[PATCH v3 0/2] Add test to distinguish between thread's signal mask and ucontext_t

[PATCH v3 0/2] Add test to distinguish between thread's signal mask and ucontext_t

[Dev Jain]

This patch series is motivated by the following observation:

Raise a signal, jump to signal handler. The ucontext_t structure dumped
by kernel to userspace has a uc_sigmask field having the mask of blocked
signals. If you run a fresh minimalistic program doing this, this field
is empty, even if you block some signals while registering the handler
with sigaction().

Here is what the man-pages have to say:

sigaction(2): "sa_mask specifies a mask of signals which should be blocked
(i.e., added to the signal mask of the thread in which the signal handler
is invoked) during execution of the signal handler. In addition, the
signal which triggered the handler will be blocked, unless the SA_NODEFER
flag is used."

signal(7): Under "Execution of signal handlers", (1.3) implies:

"The thread's current signal mask is accessible via the ucontext_t
object that is pointed to by the third argument of the signal handler."

But, (1.4) states:

"Any signals specified in act->sa_mask when registering the handler with
sigprocmask(2) are added to the thread's signal mask.  The signal being
delivered is also added to the signal mask, unless SA_NODEFER was
specified when registering the handler.  These signals are thus blocked
while the handler executes."

There clearly is no distinction being made in the man pages between
"Thread's signal mask" and ucontext_t; this logically should imply
that a signal blocked by populating struct sigaction should be visible
in ucontext_t.

Here is what the kernel code does (for Aarch64):

do_signal() -> handle_signal() -> sigmask_to_save(), which returns
&current->blocked, is passed to setup_rt_frame() -> setup_sigframe() ->
__copy_to_user(). Hence, &current->blocked is copied to ucontext_t
exposed to userspace. Returning back to handle_signal(),
signal_setup_done() -> signal_delivered() -> sigorsets() and
set_current_blocked() are responsible for using information from
struct ksignal ksig, which was populated through the sigaction()
system call in kernel/signal.c:
copy_from_user(&new_sa.sa, act, sizeof(new_sa.sa)),
to update &current->blocked; hence, the set of blocked signals for the
current thread is updated AFTER the kernel dumps ucontext_t to
userspace.

Assuming that the above is indeed the intended behaviour, because it
semantically makes sense, since the signals blocked using sigaction()
remain blocked only till the execution of the handler, and not in the
context present before jumping to the handler (but nothing can be
confirmed from the man-pages), the series introduces a test for
mangling with uc_sigmask. I will send a separate series to fix the
man-pages.

The proposed selftest has been tested out on Aarch32, Aarch64 and x86_64.

v2->v3:
 - ucontext describes current state -> ucontext describes interrupted context
 - Add a comment for blockage of USR2 even after return from handler
 - Describe blockage of signals in a better way

v1->v2:
 - Replace all occurrences of SIGPIPE with SIGSEGV
 - Fixed a mismatch between code comment and ksft log
 - Add a testcase: Raise the same signal again; it must not be queued
 - Remove unneeded <assert.h>, <unistd.h>
 - Give a detailed test description in the comments; also describe the
   exact meaning of delivered and blocked
 - Handle errors for all libc functions/syscalls
 - Mention tests in Makefile and .gitignore in alphabetical order

v1:
 - https://lore.kernel.org/all/20240607122319.768640-1-dev.jain@arm.com/

Dev Jain (2):
  selftests: Rename sigaltstack to generic signal
  selftests: Add a test mangling with uc_sigmask

 tools/testing/selftests/Makefile              |   2 +-
 .../{sigaltstack => signal}/.gitignore        |   3 +-
 .../{sigaltstack => signal}/Makefile          |   3 +-
 .../current_stack_pointer.h                   |   0
 .../selftests/signal/mangle_uc_sigmask.c      | 194 ++++++++++++++++++
 .../sas.c => signal/sigaltstack.c}            |   0
 6 files changed, 199 insertions(+), 3 deletions(-)
 rename tools/testing/selftests/{sigaltstack => signal}/.gitignore (57%)
 rename tools/testing/selftests/{sigaltstack => signal}/Makefile (53%)
 rename tools/testing/selftests/{sigaltstack => signal}/current_stack_pointer.h (100%)
 create mode 100644 tools/testing/selftests/signal/mangle_uc_sigmask.c
 rename tools/testing/selftests/{sigaltstack/sas.c => signal/sigaltstack.c} (100%)

-- 
2.34.1

[PATCH v3 1/2] selftests: Rename sigaltstack to generic signal

[Dev Jain]

Rename sigaltstack to signal, and rename the existing test to
sigaltstack.c.

Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/Makefile                                | 2 +-
 tools/testing/selftests/{sigaltstack => signal}/.gitignore      | 2 +-
 tools/testing/selftests/{sigaltstack => signal}/Makefile        | 2 +-
 .../selftests/{sigaltstack => signal}/current_stack_pointer.h   | 0
 .../selftests/{sigaltstack/sas.c => signal/sigaltstack.c}       | 0
 5 files changed, 3 insertions(+), 3 deletions(-)
 rename tools/testing/selftests/{sigaltstack => signal}/.gitignore (76%)
 rename tools/testing/selftests/{sigaltstack => signal}/Makefile (72%)
 rename tools/testing/selftests/{sigaltstack => signal}/current_stack_pointer.h (100%)
 rename tools/testing/selftests/{sigaltstack/sas.c => signal/sigaltstack.c} (100%)

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 9039f3709aff..eee1031dc18f 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -85,7 +85,7 @@ TARGETS += rtc
 TARGETS += rust
 TARGETS += seccomp
 TARGETS += sgx
-TARGETS += sigaltstack
+TARGETS += signal
 TARGETS += size
 TARGETS += sparc64
 TARGETS += splice
diff --git a/tools/testing/selftests/sigaltstack/.gitignore b/tools/testing/selftests/signal/.gitignore
similarity index 76%
rename from tools/testing/selftests/sigaltstack/.gitignore
rename to tools/testing/selftests/signal/.gitignore
index 50a19a8888ce..98a7bbc4f325 100644
--- a/tools/testing/selftests/sigaltstack/.gitignore
+++ b/tools/testing/selftests/signal/.gitignore
@@ -1,2 +1,2 @@
 # SPDX-License-Identifier: GPL-2.0-only
-sas
+sigaltstack
diff --git a/tools/testing/selftests/sigaltstack/Makefile b/tools/testing/selftests/signal/Makefile
similarity index 72%
rename from tools/testing/selftests/sigaltstack/Makefile
rename to tools/testing/selftests/signal/Makefile
index 3e96d5d47036..dd6be992fd81 100644
--- a/tools/testing/selftests/sigaltstack/Makefile
+++ b/tools/testing/selftests/signal/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS = -Wall
-TEST_GEN_PROGS = sas
+TEST_GEN_PROGS = sigaltstack
 
 include ../lib.mk
 
diff --git a/tools/testing/selftests/sigaltstack/current_stack_pointer.h b/tools/testing/selftests/signal/current_stack_pointer.h
similarity index 100%
rename from tools/testing/selftests/sigaltstack/current_stack_pointer.h
rename to tools/testing/selftests/signal/current_stack_pointer.h
diff --git a/tools/testing/selftests/sigaltstack/sas.c b/tools/testing/selftests/signal/sigaltstack.c
similarity index 100%
rename from tools/testing/selftests/sigaltstack/sas.c
rename to tools/testing/selftests/signal/sigaltstack.c
-- 
2.34.1

[PATCH v3 2/2] selftests: Add a test mangling with uc_sigmask

[Dev Jain]

This test asserts the relation between blocked signal, delivered signal,
and ucontext. The ucontext is mangled with, by adding a signal mask to
it; on return from the handler, the thread must block the corresponding
signal.

In the test description, I have also described signal delivery and blockage,
for ease of understanding what the test does.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 tools/testing/selftests/signal/.gitignore     |   1 +
 tools/testing/selftests/signal/Makefile       |   3 +-
 .../selftests/signal/mangle_uc_sigmask.c      | 194 ++++++++++++++++++
 3 files changed, 197 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/signal/mangle_uc_sigmask.c

diff --git a/tools/testing/selftests/signal/.gitignore b/tools/testing/selftests/signal/.gitignore
index 98a7bbc4f325..397fef11c89f 100644
--- a/tools/testing/selftests/signal/.gitignore
+++ b/tools/testing/selftests/signal/.gitignore
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
+mangle_uc_sigmask
 sigaltstack
diff --git a/tools/testing/selftests/signal/Makefile b/tools/testing/selftests/signal/Makefile
index dd6be992fd81..735387a53114 100644
--- a/tools/testing/selftests/signal/Makefile
+++ b/tools/testing/selftests/signal/Makefile
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0-only
 CFLAGS = -Wall
-TEST_GEN_PROGS = sigaltstack
+TEST_GEN_PROGS = mangle_uc_sigmask
+TEST_GEN_PROGS += sigaltstack
 
 include ../lib.mk
 
diff --git a/tools/testing/selftests/signal/mangle_uc_sigmask.c b/tools/testing/selftests/signal/mangle_uc_sigmask.c
new file mode 100644
index 000000000000..772109e97aaf
--- /dev/null
+++ b/tools/testing/selftests/signal/mangle_uc_sigmask.c
@@ -0,0 +1,194 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2024 ARM Ltd.
+ *
+ * Author: Dev Jain <dev.jain@arm.com>
+ *
+ * Test describing a clear distinction between signal states - delivered and
+ * blocked, and their relation with ucontext.
+ *
+ * A process can request blocking of a signal by masking it into its set of
+ * blocked signals; such a signal, when sent to the process by the kernel,
+ * will get blocked by the process and it may later unblock it and take an
+ * action. At that point, the signal will be delivered.
+ *
+ * We test the following functionalities of the kernel:
+ *
+ * ucontext_t describes the interrupted context of the thread; this implies
+ * that, in case of registering a handler and catching the corresponding
+ * signal, that state is before what was jumping into the handler.
+ *
+ * The thread's mask of blocked signals can be permanently changed, i.e, not
+ * just during the execution of the handler, by mangling with uc_sigmask
+ * from inside the handler.
+ *
+ * Assume that we block the set of signals, S1, by sigaction(), and say, the
+ * signal for which the handler was installed, is S2. When S2 is sent to the
+ * program, it will be considered "delivered", since we will act on the
+ * signal and jump to the handler. Any instances of S1 or S2 raised, while the
+ * program is executing inside the handler, will be blocked; they will be
+ * delivered immediately upon termination of the handler.
+ *
+ * For standard signals (also see real-time signals in the man page), multiple
+ * blocked instances of the same signal are not queued; such a signal will
+ * be delivered just once.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <ucontext.h>
+
+#include "../kselftest.h"
+
+void handler_verify_ucontext(int signo, siginfo_t *info, void *uc)
+{
+	int ret;
+
+	/* Kernel dumps ucontext with USR2 blocked */
+	ret = sigismember(&(((ucontext_t *)uc)->uc_sigmask), SIGUSR2);
+	ksft_test_result(ret == 1, "USR2 blocked in ucontext\n");
+
+	/*
+	 * USR2 is blocked; can be delivered neither here, nor after
+	 * exit from handler
+	 */
+	if (raise(SIGUSR2))
+		ksft_exit_fail_perror("raise");
+}
+
+void handler_segv(int signo, siginfo_t *info, void *uc)
+{
+	/*
+	 * Three cases possible:
+	 * 1. Program already terminated due to segmentation fault.
+	 * 2. SEGV was blocked even after returning from handler_usr.
+	 * 3. SEGV was delivered on returning from handler_usr.
+	 * The last option must happen.
+	 */
+	ksft_test_result_pass("SEGV delivered\n");
+}
+
+static int cnt;
+
+void handler_usr(int signo, siginfo_t *info, void *uc)
+{
+	int ret;
+
+	/*
+	 * Break out of infinite recursion caused by raise(SIGUSR1) invoked
+	 * from inside the handler
+	 */
+	++cnt;
+	if (cnt > 1)
+		return;
+
+	ksft_print_msg("In handler_usr\n");
+
+	/* SEGV blocked during handler execution, delivered on return */
+	if (raise(SIGSEGV))
+		ksft_exit_fail_perror("raise");
+
+	ksft_print_msg("SEGV bypassed successfully\n");
+
+	/*
+	 * Signal responsible for handler invocation is blocked by default;
+	 * delivered on return, leading to recursion
+	 */
+	if (raise(SIGUSR1))
+		ksft_exit_fail_perror("raise");
+
+	ksft_test_result(cnt == 1,
+			 "USR1 is blocked, cannot invoke handler right now\n");
+
+	/* Raise USR1 again; only one instance must be delivered upon exit */
+	if (raise(SIGUSR1))
+		ksft_exit_fail_perror("raise");
+
+	/* SEGV has been blocked in sa_mask, but ucontext is empty */
+	ret = sigismember(&(((ucontext_t *)uc)->uc_sigmask), SIGSEGV);
+	ksft_test_result(ret == 0, "SEGV not blocked in ucontext\n");
+
+	/* USR1 has been blocked, but ucontext is empty */
+	ret = sigismember(&(((ucontext_t *)uc)->uc_sigmask), SIGUSR1);
+	ksft_test_result(ret == 0, "USR1 not blocked in ucontext\n");
+
+	/*
+	 * Mangle ucontext; this will be copied back into &current->blocked
+	 * on return from the handler.
+	 */
+	if (sigaddset(&((ucontext_t *)uc)->uc_sigmask, SIGUSR2))
+		ksft_exit_fail_perror("sigaddset");
+}
+
+int main(int argc, char *argv[])
+{
+	struct sigaction act, act2;
+	sigset_t *set, *oldset;
+
+	ksft_print_header();
+	ksft_set_plan(7);
+
+	act.sa_flags = SA_SIGINFO;
+	act.sa_sigaction = &handler_usr;
+
+	/* Add SEGV to blocked mask */
+	if (sigemptyset(&act.sa_mask) || sigaddset(&act.sa_mask, SIGSEGV)
+	    || (sigismember(&act.sa_mask, SIGSEGV) != 1))
+		ksft_exit_fail_msg("Cannot add SEGV to blocked mask\n");
+
+	if (sigaction(SIGUSR1, &act, NULL))
+		ksft_exit_fail_perror("Cannot install handler");
+
+	act2.sa_flags = SA_SIGINFO;
+	act2.sa_sigaction = &handler_segv;
+
+	if (sigaction(SIGSEGV, &act2, NULL))
+		ksft_exit_fail_perror("Cannot install handler");
+
+	/* Invoke handler */
+	if (raise(SIGUSR1))
+		ksft_exit_fail_perror("raise");
+
+	/* USR1 must not be queued */
+	ksft_test_result(cnt == 2, "handler invoked only twice\n");
+
+	/* Mangled ucontext implies USR2 is blocked for current thread */
+	if (raise(SIGUSR2))
+		ksft_exit_fail_perror("raise");
+
+	ksft_print_msg("USR2 bypassed successfully\n");
+
+	act.sa_sigaction = &handler_verify_ucontext;
+	if (sigaction(SIGUSR1, &act, NULL))
+		ksft_exit_fail_perror("Cannot install handler");
+
+	if (raise(SIGUSR1))
+		ksft_exit_fail_perror("raise");
+
+	/*
+	 * Raising USR2 in handler_verify_ucontext is redundant since it
+	 * is blocked
+	 */
+	ksft_print_msg("USR2 still blocked on return from handler\n");
+
+	/* Confirm USR2 blockage by sigprocmask() too */
+	set = malloc(sizeof(sigset_t *));
+	if (!set)
+		ksft_exit_fail_perror("malloc");
+
+	oldset = malloc(sizeof(sigset_t *));
+	if (!oldset)
+		ksft_exit_fail_perror("malloc");
+
+	if (sigemptyset(set))
+		ksft_exit_fail_perror("sigemptyset");
+
+	if (sigprocmask(SIG_BLOCK, set, oldset))
+		ksft_exit_fail_perror("sigprocmask");
+
+	ksft_test_result(sigismember(oldset, SIGUSR2) == 1,
+			 "USR2 present in &current->blocked\n");
+
+	ksft_finished();
+}
-- 
2.34.1

Re: [PATCH v3 2/2] selftests: Add a test mangling with uc_sigmask

[Oleg Nesterov]

On 06/26, Dev Jain wrote:
>
> +int main(int argc, char *argv[])
> +{
> +	struct sigaction act, act2;
> +	sigset_t *set, *oldset;

...

> +	set = malloc(sizeof(sigset_t *));
> +	if (!set)
> +		ksft_exit_fail_perror("malloc");
> +
> +	oldset = malloc(sizeof(sigset_t *));

Why malloc() ?

Can't you simply do

	sigset_t set, oldset;

and then use sigemptyset(&set) / etc ?

Oleg.

Re: [PATCH v3 2/2] selftests: Add a test mangling with uc_sigmask

[Dev Jain]

On 6/26/24 17:24, Oleg Nesterov wrote:
> On 06/26, Dev Jain wrote:
>> +int main(int argc, char *argv[])
>> +{
>> +	struct sigaction act, act2;
>> +	sigset_t *set, *oldset;
> ...
>
>> +	set = malloc(sizeof(sigset_t *));
>> +	if (!set)
>> +		ksft_exit_fail_perror("malloc");
>> +
>> +	oldset = malloc(sizeof(sigset_t *));
> Why malloc() ?
>
> Can't you simply do
>
> 	sigset_t set, oldset;
>
> and then use sigemptyset(&set) / etc ?
>
> Oleg.


Ah yes, let us allocate it on the stack. I shall change it.

>

Re: [PATCH v3 2/2] selftests: Add a test mangling with uc_sigmask

[Mark Brown]

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On Wed, Jun 26, 2024 at 11:18:47AM +0530, Dev Jain wrote:
> This test asserts the relation between blocked signal, delivered signal,
> and ucontext. The ucontext is mangled with, by adding a signal mask to
> it; on return from the handler, the thread must block the corresponding
> signal.

This looks good on a first pass, I see there's a new version coming so
I'll go through again when that comes but I'd expect it's all good.

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