Re: Weird __put_user_asm behavior

Frank Cornelis <[e-mail]> on 12/26/2001 04:12:55 PM

To:   vda <[e-mail]>
cc:   [e-mail] (bcc: Amol Lad/HSS)

Subject:  Re: Weird __put_user_asm behavior

Hi,

> > So here it goes: when I patch the macro in include/asm-i386/uaccess.h
> > named __put_user_asm using the patch below I cannot 'strace ls' anymore.
> > The ld.so runtime linker just stops when preparing the program displaying
> > a nice message; the same message as '/lib/ld-linux.so.2' generates when
> > ran without any parameters.
> > The kernel messages of __put_user_asm go to: 0xbffffb44; then the program
> > stops on the ld.so message.
> > But, 'strace'-ing of a static compiled program does the job as expected.
> > What am I doing wrong?
>
> > --- linux-2.4.17/include/asm-i386/uaccess.h    Sat Dec 22 09:35:17 2001
> > +++ linux/include/asm-i386/uaccess.h      Mon Dec 24 12:43:22 2001
> > @@ -194,6 +194,10 @@
> >   * aliasing issues.
> >   */
> >  #define __put_user_asm(x, addr, err, itype, rtype, ltype)    \
> > +do {                                     \
> > +     if (current->ptrace & PT_PTRACED) {                \
> > +          printk(KERN_DEBUG "__put_user_asm: %#x\n", (unsigned
long)(addr)); \
> > +     }                                   \
> >       __asm__ __volatile__(                         \
>
> This must be clobbering some registers and some users of __put_user_asm
> might not expect that... did you try saving/restoring all regs around this
> 'if'?

I tried to save/restore the eax register before/after the printk function
call because this register is the only register on an i386 that a function
may alter without preserving its previous contents (even when the
function returns void).
>>>> This concept is wrong. Function is free to use/modify any register it
likes. Its not only eax.
Don't expect the value of a register (ebx/ecx etc) to remain same across AFTER a
function call.

What is weird is that an __asm__ __volatile__("nop") within the
if-statement works so it's not the if-statement itself that is causing
the problem, also works is a "push %eax\n\tpop %eax" so again it's not
the stack that is causing the problem.
But, when I try to access the (addr) parameter then execve shows the
ld.so message again. This is very odd because all it does is (in most
cases because GCC optimizes a lot) pushing a general register containing
(addr) onto the stack then call the function (I also tried a void func)
and after the call restore esp.
So, any more advice on this?

Thanks in advance,
Frank.

PS: Yes, I'm a thesis student and I really need this :).