Re: [patch+7.5] Fix assertion fail on PaX gentoo (PR 14564)

[Pedro Alves <palves at redhat dot com>]

On 09/09/2012 03:22 PM, Jan Kratochvil wrote:

>  static void
>  linux_ptrace_test_ret_to_nx (void)
>  {
> -#ifdef __i386__
> +#if defined __i386__ || defined __x86_64__
>    pid_t child, got_pid;
>    gdb_byte *return_address, *pc;
>    long l;
> @@ -101,11 +103,21 @@ linux_ptrace_test_ret_to_nx (void)
>  		 strerror (errno));
>        else
>  	{
> +#if defined __i386__
>  	  asm volatile ("pushl %0;"
>  			".globl linux_ptrace_test_ret_to_nx_instr;"
>  			"linux_ptrace_test_ret_to_nx_instr:"
>  			"ret"
>  			: : "r" (return_address) : "%esp", "memory");
> +#elif defined __x86_64__
> +	  asm volatile ("pushq %0;"
> +			".globl linux_ptrace_test_ret_to_nx_instr;"
> +			"linux_ptrace_test_ret_to_nx_instr:"
> +			"ret"
> +			: : "r" (return_address) : "%rsp", "memory");
> +#else
> +# error "!__i386__ && !__x86_64__"
> +#endif
>  	  gdb_assert_not_reached ("asm block did not terminate");
>  	}
>  
> @@ -114,13 +126,29 @@ linux_ptrace_test_ret_to_nx (void)
>  
>    got_pid = waitpid (child, &status, 0);
>    gdb_assert (got_pid == child);
> +
> +  if (WIFSIGNALED (status))
> +    {
> +      gdb_assert (WTERMSIG (status) == SIGKILL);

This, and the other assertions in the function make a little
nervous.  It seems like one should be able to make them trigger
by sending a signal to gdb's process group, or "killall gdb" at
the wrong time?  E.g., a SIGTERM, a SIGINT, or a SIGSTOP?  The
linux_test_for_tracefork family of functions seems to warn
instead of asserting if something unexpected comes out of
ptrace/waitpid.  Might be a good idea for this function too?

> +
> +      warning (_("Cannot call inferior functions, Linux kernel PaX protection "
> +		 "forbids return to non-executable pages!"));
> +      return;
> +    }
> +
>    gdb_assert (WIFSTOPPED (status));
>  
>    /* We may get SIGSEGV due to missing PROT_EXEC of the return_address.  */
>    gdb_assert (WSTOPSIG (status) == SIGTRAP || WSTOPSIG (status) == SIGSEGV);
>  
>    errno = 0;
> +#if defined __i386__
>    l = ptrace (PTRACE_PEEKUSER, child, (void *) (uintptr_t) (EIP * 4), NULL);
> +#elif defined __x86_64__
> +  l = ptrace (PTRACE_PEEKUSER, child, (void *) (uintptr_t) (RIP * 8), NULL);
> +#else
> +# error "!__i386__ && !__x86_64__"
> +#endif
>    gdb_assert (errno == 0);
>    pc = (void *) (uintptr_t) l;
>  
> @@ -154,7 +182,7 @@ linux_ptrace_test_ret_to_nx (void)
>  
>    warning (_("Cannot call inferior functions, you have broken "
>  	     "Linux kernel i386 NX (non-executable pages) support!"));
> -#endif /* __i386__ */
> +#endif /* defined __i386__ || defined __x86_64__ */
>  }
>  
>  /* Display possible problems on this system.  Display them only once per GDB
> 


-- 
Pedro Alves