Messages in this thread | | | From | alad@hss ... | Date | Wed, 26 Dec 2001 16:20:10 +0530 | Subject | Re: Weird __put_user_asm behavior |
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Frank Cornelis <fcorneli@elis.rug.ac.be> on 12/26/2001 04:12:55 PM
To: vda <vda@port.imtp.ilyichevsk.odessa.ua> cc: linux-kernel@vger.kernel.org (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 :).
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