[RFC,2/2] riscv: selftests: Add a ptrace test to check a0 of restarted syscall

Commit Message

Quan Zhou June 19, 2024, 2:01 a.m. UTC

From: Quan Zhou <[e-mail]>

This test creates two processes: a tracer and a tracee. The tracer actively
sends a SIGUSR1 signal in user mode to interrupt the read syscall being
executed by the tracee. We will reset a0/orig_a0 and then observe the value
of a0 held by the restarted read syscall.

Compared to the test program, a more common scenario is the use of the
exece syscall, which sends a signal in the kernel path to restart
the syscall.

Signed-off-by: Quan Zhou <[e-mail]>
---
 tools/testing/selftests/riscv/Makefile        |   2 +-
 tools/testing/selftests/riscv/abi/.gitignore  |   1 +
 tools/testing/selftests/riscv/abi/Makefile    |  12 ++
 .../riscv/abi/ptrace_restart_syscall.c        | 148 ++++++++++++++++++
 4 files changed, 162 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/riscv/abi/.gitignore
 create mode 100644 tools/testing/selftests/riscv/abi/Makefile
 create mode 100644 tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c

Comments

Charlie Jenkins June 20, 2024, 2:55 a.m. UTC | #1

On Wed, Jun 19, 2024 at 10:01:47AM +0800, [e-mail] wrote:
> From: Quan Zhou <[e-mail]>
> 
> This test creates two processes: a tracer and a tracee. The tracer actively
> sends a SIGUSR1 signal in user mode to interrupt the read syscall being
> executed by the tracee. We will reset a0/orig_a0 and then observe the value
> of a0 held by the restarted read syscall.

I don't quite follow what the goal of this test is. With orig_a0 being
added to the previous patch for ptrace, a more constrained test could
ensure that this value is respected.

> 
> Compared to the test program, a more common scenario is the use of the
> exece syscall, which sends a signal in the kernel path to restart
> the syscall.
> 
> Signed-off-by: Quan Zhou <[e-mail]>
> ---
>  tools/testing/selftests/riscv/Makefile        |   2 +-
>  tools/testing/selftests/riscv/abi/.gitignore  |   1 +
>  tools/testing/selftests/riscv/abi/Makefile    |  12 ++
>  .../riscv/abi/ptrace_restart_syscall.c        | 148 ++++++++++++++++++
>  4 files changed, 162 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/riscv/abi/.gitignore
>  create mode 100644 tools/testing/selftests/riscv/abi/Makefile
>  create mode 100644 tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c
> 
> diff --git a/tools/testing/selftests/riscv/Makefile b/tools/testing/selftests/riscv/Makefile
> index 7ce03d832b64..98541dc2f164 100644
> --- a/tools/testing/selftests/riscv/Makefile
> +++ b/tools/testing/selftests/riscv/Makefile
> @@ -5,7 +5,7 @@
>  ARCH ?= $(shell uname -m 2>/dev/null || echo not)
>  
>  ifneq (,$(filter $(ARCH),riscv))
> -RISCV_SUBTARGETS ?= hwprobe vector mm sigreturn
> +RISCV_SUBTARGETS ?= hwprobe vector mm sigreturn abi
>  else
>  RISCV_SUBTARGETS :=
>  endif
> diff --git a/tools/testing/selftests/riscv/abi/.gitignore b/tools/testing/selftests/riscv/abi/.gitignore
> new file mode 100644
> index 000000000000..e1e00ffb9db9
> --- /dev/null
> +++ b/tools/testing/selftests/riscv/abi/.gitignore
> @@ -0,0 +1 @@
> +abi

The gitignore should contain a list of all of the generated binaries
that should be ignored. Can you put ptrace_restart_syscall in here
instead of abi?

> diff --git a/tools/testing/selftests/riscv/abi/Makefile b/tools/testing/selftests/riscv/abi/Makefile
> new file mode 100644
> index 000000000000..634fa7de74e6
> --- /dev/null
> +++ b/tools/testing/selftests/riscv/abi/Makefile
> @@ -0,0 +1,12 @@
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (C) 2021 ARM Limited
> +# Originally tools/testing/arm64/abi/Makefile
> +
> +CFLAGS += -I$(top_srcdir)/tools/include
> +
> +TEST_GEN_PROGS := ptrace_restart_syscall
> +
> +include ../../lib.mk
> +
> +$(OUTPUT)/ptrace_restart_syscall: ptrace_restart_syscall.c
> +	$(CC) -static -o$@ $(CFLAGS) $(LDFLAGS) $^
> diff --git a/tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c b/tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c
> new file mode 100644
> index 000000000000..3e25548cb95e
> --- /dev/null
> +++ b/tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c
> @@ -0,0 +1,148 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <fcntl.h>
> +#include <signal.h>
> +#include <errno.h>
> +#include <sys/types.h>
> +#include <sys/ptrace.h>
> +#include <sys/stat.h>
> +#include <sys/user.h>
> +#include <sys/wait.h>
> +#include <sys/uio.h>
> +#include <linux/elf.h>
> +#include <linux/unistd.h>
> +#include <asm/ptrace.h>
> +
> +#include "../../kselftest_harness.h"
> +
> +#define ORIG_A0_AFTER_MODIFIED  0x5
> +#define MODIFY_A0               0x01
> +#define MODIFY_ORIG_A0          0x02
> +
> +#define perr_and_exit(fmt, ...) do {			\
> +	char buf[256];					\
> +	snprintf(buf, sizeof(buf), "%s:%d: " fmt ": %m\n",	\
> +			__func__, __LINE__, ##__VA_ARGS__);	\
> +	perror(buf);						\
> +	exit(-1);						\
> +} while (0)
> +
> +static inline void resume_and_wait_tracee(pid_t pid, int flag)
> +{
> +	int status;
> +
> +	if (ptrace(flag, pid, 0, 0))
> +		perr_and_exit("failed to resume the tracee %d", pid);
> +
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee %d", pid);
> +}
> +
> +static void ptrace_restart_syscall(int opt, int *result)
> +{
> +	int status;
> +	int p[2], fd_zero;
> +	pid_t pid;
> +
> +	struct user_regs_struct regs;
> +	struct iovec iov = {
> +		.iov_base = &regs,
> +		.iov_len = sizeof(regs),
> +	};
> +
> +	if (pipe(p))
> +		perr_and_exit("failed to create a pipe");
> +
> +	fd_zero = open("/dev/zero", O_RDONLY);
> +	if (fd_zero < 0)
> +		perr_and_exit("failed to open /dev/zero");
> +
> +	pid = fork();
> +	if (pid == 0) {
> +		char c;
> +
> +		/* Mark oneself being traced */
> +		if (ptrace(PTRACE_TRACEME, 0, 0, 0))
> +			perr_and_exit("failed to request for tracer to trace me");
> +
> +		kill(getpid(), SIGSTOP);
> +
> +		if (read(p[0], &c, 1) != 1)
> +			exit(1);
> +
> +		exit(0);
> +	} else if (pid < 0)
> +		exit(1);
> +
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee %d\n", pid);
> +
> +	/* Resume the tracee until the next syscall */
> +	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
> +
> +	/* Deliver a signal to interrupt the syscall */
> +	kill(pid, SIGUSR1);
> +
> +	/* The tracee stops at syscall exit */
> +	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
> +
> +	/* Check tracee orig_a0 before syscall restart */
> +	if (ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov))
> +		perr_and_exit("failed to get tracee registers");
> +	if (regs.orig_a0 != p[0])
> +		perr_and_exit("unexpected a0");
> +
> +	/* Modify a0/orig_a0 for the restarted syscall */
> +	switch (opt) {
> +	case MODIFY_A0:
> +		regs.a0 = fd_zero;
> +		break;
> +	case MODIFY_ORIG_A0:
> +		regs.orig_a0 = fd_zero;
> +		break;
> +	}
> +
> +	if (ptrace(PTRACE_SETREGSET, pid, NT_PRSTATUS, &iov))
> +		perr_and_exit("failed to set tracee registers");
> +
> +	/* Ignore SIGUSR1 signal */
> +	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
> +
> +	/* Stop at the entry point of the restarted syscall */
> +	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
> +
> +	/* Now, check regs.a0 of the restarted syscall */
> +	if (ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov))
> +		perr_and_exit("failed to get tracee registers");
> +	*result = regs.a0;
> +
> +	/* Resume the tracee */
> +	ptrace(PTRACE_CONT, pid, 0, 0);
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee");
> +}
> +
> +TEST(ptrace_modify_a0)
> +{
> +	int result;
> +
> +	ptrace_restart_syscall(MODIFY_A0, &result);
> +
> +	/* The tracer's modification of a0 cannot affect the restarted tracee */
> +	EXPECT_NE(ORIG_A0_AFTER_MODIFIED, result);
> +}
> +
> +TEST(ptrace_modify_orig_a0)
> +{
> +	int result;
> +
> +	ptrace_restart_syscall(MODIFY_ORIG_A0, &result);
> +
> +	/* The tracer must modify orig_a0 to actually change the tracee's a0 */
> +	EXPECT_EQ(ORIG_A0_AFTER_MODIFIED, result);

How does the value end up being 5?

- Charlie

> +}
> +
> +TEST_HARNESS_MAIN
> -- 
> 2.34.1
>

Quan Zhou June 21, 2024, 6:29 a.m. UTC | #2

On 2024/6/20 10:55, Charlie Jenkins wrote:
> On Wed, Jun 19, 2024 at 10:01:47AM +0800, [e-mail] wrote:
>> From: Quan Zhou <[e-mail]>
>>
>> This test creates two processes: a tracer and a tracee. The tracer actively
>> sends a SIGUSR1 signal in user mode to interrupt the read syscall being
>> executed by the tracee. We will reset a0/orig_a0 and then observe the value
>> of a0 held by the restarted read syscall.
> 
> I don't quite follow what the goal of this test is. With orig_a0 being
> added to the previous patch for ptrace, a more constrained test could
> ensure that this value is respected.
> 

Sry, I may not have described the patch clearly enough. This patch 
provides a channel for modifying a0 in user-space ptrace via orig_a0. 
Here, I will try to outline the whole situation:

1、When the tracer calls ptrace to modify regs->a0, can the tracee's a0 
be correctly modified?

Through testing, if the user only modifies regs->a0, it doesn't work. Why?

The execution flow of the tracee in the test program is as follows.Prior 
to this explanation:

- PTRACE_SYSCALL can make the tracee block before and after executing
   a syscall.
- The tracer sends SIGUSR1 to interrupt read, and the kernel will
   restart it.
- Please note the point marked with (*), which I believe is the cause
   of the issue.

user     kernel
     |
     |
     |
      read
         | +-> regs->orig_a0 = regs->a0; //(*1)
         |                                       <=tracer:PTRACE_SYSCALL
         | +-> syscall_enter_from_user_mode
               +-> ptrace_report_syscall_entry
                   +-> ptrace_stop
         | //stopped
         |                                       <= tracer:SIGUSR1
         |
         | //resume                              <= tracer:PTRACE_SYSCALL
         | syscall_handler...
         |
         | +-> syscall_exit_to_user_mode
               +-> syscall_exit_to_user_mode_prepare
                   +-> ptrace_report_syscall_exit
                       +-> ptrace_stop
     | //stopped
     |
     | /* Change a0/orig_a0 here and observe the restarted syscall */
     | regs->{a0/orig_a0} = fd_zero; //(*2)
     | ptrace(PTRACE_SETREGSET, ...);
         |                                      <= tracer:PTRACE_SYSCALL
         | //restarting..., skip SIGUSR1
         |
         | +-> exit_to_user_mode_loop
               +-> arch_do_signal_or_restart
                   +-> /* Settings for syscall restart */
                       regs->a0 = regs->orig_a0; //(*3)
     | //stopped
     | //and block before the syscall again, get current regs->a0
     | *result = regs->a0;
     |
     | /* Now, Check regs->a0 of restarted syscall */
     | EXPECT_NE(0x5, result); //for PTRACE_SETREGSSET a0, failed
     | EXPECT_EQ(0x5, result); //for PTRACE_SETREGSSET orig_a0, succeed

If I'm wrong, please let me know. 

Charlie Jenkins June 21, 2024, 8:20 p.m. UTC | #3

On Fri, Jun 21, 2024 at 02:29:07PM +0800, Quan Zhou wrote:
> 
> 
> On 2024/6/20 10:55, Charlie Jenkins wrote:
> > On Wed, Jun 19, 2024 at 10:01:47AM +0800, [e-mail] wrote:
> > > From: Quan Zhou <[e-mail]>
> > > 
> > > This test creates two processes: a tracer and a tracee. The tracer actively
> > > sends a SIGUSR1 signal in user mode to interrupt the read syscall being
> > > executed by the tracee. We will reset a0/orig_a0 and then observe the value
> > > of a0 held by the restarted read syscall.
> > 
> > I don't quite follow what the goal of this test is. With orig_a0 being
> > added to the previous patch for ptrace, a more constrained test could
> > ensure that this value is respected.
> > 
> 
> Sry, I may not have described the patch clearly enough. This patch provides
> a channel for modifying a0 in user-space ptrace via orig_a0. Here, I will
> try to outline the whole situation:
> 
> 1、When the tracer calls ptrace to modify regs->a0, can the tracee's a0 be
> correctly modified?
> 
> Through testing, if the user only modifies regs->a0, it doesn't work. Why?
> 
> The execution flow of the tracee in the test program is as follows.Prior to
> this explanation:
> 
> - PTRACE_SYSCALL can make the tracee block before and after executing
>   a syscall.
> - The tracer sends SIGUSR1 to interrupt read, and the kernel will
>   restart it.
> - Please note the point marked with (*), which I believe is the cause
>   of the issue.
> 
> user     kernel
>     |
>     |
>     |
>      read
>         | +-> regs->orig_a0 = regs->a0; //(*1)
>         |                                       <=tracer:PTRACE_SYSCALL
>         | +-> syscall_enter_from_user_mode
>               +-> ptrace_report_syscall_entry
>                   +-> ptrace_stop
>         | //stopped
>         |                                       <= tracer:SIGUSR1
>         |
>         | //resume                              <= tracer:PTRACE_SYSCALL
>         | syscall_handler...
>         |
>         | +-> syscall_exit_to_user_mode
>               +-> syscall_exit_to_user_mode_prepare
>                   +-> ptrace_report_syscall_exit
>                       +-> ptrace_stop
>     | //stopped
>     |
>     | /* Change a0/orig_a0 here and observe the restarted syscall */
>     | regs->{a0/orig_a0} = fd_zero; //(*2)
>     | ptrace(PTRACE_SETREGSET, ...);
>         |                                      <= tracer:PTRACE_SYSCALL
>         | //restarting..., skip SIGUSR1
>         |
>         | +-> exit_to_user_mode_loop
>               +-> arch_do_signal_or_restart
>                   +-> /* Settings for syscall restart */
>                       regs->a0 = regs->orig_a0; //(*3)
>     | //stopped
>     | //and block before the syscall again, get current regs->a0
>     | *result = regs->a0;
>     |
>     | /* Now, Check regs->a0 of restarted syscall */
>     | EXPECT_NE(0x5, result); //for PTRACE_SETREGSSET a0, failed
>     | EXPECT_EQ(0x5, result); //for PTRACE_SETREGSSET orig_a0, succeed
> 
> If I'm wrong, please let me know. 

Quan Zhou June 24, 2024, 3:24 a.m. UTC | #4

On 2024/6/22 04:20, Charlie Jenkins wrote:
> On Fri, Jun 21, 2024 at 02:29:07PM +0800, Quan Zhou wrote:
>>
>>
>> On 2024/6/20 10:55, Charlie Jenkins wrote:
>>> On Wed, Jun 19, 2024 at 10:01:47AM +0800, [e-mail] wrote:
>>>> From: Quan Zhou <[e-mail]>
>>>>
>>>> This test creates two processes: a tracer and a tracee. The tracer actively
>>>> sends a SIGUSR1 signal in user mode to interrupt the read syscall being
>>>> executed by the tracee. We will reset a0/orig_a0 and then observe the value
>>>> of a0 held by the restarted read syscall.
>>>
>>> I don't quite follow what the goal of this test is. With orig_a0 being
>>> added to the previous patch for ptrace, a more constrained test could
>>> ensure that this value is respected.
>>>
>>
>> Sry, I may not have described the patch clearly enough. This patch provides
>> a channel for modifying a0 in user-space ptrace via orig_a0. Here, I will
>> try to outline the whole situation:
>>
>> 1、When the tracer calls ptrace to modify regs->a0, can the tracee's a0 be
>> correctly modified?
>>
>> Through testing, if the user only modifies regs->a0, it doesn't work. Why?
>>
>> The execution flow of the tracee in the test program is as follows.Prior to
>> this explanation:
>>
>> - PTRACE_SYSCALL can make the tracee block before and after executing
>>    a syscall.
>> - The tracer sends SIGUSR1 to interrupt read, and the kernel will
>>    restart it.
>> - Please note the point marked with (*), which I believe is the cause
>>    of the issue.
>>
>> user     kernel
>>      |
>>      |
>>      |
>>       read
>>          | +-> regs->orig_a0 = regs->a0; //(*1)
>>          |                                       <=tracer:PTRACE_SYSCALL
>>          | +-> syscall_enter_from_user_mode
>>                +-> ptrace_report_syscall_entry
>>                    +-> ptrace_stop
>>          | //stopped
>>          |                                       <= tracer:SIGUSR1
>>          |
>>          | //resume                              <= tracer:PTRACE_SYSCALL
>>          | syscall_handler...
>>          |
>>          | +-> syscall_exit_to_user_mode
>>                +-> syscall_exit_to_user_mode_prepare
>>                    +-> ptrace_report_syscall_exit
>>                        +-> ptrace_stop
>>      | //stopped
>>      |
>>      | /* Change a0/orig_a0 here and observe the restarted syscall */
>>      | regs->{a0/orig_a0} = fd_zero; //(*2)
>>      | ptrace(PTRACE_SETREGSET, ...);
>>          |                                      <= tracer:PTRACE_SYSCALL
>>          | //restarting..., skip SIGUSR1
>>          |
>>          | +-> exit_to_user_mode_loop
>>                +-> arch_do_signal_or_restart
>>                    +-> /* Settings for syscall restart */
>>                        regs->a0 = regs->orig_a0; //(*3)
>>      | //stopped
>>      | //and block before the syscall again, get current regs->a0
>>      | *result = regs->a0;
>>      |
>>      | /* Now, Check regs->a0 of restarted syscall */
>>      | EXPECT_NE(0x5, result); //for PTRACE_SETREGSSET a0, failed
>>      | EXPECT_EQ(0x5, result); //for PTRACE_SETREGSSET orig_a0, succeed
>>
>> If I'm wrong, please let me know. 

Patch

diff --git a/tools/testing/selftests/riscv/Makefile b/tools/testing/selftests/riscv/Makefile
index 7ce03d832b64..98541dc2f164 100644
--- a/tools/testing/selftests/riscv/Makefile
+++ b/tools/testing/selftests/riscv/Makefile
@@ -5,7 +5,7 @@ 
 ARCH ?= $(shell uname -m 2>/dev/null || echo not)
 
 ifneq (,$(filter $(ARCH),riscv))
-RISCV_SUBTARGETS ?= hwprobe vector mm sigreturn
+RISCV_SUBTARGETS ?= hwprobe vector mm sigreturn abi
 else
 RISCV_SUBTARGETS :=
 endif
diff --git a/tools/testing/selftests/riscv/abi/.gitignore b/tools/testing/selftests/riscv/abi/.gitignore
new file mode 100644
index 000000000000..e1e00ffb9db9
--- /dev/null
+++ b/tools/testing/selftests/riscv/abi/.gitignore
@@ -0,0 +1 @@ 
+abi
diff --git a/tools/testing/selftests/riscv/abi/Makefile b/tools/testing/selftests/riscv/abi/Makefile
new file mode 100644
index 000000000000..634fa7de74e6
--- /dev/null
+++ b/tools/testing/selftests/riscv/abi/Makefile
@@ -0,0 +1,12 @@ 
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2021 ARM Limited
+# Originally tools/testing/arm64/abi/Makefile
+
+CFLAGS += -I$(top_srcdir)/tools/include
+
+TEST_GEN_PROGS := ptrace_restart_syscall
+
+include ../../lib.mk
+
+$(OUTPUT)/ptrace_restart_syscall: ptrace_restart_syscall.c
+	$(CC) -static -o$@ $(CFLAGS) $(LDFLAGS) $^
diff --git a/tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c b/tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c
new file mode 100644
index 000000000000..3e25548cb95e
--- /dev/null
+++ b/tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c
@@ -0,0 +1,148 @@ 
+// SPDX-License-Identifier: GPL-2.0-only
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/ptrace.h>
+#include <sys/stat.h>
+#include <sys/user.h>
+#include <sys/wait.h>
+#include <sys/uio.h>
+#include <linux/elf.h>
+#include <linux/unistd.h>
+#include <asm/ptrace.h>
+
+#include "../../kselftest_harness.h"
+
+#define ORIG_A0_AFTER_MODIFIED  0x5
+#define MODIFY_A0               0x01
+#define MODIFY_ORIG_A0          0x02
+
+#define perr_and_exit(fmt, ...) do {			\
+	char buf[256];					\
+	snprintf(buf, sizeof(buf), "%s:%d: " fmt ": %m\n",	\
+			__func__, __LINE__, ##__VA_ARGS__);	\
+	perror(buf);						\
+	exit(-1);						\
+} while (0)
+
+static inline void resume_and_wait_tracee(pid_t pid, int flag)
+{
+	int status;
+
+	if (ptrace(flag, pid, 0, 0))
+		perr_and_exit("failed to resume the tracee %d", pid);
+
+	if (waitpid(pid, &status, 0) != pid)
+		perr_and_exit("failed to wait for the tracee %d", pid);
+}
+
+static void ptrace_restart_syscall(int opt, int *result)
+{
+	int status;
+	int p[2], fd_zero;
+	pid_t pid;
+
+	struct user_regs_struct regs;
+	struct iovec iov = {
+		.iov_base = &regs,
+		.iov_len = sizeof(regs),
+	};
+
+	if (pipe(p))
+		perr_and_exit("failed to create a pipe");
+
+	fd_zero = open("/dev/zero", O_RDONLY);
+	if (fd_zero < 0)
+		perr_and_exit("failed to open /dev/zero");
+
+	pid = fork();
+	if (pid == 0) {
+		char c;
+
+		/* Mark oneself being traced */
+		if (ptrace(PTRACE_TRACEME, 0, 0, 0))
+			perr_and_exit("failed to request for tracer to trace me");
+
+		kill(getpid(), SIGSTOP);
+
+		if (read(p[0], &c, 1) != 1)
+			exit(1);
+
+		exit(0);
+	} else if (pid < 0)
+		exit(1);
+
+	if (waitpid(pid, &status, 0) != pid)
+		perr_and_exit("failed to wait for the tracee %d\n", pid);
+
+	/* Resume the tracee until the next syscall */
+	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
+
+	/* Deliver a signal to interrupt the syscall */
+	kill(pid, SIGUSR1);
+
+	/* The tracee stops at syscall exit */
+	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
+
+	/* Check tracee orig_a0 before syscall restart */
+	if (ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov))
+		perr_and_exit("failed to get tracee registers");
+	if (regs.orig_a0 != p[0])
+		perr_and_exit("unexpected a0");
+
+	/* Modify a0/orig_a0 for the restarted syscall */
+	switch (opt) {
+	case MODIFY_A0:
+		regs.a0 = fd_zero;
+		break;
+	case MODIFY_ORIG_A0:
+		regs.orig_a0 = fd_zero;
+		break;
+	}
+
+	if (ptrace(PTRACE_SETREGSET, pid, NT_PRSTATUS, &iov))
+		perr_and_exit("failed to set tracee registers");
+
+	/* Ignore SIGUSR1 signal */
+	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
+
+	/* Stop at the entry point of the restarted syscall */
+	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
+
+	/* Now, check regs.a0 of the restarted syscall */
+	if (ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov))
+		perr_and_exit("failed to get tracee registers");
+	*result = regs.a0;
+
+	/* Resume the tracee */
+	ptrace(PTRACE_CONT, pid, 0, 0);
+	if (waitpid(pid, &status, 0) != pid)
+		perr_and_exit("failed to wait for the tracee");
+}
+
+TEST(ptrace_modify_a0)
+{
+	int result;
+
+	ptrace_restart_syscall(MODIFY_A0, &result);
+
+	/* The tracer's modification of a0 cannot affect the restarted tracee */
+	EXPECT_NE(ORIG_A0_AFTER_MODIFIED, result);
+}
+
+TEST(ptrace_modify_orig_a0)
+{
+	int result;
+
+	ptrace_restart_syscall(MODIFY_ORIG_A0, &result);
+
+	/* The tracer must modify orig_a0 to actually change the tracee's a0 */
+	EXPECT_EQ(ORIG_A0_AFTER_MODIFIED, result);
+}
+
+TEST_HARNESS_MAIN