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

[Stefan Liebler <stli@linux.ibm.com>]

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On 8/27/19 9:11 PM, Adhemerval Zanella wrote:
> 
> 
> On 27/08/2019 12:14, Florian Weimer wrote:
>> * Adhemerval Zanella:
>>
>>> 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
>>
>> The test does the right thing if the path does not exist, so I'm not
>> sure if it is necessary.
> 
> Even though, a check for a Linux specific path should be restricted to
> Linux builds only.
> 
>>
>> support_can_ptrace would be misleading because even with YAMA scope
>> support, tracing direct subprocesses will still work.
> 
> Indeed, a better prototype would be indeed:
> 
> /* Return the current YAMA mode set on the machine (0 to 3) or -1
>     if YAMA is not supported.  */
> int support_ptrace_scope (void);
> 
>>
>> I fact, I think we might be able to get this test to work even with
>> fairly restrictive YAMA scopes, by proper ordering of forks and using
>> execve to run tst-pldd.
> 
> The problem with ptrace_scope 1 is tst-pldd will either make its fork
> helper process a pldd descendant or pass pldd pid to the forked process
> so it can call prctl (PR_SET_PTRACER, ...).  In either case I am not
> really sure it is possible without change pldd process itself (since
> it does not have an interactive mode), nor if the complexity to support
> this specific scenarios pays off.
> 
> The ptrace_scope 2 is even more tricky since it requires CAP_SYS_PTRACE.
> 
> In any case I think by restricting the test to run on ptrace_scope 0
> is a fair assumption.
> 
>>
>> Thanks,
>> Florian
>>

Please have a look at the adjusted the patch.

I've introduced new support functions.
And if ptrace_scope is 1 "restricted ptrace", I've just call prctl 
(PR_SET_PTRACER, PR_SET_PTRACER_ANY,...) on the target process.
This way the target process does not need to know the pid of pldd and 
pldd is allowed to attach to the target process.

If ptrace_scope is 2 or 3, the test is marked as UNSUPPORTED.

Thanks.
Stefan

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commit 7eba88e4f44df6f4a6d174e566b1796f2abc490c
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".  In case of "restricted ptrace", we can
    effectively disable the restriction by using prctl (PR_SET_PTRACER_ANY).
    
    If ptrace_scope exists and has a higher restriction, then the test
    is marked as UNSUPPORTED.
    
    ChangeLog:
    
            * elf/tst-pldd.c (do_test): Add UNSUPPORTED check.
            (target_process): Disable restricted ptrace.
            * 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..728272d177 100644
--- a/elf/tst-pldd.c
+++ b/elf/tst-pldd.c
@@ -30,10 +30,20 @@
 #include <support/capture_subprocess.h>
 #include <support/check.h>
 #include <support/support.h>
+#include <support/ptrace.h>
+
+static int ptrace_scope;
 
 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 ();
 }
 
@@ -52,6 +62,11 @@ in_str_list (const char *libname, const char *const strlist[])
 static int
 do_test (void)
 {
+  /* Check if our subprocess can be debugged with ptrace.  */
+  ptrace_scope = support_ptrace_scope ();
+  if (ptrace_scope >= 2)
+    FAIL_UNSUPPORTED ("/proc/sys/kernel/yama/ptrace_scope >= 2");
+
   /* Create a copy of current test to check with pldd.  */
   struct support_subprocess target = support_subprocess (target_process, NULL);
 
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..82f79ff25c
--- /dev/null
+++ b/support/ptrace.h
@@ -0,0 +1,36 @@
+/* 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);
+
+/* Effectively disables YAMA restriction for the calling process
+   if support_ptrace_scope returns 1 "restricted ptrace".  */
+void support_ptrace_process_set_ptracer_any (void);
+
+__END_DECLS
+
+#endif
diff --git a/support/support_ptrace.c b/support/support_ptrace.c
new file mode 100644
index 0000000000..e9410384b5
--- /dev/null
+++ b/support/support_ptrace.c
@@ -0,0 +1,58 @@
+/* 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;
+}
+
+void
+support_ptrace_process_set_ptracer_any (void)
+{
+#ifdef __linux__
+  int ret = prctl (PR_SET_PTRACER, PR_SET_PTRACER_ANY, 0, 0, 0);
+  if (ret != 0)
+    FAIL_EXIT1 ("Failed to disable YAMA restriction. (prctl returned %d: %m)",
+		ret);
+#else
+  FAIL_UNSUPPORTED ("prctl (PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...) "
+		    "not supported!");
+#endif
+}