feat: add select and pselect6 syscall

[Godones]

WARN: 测试程序无法运行!!! 子进程会出现页错误

[Copilot]

Pull Request Overview

This PR implements new select and pselect6 syscalls in the kernel, adds a user-space test application for select, and exposes poll utilities required by the new syscalls.

• Introduce a simple select test program and its DADK build configuration
• Implement SysSelect and SysPselect6 kernel handlers and register them
• Expose do_sys_poll and poll_select_set_timeout as public helpers and enable the let_chains feature

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
user/dadk/config/test_select_0_1_1.toml	Add DADK config for the test_select application
user/apps/test_select/main.c	Add user-space select demo utilizing eventfd
user/apps/test_select/Makefile	Add build/install/clean targets for test_select
kernel/src/lib.rs	Enable let_chains nightly feature for Rust patterns
kernel/src/filesystem/vfs/syscall/sys_select.rs	Implement select syscall and its helper functions
kernel/src/filesystem/vfs/syscall/sys_pselect6.rs	Implement pselect6 syscall wrapper
kernel/src/filesystem/vfs/syscall/mod.rs	Register the new syscall modules
kernel/src/filesystem/poll.rs	Make do_sys_poll and poll_select_set_timeout public

Comments suppressed due to low confidence (2)

kernel/src/filesystem/vfs/syscall/sys_select.rs:62

• The call to size_of::<PosixTimeval>() requires an import of core::mem::size_of; add use core::mem::size_of; at the top to avoid an unresolved name error.

            size_of::<PosixTimeval>(),

kernel/src/filesystem/vfs/syscall/sys_pselect6.rs:30

• The call to size_of::<SigSet>() also needs use core::mem::size_of; imported to compile.

                UserBufferReader::new(sigmask_ptr as *const SigSet, size_of::<SigSet>(), true)?;

[fslongjin]

WARN: 测试程序无法运行!!! 子进程会出现页错误

2025.7.10更新

是进程拷贝信号处理程序的bug。busybox启动的时候设置了SIGCHLD的处理函数。结果fork的时候子进程错误的继承了这个值。但是其实子进程没这个信号处理函数。导致收到SIGCHLD信号的时候崩溃。

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(原始评论）

可能跟进程fork&退出的时候unmap页面/pagecache有关？
cc @sparkzky @MemoryShore
只要我不让子进程退出就不会出现segment fault,以及No mapped VMA的报错。

[sparkzky]

WARN: 测试程序无法运行!!! 子进程会出现页错误

2025.7.10更新

是进程拷贝信号处理程序的bug。busybox启动的时候设置了SIGCHLD的处理函数。结果fork的时候子进程错误的继承了这个值。但是其实子进程没这个信号处理函数。导致收到SIGCHLD信号的时候崩溃。

(原始评论）

可能跟进程fork&退出的时候unmap页面/pagecache有关？ cc @sparkzky @MemoryShore 只要我不让子进程退出就不会出现segment fault,以及No mapped VMA的报错。

好像这个信号处理函数还是有必要的?后面busybox一直在调用SYS_RT_SIGTIMEDWAIT好像就是没有接收到子进程的SIGCHLD信号？