Re: [PATCH] Add UNSUPPORTED check in elf/tst-pldd.

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 27/08/2019 07:19, Stefan Liebler wrote:
> Hi,
> 
> the testcase forks a child process and runs pldd with PID of
> this child.  On systems where /proc/sys/kernel/yama/ptrace_scope
> differs from zero, pldd will fail with
> /usr/bin/pldd: cannot attach to process 3: Operation not permitted
> 
> This patch checks if ptrace_scope is zero and otherwise marks the
> test as UNSUPPORTED.
> 
> Bye
> Stefan
> 
> ChangeLog:
> 
>     * elf/tst-pldd.c (do_test): Add UNSUPPORTED check.
> 
> 20190826_tst-pldd.patch
> 
> commit 9c0b03c38bdd31618909da46b8bd4e09b5a236d2
> Author: Stefan Liebler <stli@linux.ibm.com>
> Date:   Mon Aug 26 15:45:07 2019 +0200
> 
>     Add UNSUPPORTED check in elf/tst-pldd.
>     
>     The testcase forks a child process and runs pldd with PID of
>     this child.  On systems where /proc/sys/kernel/yama/ptrace_scope
>     differs from zero, pldd will fail with
>     /usr/bin/pldd: cannot attach to process 3: Operation not permitted
>     
>     This patch checks if ptrace_scope is zero and otherwise marks the
>     test as UNSUPPORTED.
>     
>     ChangeLog:
>     
>             * elf/tst-pldd.c (do_test): Add UNSUPPORTED check.
> 
> diff --git a/elf/tst-pldd.c b/elf/tst-pldd.c
> index 6b7c94a1c0..3f211dc342 100644
> --- a/elf/tst-pldd.c
> +++ b/elf/tst-pldd.c
> @@ -52,6 +52,24 @@ in_str_list (const char *libname, const char *const strlist[])
>  static int
>  do_test (void)
>  {
> +  /* Check if all processes can be debugged with ptrace.  */
> +  {
> +    FILE *f = fopen ("/proc/sys/kernel/yama/ptrace_scope", "r");
> +    if (f != NULL)
> +      {
> +	/* If ptrace_scope exists, then it has to be 0 which means
> +	   "classic ptrace permissions".  A process can PTRACE_ATTACH
> +	   to any other process running under the same uid, as long as
> +	   it is dumpable.  Otherwise pldd will fail to attach to the
> +	   subprocess.  */
> +	int i = 99;
> +	fscanf (f, "%d", &i);
> +	fclose (f);
> +	if (i != 0)
> +	  FAIL_UNSUPPORTED ("/proc/sys/kernel/yama/ptrace_scope != 0");
> +      }
> +  }
> +

This is a Linuxism and I think we should create a 'support_can_ptrace' similar
to 'support_can_chroot'.  The logic to detect it seems correct, I would just
check fscanf returned to value and use xfclose.  It would be something like

bool
support_can_ptrace (void)
{
  bool ret = true;

#ifdef __linux__
  /* YAMA may be not enabled.  If it is then ptrace_scope it has to be 0
     which means "classic ptrace permissions".  A process can 
     PTRACE_ATTACH to any other process running under the same uid, as
     long as it is dumpable.  Otherwise pldd will fail to attach to the
     subprocess.  */
  FILE *f = fopen ("/proc/sys/kernel/yama/ptrace_scope", "r");
  if (f == NULL)
    return true;

  int i = 99;
  TEST_COMPARE (fscanf (f, "%d", &i), 1);
  xfclose (f);
  ret = i == 0;
#endif

  return ret;
}

And I think we might eventually need to handle seccomp as well.

>    /* Create a copy of current test to check with pldd.  */
>    struct support_subprocess target = support_subprocess (target_process, NULL);
>  
>