feat: 实现基于异常表的安全用户空间内存访问机制

[fslongjin]

• 新增异常表机制，在系统调用中安全处理用户空间内存访问错误
• 实现带异常表保护的memcpy和memset函数，防止无效用户地址导致内核在内存拷贝处以及pagefault处理程序之间反复横跳
• 重构用户空间访问API，提供安全的UserBuffer包装类型
• 更新页错误处理程序，支持异常表修复路径
• 添加异常表测试程序，验证各种边界情况
• 更新内存管理文档，详细说明异常表设计原理和使用场景

[Copilot]

Pull Request Overview

This PR implements a comprehensive exception table mechanism for safe user space memory access in DragonOS, addressing a critical kernel stability issue where invalid user addresses could cause kernel panics. The implementation adds exception table entries to memory copy operations, allowing the page fault handler to gracefully recover from invalid user memory accesses by redirecting to fixup code.

Key changes:

• Added exception table infrastructure with binary search for fast fixup lookup during page faults
• Implemented protected memory operations (copy_from_user_protected, copy_to_user_protected, memset_with_exception_table) with inline assembly for x86_64
• Introduced UserBuffer wrapper type that guarantees all user space accesses go through exception-protected paths
• Refactored system call handlers (sigaction, process management) to use the new protected APIs

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
user/apps/c_unitest/test_exception_table.c	Test suite verifying exception table behavior with invalid pointers across multiple syscalls
kernel/src/syscall/user_buffer.rs	New UserBuffer type providing safe read/write/clear operations for user space memory
kernel/src/syscall/user_access.rs	Added protected copy functions and integration with UserBuffer API
kernel/src/syscall/mod.rs	Added user_buffer module to syscall subsystem
kernel/src/process/mod.rs	Updated to use clear_user_protected for thread cleanup
kernel/src/mm/mod.rs	Added trait methods for exception-table-protected memcpy and memset
kernel/src/ipc/syscall/sys_sigaction.rs	Refactored to use protected buffer access, removing unsafe pointer dereferences
kernel/src/ipc/signal_types.rs	Modified UserSigaction derive attributes for compatibility
kernel/src/exception/mod.rs	Added extable module to exception handling
kernel/src/exception/extable.rs	Core exception table search implementation with binary search algorithm
kernel/src/arch/x86_64/mm/mod.rs	x86_64-specific implementations of protected copy and memset using inline assembly
kernel/src/arch/x86_64/mm/fault.rs	Enhanced page fault handler to check exception table and perform fixups
kernel/src/arch/x86_64/link.lds	Added __ex_table section to linker script for exception table storage
docs/kernel/memory_management/extable_safe_copy_design.md	Comprehensive design documentation explaining mechanism and use cases
docs/kernel/memory_management/index.rst	Updated documentation index to include exception table design

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