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

[Stefan Liebler <stli at linux dot ibm dot com>]

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

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.

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);
+}
+
 /* 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");
+  }
 
-    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);
-  }
+  /* 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);
 
   /* 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;
 
 	/* 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;
 }
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 \
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
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;
+}