 2.6 (which use
> > > different module loaders).
> > >
> > > Having said that, two caveats:
> > >
> > >  - In 2.6, sys_call_table is no longer exported, so your code can't
> > >    work (and that's intentional, see below).
> >
> > I always put the sys_call_table address as a module parameter into
> > kernel in version above 2.4.20, hehe. Ugly?
> >
> > >
> > >  - Kernel developers generally frown on modules that try to intercept
> > >    syscalls.  For one thing, it's potentially racy in an SMP
> > >    environment and for another, it's questionable whether it's even
> > >    legal to do so, at least if the module is proprietary (not offering
> > >    a legal opinion here, just raising a potential red flag).
> >
> > Nod. I am very happy to export our kernel module source code under GPL license.
> >
> > >
> > > On a related topic, you may find it easier to develop such code with
> > > the Ski simulator [1].  It's very easy to setup and would let you
> > > single-step through the code in question, so you can see exactly
> > > what's going on.
> > >
> > >         --david
> > >
> > > [1] http://www.hpl.hp.com/research/linux/ski/
> > >
> >
>