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Re: [PATCH] Add UNSUPPORTED check in elf/tst-pldd.
- From: Stefan Liebler <stli at linux dot ibm dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: libc-alpha at sourceware dot org, Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Date: Mon, 2 Sep 2019 17:28:22 +0200
- Subject: Re: [PATCH] Add UNSUPPORTED check in elf/tst-pldd.
- References: <fbb0d1e8-4f39-59ed-801c-d53e14d0320a@linux.ibm.com> <d70954b6-e749-5546-bcfd-509a597b8a14@linaro.org> <877e6yiqsn.fsf@oldenburg2.str.redhat.com> <3b9cb5e4-7c9a-c9a4-449e-43ba98a6ad01@linaro.org> <32961942-e67b-c356-a92d-b6e45c8aaf9d@linux.ibm.com> <87blw9fxrp.fsf@oldenburg2.str.redhat.com> <1d419974-c973-c4c1-f1cd-4bbbf8b074f8@linux.ibm.com> <87tva08ijw.fsf@oldenburg2.str.redhat.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;
+}