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Message-ID: <8ee88532-0aad-4e57-8a91-e5d12cf86335@iscas.ac.cn>
Date: Mon, 24 Jun 2024 11:24:42 +0800
From: Quan Zhou <zhouquan@...as.ac.cn>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, oleg@...hat.com, paul.walmsley@...ive.com,
palmer@...belt.com, aou@...s.berkeley.edu, andy.chiu@...ive.com,
shuah@...nel.org
Subject: Re: [RFC PATCH 2/2] riscv: selftests: Add a ptrace test to check a0
of restarted syscall
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@...as.ac.cn wrote:
>>>> From: Quan Zhou <zhouquan@...as.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@...osinc.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@...osinc.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 = ®s,
> + .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!
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