ext4_view/
reader.rs

1// Copyright 2024 Google LLC
2//
3// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
4// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
5// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
6// option. This file may not be copied, modified, or distributed
7// except according to those terms.
8
9use crate::error::BoxedError;
10use alloc::boxed::Box;
11use alloc::vec::Vec;
12use core::error::Error;
13use core::fmt::{self, Display, Formatter};
14
15#[cfg(feature = "std")]
16use {
17    std::fs::File,
18    std::io::{Seek, SeekFrom},
19};
20
21/// Interface used by [`Ext4`] to read the filesystem data from a storage
22/// file or device.
23///
24/// [`Ext4`]: crate::Ext4
25pub trait Ext4Read {
26    /// Read bytes into `dst`, starting at `start_byte`.
27    ///
28    /// Exactly `dst.len()` bytes will be read; an error will be
29    /// returned if there is not enough data to fill `dst`, or if the
30    /// data cannot be read for any reason.
31    fn read(
32        &mut self,
33        start_byte: u64,
34        dst: &mut [u8],
35    ) -> Result<(), BoxedError>;
36}
37
38#[cfg(feature = "std")]
39impl Ext4Read for File {
40    fn read(
41        &mut self,
42        start_byte: u64,
43        dst: &mut [u8],
44    ) -> Result<(), BoxedError> {
45        use std::io::Read;
46
47        self.seek(SeekFrom::Start(start_byte)).map_err(Box::new)?;
48        self.read_exact(dst).map_err(Box::new)?;
49        Ok(())
50    }
51}
52
53/// Error type used by the [`Vec<u8>`] impl of [`Ext4Read`].
54#[derive(Clone, Copy, Debug, Eq, PartialEq)]
55pub struct MemIoError {
56    start: u64,
57    read_len: usize,
58    src_len: usize,
59}
60
61impl Display for MemIoError {
62    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
63        write!(
64            f,
65            "failed to read {} bytes at offset {} from a slice of length {}",
66            self.read_len, self.start, self.src_len
67        )
68    }
69}
70
71impl Error for MemIoError {}
72
73impl Ext4Read for Vec<u8> {
74    fn read(
75        &mut self,
76        start_byte: u64,
77        dst: &mut [u8],
78    ) -> Result<(), BoxedError> {
79        read_from_bytes(self, start_byte, dst).ok_or_else(|| {
80            Box::new(MemIoError {
81                start: start_byte,
82                read_len: dst.len(),
83                src_len: self.len(),
84            })
85            .into()
86        })
87    }
88}
89
90fn read_from_bytes(src: &[u8], start_byte: u64, dst: &mut [u8]) -> Option<()> {
91    let start = usize::try_from(start_byte).ok()?;
92    let end = start.checked_add(dst.len())?;
93    let src = src.get(start..end)?;
94    dst.copy_from_slice(src);
95
96    Some(())
97}
98
99#[cfg(test)]
100mod tests {
101    use super::*;
102
103    #[test]
104    fn test_vec_read() {
105        let mut src = vec![1, 2, 3];
106
107        let mut dst = [0; 3];
108        src.read(0, &mut dst).unwrap();
109        assert_eq!(dst, [1, 2, 3]);
110
111        let mut dst = [0; 2];
112        src.read(1, &mut dst).unwrap();
113        assert_eq!(dst, [2, 3]);
114
115        let err = src.read(4, &mut dst).unwrap_err();
116        assert_eq!(
117            format!("{err}"),
118            format!(
119                "failed to read 2 bytes at offset 4 from a slice of length 3"
120            )
121        );
122    }
123}
ext4_view/reader.rs

ext4_view/
reader.rs
