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    SubjectRe: Weird __put_user_asm behavior

    > > 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 runtime linker just stops when preparing the program displaying
    > > a nice message; the same message as '/lib/' generates when
    > > ran without any parameters.
    > > The kernel messages of __put_user_asm go to: 0xbffffb44; then the program
    > > stops on the 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).
    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 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,

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

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