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

On 2024/6/20 10:55, Charlie Jenkins wrote:

On Wed, Jun 19, 2024 at 10:01:47AM +0800, zhouquan@iscas.ac.cn wrote:

...

From: Quan Zhou zhouquan@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?

The current test may not intuitively reflect the issue. If possible, I will provide a more comprehensive test based on everyone's suggestions.

Thanks, Quan

...

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 zhouquan@iscas.ac.cn

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?

...yeah, I will fix it later.

...

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

The tracer ultimately sets `fd_zero` to the restarted syscall.

Since 0, 1, and 2 are standard input, output, and error, the file descriptors will be allocated in this order: `p[0] -> p[1] -> fd_zero`. Thus, fd_zero will be 5.

...

+}

+TEST_HARNESS_MAIN

2.34.1