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

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

On 02/09/2019 12:28, Stefan Liebler wrote:
> On 8/29/19 10:47 AM, Florian Weimer wrote:
>> * Stefan Liebler:
>>
>>> On 8/28/19 11:24 AM, Florian Weimer wrote:
>>>> * Stefan Liebler:
>>>>
>>>>>    static void
>>>>>    target_process (void *arg)
>>>>>    {
>>>>> +  if (ptrace_scope == 1)
>>>>> +    {
>>>>> +      /* YAMA is configured to "restricted ptrace".
>>>>> +     Disable the restriction for this subprocess.  */
>>>>> +      support_ptrace_process_set_ptracer_any ();
>>>>> +    }
>>>>> +
>>>>>      pause ();
>>>>>    }
>>>>
>>>> I think this has a race condition if pldd attaches to the process before
>>>> the support_ptrace_process_set_ptracer_any call.  I have no idea how
>>>> hard it is in practice to hit this race.  It should be possible to use a
>>>> process-shared barrier or some other form of synchronization to avoid
>>>> this issue.
>>>>
>>>> Thanks,
>>>> Florian
>>>>
>>>
>>> I've added a synchronization with stdatomic.h on a shared memory mapping.
>>> I've not used pthread* functions as I don't want to link against
>>> libpthread.so. Then further adjustments are needed.
>>>
>>> Or should I just restrict the test ptrace_scope 0 as Adhemerval has
>>> proposed in his post?
>>
>> Is it possible to create a process tree like this?
>>
>>
>>    parent (performs output checks)
>>      subprocess 1 (becomes pldd via execve)
>>        subprocess 2
>>
>> If you execve pldd from subprocess 1, wouldn't subprocess 2 in its
>> ptrace scope for ptrace_scope < 2?
> Yes, this is possible.
> I've rearranged the subprocesses. See attached patch.
> Now we have a new function pldd_process which forks target_process,
> stores the pid of target_prcess to a shared memory mapping as do_test needs to know this pid.
> 
> Afterwards it execve to pldd which successfully ptrace target_process in case of "restricted ptrace".
> 
> Please review the usage of support-subprocess-functions.
> 
> Bye,
> Stefan
>>
>> Thanks,
>> Florian
>>
> 
> 
> 20190902_tst-pldd.patch
> 
> commit ad51263d51d12ce6ca2ce9304efe5ba05b3912b1
> 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 exists, is zero "classic ptrace permissions"
>     or one "restricted ptrace".  If ptrace_scope exists and has a higher
>     restriction, then the test is marked as UNSUPPORTED.
>     
>     The case "restricted ptrace" is handled by rearranging the processes involved
>     during the test.  Now we have the following process tree:
>     -parent: do_test (performs output checks)
>     --subprocess 1: pldd_process (becomes pldd via execve)
>     ---subprocess 2: target_process (ptraced via pldd)
>     
>     ChangeLog:
>     
>             * elf/tst-pldd.c (do_test): Add UNSUPPORTED check.
>             Rearrange subprocesses.
>             (pldd_process): New function.
>             * support/Makefile (libsupport-routines): Add support_ptrace.
>             * support/ptrace.h: New file.
>             * support/support_ptrace.c: Likewise.

LGTM with just a change below, thanks.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

> 
> diff --git a/elf/tst-pldd.c b/elf/tst-pldd.c
> index 6b7c94a1c0..eb1b9cb5f5 100644
> --- a/elf/tst-pldd.c
> +++ b/elf/tst-pldd.c
> @@ -30,6 +30,12 @@
>  #include <support/capture_subprocess.h>
>  #include <support/check.h>
>  #include <support/support.h>
> +#include <support/ptrace.h>
> +#include <support/xunistd.h>
> +#include <sys/mman.h>
> +#include <errno.h>
> +#include <signal.h>
> +
>  
>  static void
>  target_process (void *arg)
> @@ -37,6 +43,34 @@ target_process (void *arg)
>    pause ();
>  }
>  
> +static void
> +pldd_process (void *arg)
> +{
> +  pid_t *target_pid_ptr = (pid_t *) arg;
> +
> +  /* Create a copy of current test to check with pldd.  As the
> +     target_process is a child of this pldd_process, pldd is also able
> +     to attach to target_process if YAMA is configured to 1 =
> +     "restricted ptrace".  */
> +  struct support_subprocess target = support_subprocess (target_process, NULL);
> +
> +  /* Store the pid of target-process as do_test needs it in order to
> +     e.g. terminate it at end of the test.  */
> +  *target_pid_ptr = target.pid;
> +
> +  /* Three digits per byte plus null terminator.  */
> +  char pid[3 * sizeof (uint32_t) + 1];
> +  snprintf (pid, array_length (pid), "%d", target.pid);
> +
> +  char *prog = xasprintf ("%s/pldd", support_bindir_prefix);
> +
> +  /* Run pldd and use the pid of target_process as argument.  */
> +  execve (prog, (char *const []) { (char *) prog, pid, NULL },
> +	  (char *const []) { NULL });
> +
> +  FAIL_EXIT1 ("Returned from execve: errno=%d=%m\n", errno);
> +}
> +

Ok.

>  /* The test runs in a container because pldd does not support tracing
>     a binary started by the loader iself (as with testrun.sh).  */
>  
> @@ -52,25 +86,22 @@ in_str_list (const char *libname, const char *const strlist[])
>  static int
>  do_test (void)
>  {
> -  /* Create a copy of current test to check with pldd.  */
> -  struct support_subprocess target = support_subprocess (target_process, NULL);
> -
> -  /* Run 'pldd' on test subprocess.  */
> -  struct support_capture_subprocess pldd;
> +  /* Check if our subprocess can be debugged with ptrace.  */
>    {
> -    /* Three digits per byte plus null terminator.  */
> -    char pid[3 * sizeof (uint32_t) + 1];
> -    snprintf (pid, array_length (pid), "%d", target.pid);
> -
> -    char *prog = xasprintf ("%s/pldd", support_bindir_prefix);
> -
> -    pldd = support_capture_subprogram (prog,
> -      (char *const []) { (char *) prog, pid, NULL });
> +    int ptrace_scope = support_ptrace_scope ();
> +    if (ptrace_scope >= 2)
> +      FAIL_UNSUPPORTED ("/proc/sys/kernel/yama/ptrace_scope >= 2");
> +  }
>  

Ok.

> -    support_capture_subprocess_check (&pldd, "pldd", 0, sc_allow_stdout);
> +  pid_t *target_pid_ptr = (pid_t *) xmmap (NULL, sizeof (pid_t),
> +					   PROT_READ | PROT_WRITE,
> +					   MAP_SHARED | MAP_ANONYMOUS, -1);
>  
> -    free (prog);
> -  }

Ok.

> +  /* Run 'pldd' on test subprocess which will be created in pldd_process.
> +     The pid of the subprocess will be written to target_pid_ptr.  */
> +  struct support_capture_subprocess pldd;
> +  pldd = support_capture_subprocess (pldd_process, target_pid_ptr);
> +  support_capture_subprocess_check (&pldd, "pldd", 0, sc_allow_stdout);

Ok, this is will updated by the pldd_process.

>  
>    /* Check 'pldd' output.  The test is expected to be linked against only
>       loader and libc.  */
> @@ -85,7 +116,7 @@ do_test (void)
>      /* First line is in the form of <pid>: <full path of executable>  */
>      TEST_COMPARE (fscanf (out, "%u: " STRINPUT (512), &pid, buffer), 2);
>  
> -    TEST_COMPARE (pid, target.pid);
> +    TEST_COMPARE (pid, *target_pid_ptr);
>      TEST_COMPARE (strcmp (basename (buffer), "tst-pldd"), 0);
>  
>      /* It expects only one loader and libc loaded by the program.  */
> @@ -93,7 +124,7 @@ do_test (void)
>      while (fgets (buffer, array_length (buffer), out) != NULL)
>        {
>  	/* Ignore vDSO.  */
> -        if (buffer[0] != '/')
> +	if (buffer[0] != '/')
>  	  continue;
>  

Ok.

>  	/* Remove newline so baseline (buffer) can compare against the
> @@ -128,7 +159,9 @@ do_test (void)
>    }
>  
>    support_capture_subprocess_free (&pldd);
> -  support_process_terminate (&target);
> +  if (kill (*target_pid_ptr, SIGKILL) != 0)
> +    FAIL_EXIT1 ("Unable to kill target_process: errno=%d=%m\n", errno);
> +  xmunmap (target_pid_ptr, sizeof (pid_t));
>  
>    return 0;
>  }

Ok.

> diff --git a/support/Makefile b/support/Makefile
> index ab66913a02..34d14eb1bb 100644
> --- a/support/Makefile
> +++ b/support/Makefile
> @@ -56,6 +56,7 @@ libsupport-routines = \
>    support_format_hostent \
>    support_format_netent \
>    support_isolate_in_subprocess \
> +  support_ptrace \
>    support_openpty \
>    support_paths \
>    support_quote_blob \

Ok.

> diff --git a/support/ptrace.h b/support/ptrace.h
> new file mode 100644
> index 0000000000..90006a6b75
> --- /dev/null
> +++ b/support/ptrace.h
> @@ -0,0 +1,32 @@
> +/* Support functions handling ptrace_scope.
> +   Copyright (C) 2019 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef SUPPORT_PTRACE_H
> +#define SUPPORT_PTRACE_H
> +
> +#include <sys/cdefs.h>
> +
> +__BEGIN_DECLS
> +
> +/* Return the current YAMA mode set on the machine (0 to 3) or -1
> +   if YAMA is not supported.  */
> +int support_ptrace_scope (void);
> +
> +__END_DECLS
> +
> +#endif

I think it should named xptrace.h.

> diff --git a/support/support_ptrace.c b/support/support_ptrace.c
> new file mode 100644
> index 0000000000..2084e5a189
> --- /dev/null
> +++ b/support/support_ptrace.c
> @@ -0,0 +1,44 @@
> +/* Support functions handling ptrace_scope.
> +   Copyright (C) 2019 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#include <support/check.h>
> +#include <support/xstdio.h>
> +#include <support/ptrace.h>
> +#include <sys/prctl.h>
> +
> +int
> +support_ptrace_scope (void)
> +{
> +  int ptrace_scope = -1;
> +
> +#ifdef __linux__
> +  /* YAMA may be not enabled.  Otherwise it contains a value from 0 to 3:
> +     - 0 classic ptrace permissions
> +     - 1 restricted ptrace
> +     - 2 admin-only attach
> +     - 3 no attach  */
> +  FILE *f = fopen ("/proc/sys/kernel/yama/ptrace_scope", "r");
> +  if (f != NULL)
> +    {
> +      TEST_COMPARE (fscanf (f, "%d", &ptrace_scope), 1);
> +      xfclose (f);
> +    }
> +#endif
> +
> +  return ptrace_scope;
> +}
> 

Ok.