From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: libc-alpha@sourceware.org
Subject: Re: [PATCH] Add UNSUPPORTED check in elf/tst-pldd.
Date: Wed, 28 Aug 2019 12:19:00 -0000 [thread overview]
Message-ID: <762c495b-5155-1553-4414-61dc6e2238c6@linaro.org> (raw)
In-Reply-To: <32961942-e67b-c356-a92d-b6e45c8aaf9d@linux.ibm.com>
On 28/08/2019 06:06, Stefan Liebler wrote:
> 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
>
> 20190828_tst-pldd.patch
>
> 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.
LGTM with a change below.
>
> 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);
>
I think we can restrict the test to ptrace_scope 0 for now, since as Florian
has brought we will need some synchronization with parent after the
support_ptrace_process_set_ptracer_any call.
> 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..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
Ok.
> 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
> +}
>
Ok.
prev parent reply other threads:[~2019-08-28 12:19 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-27 10:19 Stefan Liebler
2019-08-27 15:06 ` Adhemerval Zanella
2019-08-27 15:14 ` Florian Weimer
2019-08-27 19:11 ` Adhemerval Zanella
2019-08-28 9:06 ` Stefan Liebler
2019-08-28 9:24 ` Florian Weimer
2019-08-28 14:42 ` Stefan Liebler
2019-08-29 8:47 ` Florian Weimer
2019-09-02 15:28 ` Stefan Liebler
2019-09-17 13:31 ` Adhemerval Zanella
2019-09-17 15:17 ` Stefan Liebler
2019-09-18 10:45 ` Stefan Liebler
2019-09-18 15:18 ` Joseph Myers
2019-09-19 10:28 ` Stefan Liebler
2019-09-02 19:37 ` Adhemerval Zanella
2019-09-03 6:30 ` Stefan Liebler
2019-09-03 13:34 ` Adhemerval Zanella
2019-09-06 3:21 ` Carlos O'Donell
2019-09-10 8:46 ` Stefan Liebler
2019-09-10 13:33 ` Adhemerval Zanella
2019-09-11 7:05 ` Stefan Liebler
2019-08-28 12:19 ` Adhemerval Zanella [this message]
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