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

[Quan Zhou]

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, zhouquan at iscas.ac.cn wrote:
>>>> From: Quan Zhou <zhouquan at iscas.ac.cn>
>>>>
>>>> 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. 🙂
>>
>> 2、Actually, I discovered the issue while using the execve function.
>> When I tried to modify the first parameter of execve in the tracer,
>> I found it didn't work.
>>
>> As for why not use execve for testing, there are two reasons:
>>
>> 1) The root cause of this issue is that when a syscall is interrupted
>> and then resumed, it restarts with orig_a0 instead of a0, so modifying
>> a0 doesn't work. I want to focus the test on the "restarted syscall".
>>
>> 2) Compared to the current test scenario, execve is terminated by ptrace
>> earlier, so I chose a later point. In fact, setting regs->a0 in the path
>> between (*1) and (*3) is ineffective because it will eventually be
>> overwritten by orig_a0, correct?
> 
> Thank you for the thorough explanation! I feel like a test case like the
> following achieves the same goal but without needing the pipes and the
> file. What do you think?
> 
>>From 5f13cdf8f7312b2b3cc98fbfbb3c95fcef62c0f0 Mon Sep 17 00:00:00 2001
> From: Charlie Jenkins <charlie at rivosinc.com>
> Date: Fri, 21 Jun 2024 12:58:29 -0700
> Subject: [PATCH] riscv: selftests: Add a ptrace test to check a0 of restarted
>   syscall
> 
> Add a riscv selftest that checks that a0 of syscalls are able to be
> replaced. When entering a syscall, a0 contains the first argument to the
> syscall and upon exiting, a0 contains the return value. To replace the
> a0 argument instead of the a0 return value with ptrace after halting the
> program with PTRACE_SYSCALL, orig_a0 must be used. This test checks that
> orig_a0 allows a syscall argument to be modified, and that changing a0
> does not change the syscall argument.
> 
> Signed-off-by: Charlie Jenkins <charlie at rivosinc.com>
> ---
>   .../riscv/abi/ptrace_restart_syscall.c        | 123 ++++++++++++++++++
>   1 file changed, 123 insertions(+)
>   create mode 100644 tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c
> 
> 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..e74ae02c6850
> --- /dev/null
> +++ b/tools/testing/selftests/riscv/abi/ptrace_restart_syscall.c
> @@ -0,0 +1,123 @@
> +// 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 DEFAULT_A0		0x6
> +
> +#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\n", pid);
> +
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee %d\n", pid);
> +}
> +
> +static void ptrace_restart_syscall(int opt, int *result)
> +{
> +	int status;
> +	pid_t pid;
> +
> +	struct user_regs_struct regs;
> +	struct iovec iov = {
> +		.iov_base = &regs,
> +		.iov_len = sizeof(regs),
> +	};
> +
> +	pid = fork();
> +	if (pid == 0) {
> +		/* Mark oneself being traced */
> +		long val = ptrace(PTRACE_TRACEME, 0, 0, 0);
> +		if (val)
> +			perr_and_exit("failed to request for tracer to trace me: %ld\n", val);
> +
> +		kill(getpid(), SIGSTOP);
> +
> +		/* Perform exit syscall that will be intercepted */
> +		exit(DEFAULT_A0);
> +	} else if (pid < 0) {
> +		exit(1);
> +	}
> +
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee %d\n", pid);
> +
> +	/* 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\n");
> +
> +	/* Modify a0/orig_a0 for the restarted syscall */
> +	switch (opt) {
> +	case MODIFY_A0:
> +		regs.a0 = ORIG_A0_AFTER_MODIFIED;
> +		break;
> +	case MODIFY_ORIG_A0:
> +		regs.orig_a0 = ORIG_A0_AFTER_MODIFIED;
> +		break;
> +	}
> +
> +	if (ptrace(PTRACE_SETREGSET, pid, NT_PRSTATUS, &iov))
> +		perr_and_exit("failed to set tracee registers\n");
> +
> +	/* Resume the tracee */
> +	ptrace(PTRACE_CONT, pid, 0, 0);
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee\n");
> +
> +	*result = WEXITSTATUS(status);
> +}
> +
> +TEST(ptrace_modify_a0)
> +{
> +	int result;
> +
> +	ptrace_restart_syscall(MODIFY_A0, &result);
> +
> +	/* The tracer's modification of a0 cannot affect the restarted tracee */
> +	EXPECT_EQ(DEFAULT_A0, 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

Great, this test can reflect issues more concisely compared to the 
previous one. I will update it in the next PATCH.

Thanks a lot!