feat: implement preadv, pwritev, preadv2, pwritev2 syscalls

docs/preadv_pwritev_design.md

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+# Design Document: preadv/pwritev Syscall Implementation
+
+## Overview
+
+This document describes the implementation plan for the Linux `preadv(2)`, `pwritev(2)`, `preadv2(2)`, and `pwritev2(2)` syscalls in litebox. These syscalls enable vectored I/O at a specified offset without affecting the file pointer position.
+
+## Background
+
+LiteBox already has `readv` and `writev` syscalls implemented, which perform scatter-gather I/O using the file's current position. The `preadv`/`pwritev` variants add the ability to specify an offset, similar to how `pread`/`pwrite` extend `read`/`write`.
+
+The `v2` variants add support for RWF_* flags that modify the I/O behavior.
+
+## Syscall Signatures
+
+### preadv
+
+```c
+ssize_t preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+```
+
+On x86-64, the offset is split into low/high parts:
+```c
+ssize_t preadv(unsigned long fd, const struct iovec *vec, unsigned long vlen, 
+               unsigned long pos_l, unsigned long pos_h);
+```
+
+### preadv2
+
+```c
+ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt, off_t offset, int flags);
+```
+
+### pwritev
+
+```c
+ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+```
+
+### pwritev2
+
+```c
+ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt, off_t offset, int flags);
+```
+
+## Parameters
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `fd` | `int` | File descriptor |
+| `iov` | `const struct iovec *` | Array of iovec structures |
+| `iovcnt` | `int` | Number of elements in iov array |
+| `offset` | `off_t` | File offset to read/write at |
+| `flags` | `int` | RWF_* flags (v2 variants only) |
+
+## RWF Flags (v2 variants)
+
+| Flag | Value | Description |
+|------|-------|-------------|
+| `RWF_HIPRI` | `0x00000001` | High priority request, may use polling |
+| `RWF_DSYNC` | `0x00000002` | Per-I/O O_DSYNC equivalent |
+| `RWF_SYNC` | `0x00000004` | Per-I/O O_SYNC equivalent |
+| `RWF_NOWAIT` | `0x00000008` | Don't block, return EAGAIN if would block |
+| `RWF_APPEND` | `0x00000010` | Per-I/O O_APPEND |
+
+For our initial implementation:
+- `RWF_NOWAIT`: Ignored (LiteBox I/O is typically synchronous)
+- `RWF_DSYNC`, `RWF_SYNC`: Ignored (data is persisted synchronously)
+- `RWF_HIPRI`: Ignored (no polling support)
+- `RWF_APPEND`: Should be supported for pwritev2
+
+Unknown flags should return EOPNOTSUPP.
+
+## Special Behavior
+
+### offset == -1 for v2 variants
+
+When offset is -1 in preadv2/pwritev2, the syscall behaves like readv/writev (uses current file position). This is a Linux-specific behavior to allow using RWF flags with non-positioned I/O.
+
+## Return Values
+
+- On success: Total number of bytes read/written
+- On error: -1 with errno set
+
+## Error Handling
+
+| Error | Condition |
+|-------|-----------|
+| `EBADF` | Invalid file descriptor or fd not open for reading/writing |
+| `EINVAL` | Invalid offset (negative), invalid iovcnt, or sum of iov_len overflows |
+| `EFAULT` | iov points outside accessible address space |
+| `ESPIPE` | fd refers to a pipe or socket (can't seek) |
+| `EOPNOTSUPP` | Unknown flags specified |
+
+## Implementation Plan
+
+### Phase 1: Add Types to `litebox_common_linux/src/lib.rs`
+
+1. Add `RwfFlags` bitflags type for RWF_* flags
+2. Add `Preadv` variant to `SyscallRequest` enum
+3. Add `Pwritev` variant to `SyscallRequest` enum
+4. Add `Preadv2` variant to `SyscallRequest` enum  
+5. Add `Pwritev2` variant to `SyscallRequest` enum
+6. Add syscall number parsing for all four syscalls
+
+### Phase 2: Add Syscall Handlers to `litebox_shim_linux`
+
+1. Add dispatch cases in `litebox_shim_linux/src/lib.rs`
+2. Implement `sys_preadv()` in `litebox_shim_linux/src/syscalls/file.rs`
+3. Implement `sys_pwritev()` in `litebox_shim_linux/src/syscalls/file.rs`
+4. Implement `sys_preadv2()` in `litebox_shim_linux/src/syscalls/file.rs`
+5. Implement `sys_pwritev2()` in `litebox_shim_linux/src/syscalls/file.rs`
+
+### Phase 3: Implementation Details
+
+#### sys_preadv Implementation
+
+```
+1. Validate fd (must be valid, open for reading)
+2. Validate offset (must be non-negative)
+3. Validate iovcnt (must be > 0, <= IOV_MAX)
+4. Read iovec array from user space
+5. Get file descriptor from file table
+6. For each iovec element:
+   a. Read from file at current offset
+   b. Copy data to user buffer
+   c. Advance offset by bytes read
+   d. Stop if fewer bytes than requested (EOF or error)
+7. Return total bytes read
+```
+
+#### sys_pwritev Implementation
+
+```
+1. Validate fd (must be valid, open for writing)
+2. Validate offset (must be non-negative)
+3. Validate iovcnt (must be > 0, <= IOV_MAX)
+4. Read iovec array from user space
+5. Get file descriptor from file table
+6. For each iovec element:
+   a. Copy data from user buffer
+   b. Write to file at current offset
+   c. Advance offset by bytes written
+   d. Stop if fewer bytes than requested (error or partial write)
+7. Return total bytes written
+```
+
+#### sys_preadv2/sys_pwritev2 Implementation
+
+Same as above, but:
+1. Parse and validate RWF flags
+2. If offset == -1, delegate to sys_readv/sys_writev
+3. Otherwise proceed as preadv/pwritev with flags handling
+
+## Key Design Decisions
+
+### 1. Reuse Existing readv/writev Infrastructure
+
+The existing `sys_readv` and `sys_writev` implementations read/write using the file's current position. Our preadv/pwritev can share the iovec processing logic but pass an explicit offset to the underlying fs.read/fs.write calls.
+
+### 2. Offset Parameter on x86-64
+
+On 64-bit Linux, the offset is passed as two parameters (pos_l, pos_h) for historical ABI reasons, but on x86-64 the high part is always 0 and can be ignored. We'll accept both parameters in the syscall parsing but use only pos_l as a 64-bit offset.
+
+### 3. Flag Handling Strategy
+
+For initial implementation, we'll accept all valid RWF flags but implement only essential behavior. Unknown flags return EOPNOTSUPP as required by the API contract.
+
+## Files to Modify
+
+1. `litebox_common_linux/src/lib.rs` - Add types and syscall parsing
+2. `litebox_shim_linux/src/lib.rs` - Add syscall dispatch
+3. `litebox_shim_linux/src/syscalls/file.rs` - Implement handlers
+
+## Test Plan
+
+### Unit Tests
+
+1. `test_preadv_basic` - Basic vectored read at offset
+2. `test_pwritev_basic` - Basic vectored write at offset
+3. `test_preadv_offset_unchanged` - Verify file position not affected
+4. `test_pwritev_offset_unchanged` - Verify file position not affected
+5. `test_preadv2_flags` - Test with various RWF flags
+6. `test_pwritev2_flags` - Test with various RWF flags
+7. `test_preadv2_offset_minus_one` - Test offset=-1 behavior
+8. `test_preadv_espipe` - Error on pipe/socket
+9. `test_preadv_invalid_offset` - Error on negative offset
+
+## References
+
+- Linux man pages: `man 2 preadv`, `man 2 preadv2`
+- Linux source: `/workspace/linux/fs/read_write.c`
+- Asterinas implementation: `/workspace/asterinas/kernel/src/syscall/preadv.rs`
+- Existing readv/writev in litebox

docs/preadv_pwritev_progress.md

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+# preadv/pwritev Implementation Progress Report
+
+This is an append-only progress report for the preadv/pwritev syscall implementation.
+
+---
+
+## Entry 1 - Project Setup
+
+**Status**: Completed
+
+### Completed
+- [x] Created feature branch `wdcui/preadv-pwritev` from main
+- [x] Reviewed existing readv/writev implementation in litebox
+- [x] Reviewed Linux source code for preadv/pwritev syscalls
+- [x] Reviewed Asterinas implementation for reference
+- [x] Created design document
+
+### Next Steps
+- Add RwfFlags bitflags type to litebox_common_linux
+- Add syscall variants to SyscallRequest enum
+- Add syscall number parsing
+
+---
+
+## Entry 2 - Implementation Complete
+
+**Status**: Completed
+
+### Changes Made
+
+#### litebox_common_linux/src/lib.rs
+- Added `RwfFlags` bitflags type with RWF_HIPRI, RWF_DSYNC, RWF_SYNC, RWF_NOWAIT, RWF_APPEND
+- Added SyscallRequest variants: Preadv, Pwritev, Preadv2, Pwritev2
+- Added syscall number parsing for preadv, pwritev, preadv2, pwritev2
+- Added RwfFlags to `reinterpret_truncated_from_usize_for!` macro
+- Handles x86 vs x86_64 offset parameter differences
+
+#### litebox_shim_linux/src/lib.rs
+- Added dispatch cases for Preadv, Pwritev, Preadv2, Pwritev2
+
+#### litebox_shim_linux/src/syscalls/file.rs
+- Implemented sys_preadv(): vectored read at offset
+- Implemented sys_pwritev(): vectored write at offset
+- Implemented sys_preadv2(): vectored read with flags, offset=-1 falls back to readv
+- Implemented sys_pwritev2(): vectored write with flags, offset=-1 falls back to writev
+- Implemented do_preadv() and do_pwritev() helper functions
+- Returns ESPIPE for pipes/sockets (positioned I/O not supported)
+- Returns EINVAL for negative offsets and invalid flags
+- Returns EOPNOTSUPP for unknown flags
+
+#### litebox_shim_linux/src/syscalls/tests.rs
+- Added test_preadv_pwritev_basic: tests basic preadv and pwritev operations
+- Added test_preadv2_offset_minus_one: tests preadv2 fallback to readv when offset=-1
+- Added test_preadv_invalid_offset: tests EINVAL for negative offset
+- Added test_preadv2_invalid_flags: tests EINVAL for RWF_APPEND on read
+
+### Test Results
+- All 174 tests pass
+- No clippy warnings
+- Code formatted with cargo fmt
+
+---
+