On 12/08, Ananth N Mavinakayanahalli wrote:
>
> On Mon, Dec 07, 2009 at 07:05:40PM +0100, Oleg Nesterov wrote:
> > On 12/07, Oleg Nesterov wrote:
> > >
> > > On 12/07, Jan Kratochvil wrote:
> > > >
> > > > On Mon, 07 Dec 2009 15:24:51 +0100, Oleg Nesterov wrote:
> > > > > But. &raise_sigusr2 is not equal to the "actual" address of
> > > > > &raise_sigusr2(),
> > > > > this value points to the "thunk" (I do not know the correct English
> > > > > term)
> > > >
> > > > ppc64 calls it "function descriptor" (GDB
> > > > ppc64_linux_convert_from_func_ptr_addr):
> > > > For PPC64, a function descriptor is a TOC entry,
> > >
> > > Thanks Jan.
> > >
> > > > in a data section,
> > >
> > > Yes!
> > >
> > > Now I can't understand how this test-case could ever work on ppc.
> > > step-jump-cont does:
> > >
> > > regs->nip = raise_sigusr2; <--- points to data section
> > > ptrace(PTRACE_CONT);
> > >
> > > of course, the tracee gets SIGSEGV, this section is not executable.
> >
> > Hmm. Looks like, "powerpc" means a lot of different hardware, and
> > _PAGE_EXEC may be 0. I didn't notice this when I quickly grepped
> > arch/powerpc/
> >
> > IOW, perhaps on some machines r implies x ?
> >
> > Is yes, this can explain why the results differ on different
> > machines.
>
> Well, powerpc 32-bit adheres to the SVR4 ABI, while powerpc 64-bit uses
> the PPC64-ELF ABI (http://refspecs.linuxfoundation.org/ELF/ppc64/). The
> 64bit ABI uses function descriptors and the 'func_name' is the data
> address,
Cai, Ananth, thank you.
So. I think we can forget about the possible kernel problems (and
in any case we can rule out utrace).
The test-case just wrong and should be fixed. The tracee can't execute
the function descriptor in data section, that is why it gets SIGSEGV.
> while the '.func_name' is the text address.
tried to change the code to
REGS_ACCESS (regs, nip) = (unsigned long) .raise_sigusr2
but gcc doesn't like this ;)
> (See
> handle_rt_signal64 in arch/powerpc/kernel/signal_64.c and
> kprobe_lookup_name in arch/powerpc/include/asm/kprobes.h.
Thanks... looking at handle_rt_signal64(), looks like we should
also set regs->gpr[2] = funct_desc_ptr->toc if we change regs->nip
I hope someone who understand powerpc could fix the test-case ;)
Oleg.
