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Re: [PATCH] [RFC] PR target/52813 and target/11807


On Wed, Dec 19, 2018 at 08:40:13AM +0200, Dimitar Dimitrov wrote:
> On Mon, Dec 17 2018 20:15:02 EET Bernd Edlinger wrote:
> > out of curiosity I looked at the clobber statement in
> > gdb/nat/linux-ptrace.c:
> > 
> >            asm volatile ("pushq %0;"
> >                          ".globl linux_ptrace_test_ret_to_nx_instr;"
> >                          "linux_ptrace_test_ret_to_nx_instr:"
> >                          "ret"
> >                          : : "r" ((uint64_t) (uintptr_t) return_address)
> >                          : "%rsp", "memory");
> > 
> > it turns out to be a far jump, instruction.
> 
> GDB functionality should not be affected if SP clobber is removed, even if the 
> generated code is slightly different. Please see this comment:
> http://sourceware.org/ml/gdb-patches/2018-12/msg00204.html
> 
> As I understand it, this particular code is never meant to return. It should 
> either stop due to the NX mapping of return_address/%0, or hit the breakpoint 
> placed at return_address/%0.

If it doesn't return it is undefined behaviour, so anything might happen
and that is perfectly alright.

Defining labels is an asm is undefined, too.

Maybe real assembler code is wanted here?  I.e. a .s file.


Segher


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