On Thu, 2009-11-19 at 10:58 +0100, Louis Rilling wrote:
> > int clone_with_pids(long flags_low, struct clone_args *clone_args, long args_size,
> > int *pids)
> > {
> > long retval;
> >
> > __asm__ __volatile__(
> > "movq %3, %%r10\n\t" /* pids in r10*/
> > "pushq %%rbp\n\t" /* save value of ebp */
> > :
> > :"D" (flags_low), /* rdi */
> > "S" (clone_args),/* rsi */
> > "d" (args_size), /* rdx */
> > "a" (pids) /* use rax, which gets moved to r10 */
> > );
>
> 1. The fourth C arg is not in rax, but in rcx.
Hey Louis,
So, try as I might, I couldn't get that to work. I thought it was rcx,
too.
So, changing that instruction to:
"movq %3, %%rcx\n\t" /* pids in r10*/
and putting 0x11111, etc... in for the args the strace output for the
syscall looks like this:
syscall_299(0x11111, 0x22222, 0x33333, 0x1, 0x1, 0x2, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0) = -1 (errno 22)
and I get -EFAULT back from the function doing the copy_from_user() of
the pids argument, even when using good values.
If I use the asm posted above, I get this:
syscall_299(0x11111, 0x22222, 0x33333, 0x44444, 0x1, 0x2, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0) = -1 (errno 22)
Or, this from a real call:
syscall_299(0x1100011, 0x7fff19f0fd40, 0x38, 0x602070, 0x1, 0x2,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0[2992, 377]: Child:
I had to find r10 basically by trial and error. I have no idea why it
works.
> >
> > __asm__ __volatile__(
> > "syscall\n\t" /* Linux/x86_64 system call */
> > "testq %0,%0\n\t" /* check return value */
> > "jne 1f\n\t" /* jump if parent */
> > "popq %%rbx\n\t" /* get subthread function */
> > "call *%%rbx\n\t" /* start subthread function */
> > "movq %2,%0\n\t"
> > "syscall\n" /* exit system call: exit subthread */
> > "1:\n\t"
> > "popq %%rbp\t" /* restore parent's ebp */
> > :"=a" (retval)
> > :"0" (__NR_clone3), "i" (__NR_exit)
> > :"ebx", "ecx", "edx"
> > );
>
> 2. You should probably not separate this into two asm statements. In particular,
> the compiler has no way to know that r10 should be preserved between the two
> statements, and may be confused by the change of rsp.
Yeah, I wondered about that. Suka, we should probably fix your tests
and the i386 code, too.
> 3. r10 and r11 should be listed as clobbered.
D'oh! I didn't even touch the bottom registers because it continued to
work from the i386 version that I stole from Suka.
> 4. I fail to see the magic that puts the subthread function pointer in the
> stack.
>
> 5. Maybe rdi should contain the subthread argument before calling the subthread?
>
> 6. rdi, rsi, rdx, rcx, r8 and r9 should be added to the clobber list because of
> the call to the subthread function.
>
> 7. rsi could be used in place of rbx to hold the function pointer, which would
> allow you to remove ebx from the clobber list.
>
> 8. I don't see why rbp should be saved. The ABI says it must be saved by the
> callee.
>
> 9. Before calling exit(), maybe put some exit code in rdi?
Thanks for looking through this, Louis. I'll send out another version
in a bit.
-- Dave
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