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Merged
folkertdev merged 1 commit into from
Jun 10, 2025
Merged

use uninitialized output buffers for the fuzzers #112

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25 changes: 14 additions & 11 deletions fuzz/fuzz_targets/compress_then_decompress_chunked.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,17 +13,16 @@ use libfuzzer_sys::fuzz_target;

/// compress the data with the stock C bzip2
fn compress_c(data: &[u8], compression_level: u8, work_factor: u8) -> Vec<u8> {

// output buffer for compression, will get resized later if needed
let mut output = vec![0u8; 1024];
let mut output = Vec::<u8>::with_capacity(1024);

let mut stream = libbz2_rs_sys::bz_stream {
next_in: data.as_ptr() as *mut _,
avail_in: data.len() as _,
total_in_lo32: 0,
total_in_hi32: 0,
avail_out: output.capacity() as _,
next_out: output.as_mut_ptr() as *mut _,
avail_out: output.len() as _,
total_out_lo32: 0,
total_out_hi32: 0,
state: std::ptr::null_mut(),
Expand All @@ -46,10 +45,14 @@ fn compress_c(data: &[u8], compression_level: u8, work_factor: u8) -> Vec<u8> {
let error = loop {
match unsafe { libbz2_rs_sys::BZ2_bzCompress(&mut stream, BZ_FINISH) } {
BZ_FINISH_OK => {
let used = output.len() - stream.avail_out as usize;
let used = output.capacity() - stream.avail_out as usize;

// Safety: we've written this many (initialized!) bytes to the output.
unsafe { output.set_len(used) };

// The output buffer is full, resize it
let add_space: u32 = Ord::max(1024, output.len().try_into().unwrap());
output.resize(output.len() + add_space as usize, 0);
let add_space: u32 = Ord::max(1024, output.capacity().try_into().unwrap());
output.reserve(add_space as usize);

// If resize() reallocates, it may have moved in memory
stream.next_out = output.as_mut_ptr().cast::<i8>().wrapping_add(used);
Expand All @@ -70,11 +73,11 @@ fn compress_c(data: &[u8], compression_level: u8, work_factor: u8) -> Vec<u8> {
assert_eq!(error, BZ_OK);

// truncate the output buffer down to the actual number of compressed bytes
output.truncate(
((u64::from(stream.total_out_hi32) << 32) + u64::from(stream.total_out_lo32))
.try_into()
.unwrap(),
);
let total = u64::from(stream.total_out_hi32) << 32 | u64::from(stream.total_out_lo32);
unsafe { output.set_len(usize::try_from(total).unwrap()) };

// Just check that this byte is in fact initialized.
std::hint::black_box(output.last() == Some(&0));

unsafe {
// cleanup, should always succeed
Expand Down
27 changes: 20 additions & 7 deletions fuzz/fuzz_targets/decompress_chunked.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -46,10 +46,13 @@ fuzz_target!(|fuzz_data: &[u8]| -> Corpus {
Corpus::Keep
};

// initialize output buffer, resizing will happen later if needed
// using the input length is just a quick heuristic and not otherwise meaningful or necessary
let mut output = vec![0u8; fuzz_data.len()];
let output_len = output.len() as _;
// Initialize output buffer, resizing will happen later if needed
// using the input length is just a quick heuristic and not otherwise meaningful or necessary.
//
// Deliberately use uninitialized memory for the output, to ensure we never
// read/use uninitialized data.
let mut output = Vec::<u8>::with_capacity(fuzz_data.len());
let output_len = output.capacity() as _;

// set output buffer in decompression context
stream.next_out = output.as_mut_ptr().cast::<core::ffi::c_char>();
Expand All @@ -73,10 +76,14 @@ fuzz_target!(|fuzz_data: &[u8]| -> Corpus {
// there is still input from this chunk left to process but no more output buffer
// this means we have to increase the output buffer and retry the decompress
if stream.avail_in > 0 && stream.avail_out == 0 {
let used = output.len() - stream.avail_out as usize;
let used = output.capacity() - stream.avail_out as usize;

// Safety: we've written this many (initialized!) bytes to the output.
unsafe { output.set_len(used) };

// The dest buffer is full, resize it
let add_space: u32 = Ord::max(4096, output.len().try_into().unwrap());
output.resize(output.len() + add_space as usize, 0);
let add_space: u32 = Ord::max(4096, output.capacity().try_into().unwrap());
output.reserve(add_space as usize);

// If resize() reallocates, it may have moved in memory
stream.next_out = output.as_mut_ptr().cast::<i8>().wrapping_add(used);
Expand Down Expand Up @@ -108,6 +115,12 @@ fuzz_target!(|fuzz_data: &[u8]| -> Corpus {
}
}

let total = u64::from(stream.total_out_hi32) << 32 | u64::from(stream.total_out_lo32);
unsafe { output.set_len(usize::try_from(total).unwrap()) };

// Just check that this byte is in fact initialized.
std::hint::black_box(output.last() == Some(&0));

unsafe {
// clean up state, should always succeed
let err = BZ2_bzDecompressEnd(&mut stream);
Expand Down
44 changes: 24 additions & 20 deletions test-libbz2-rs-sys/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -260,7 +260,8 @@ pub unsafe fn decompress_c_with_capacity(
opaque: std::ptr::null_mut::<libc::c_void>(),
};

let mut dest = vec![0u8; capacity];
// Deliberately use uninitialized memory for the output.
let mut dest = Vec::<u8>::with_capacity(capacity);

strm.bzalloc = None;
strm.bzfree = None;
Expand All @@ -271,7 +272,7 @@ pub unsafe fn decompress_c_with_capacity(
return (ret, vec![]);
}
strm.avail_in = source_len;
strm.avail_out = dest.len() as _;
strm.avail_out = dest.capacity() as _;
strm.next_in = source as *mut libc::c_char;
strm.next_out = dest.as_mut_ptr().cast::<core::ffi::c_char>();

Expand All @@ -282,10 +283,14 @@ pub unsafe fn decompress_c_with_capacity(
BZ2_bzDecompressEnd(&mut strm);
break BZ_UNEXPECTED_EOF;
} else {
let used = dest.len() - strm.avail_out as usize;
let used = dest.capacity() - strm.avail_out as usize;

// We've written this many (initialized!) bytes to the output.
dest.set_len(used);

// The dest buffer is full.
let add_space: u32 = Ord::max(1024, dest.len().try_into().unwrap());
dest.resize(dest.len() + add_space as usize, 0);
let add_space: u32 = Ord::max(1024, dest.capacity().try_into().unwrap());
dest.reserve(add_space as usize);

// If resize() reallocates, it may have moved in memory.
strm.next_out = dest.as_mut_ptr().cast::<i8>().wrapping_add(used);
Expand All @@ -305,11 +310,8 @@ pub unsafe fn decompress_c_with_capacity(
}
};

dest.truncate(
((u64::from(strm.total_out_hi32) << 32) + u64::from(strm.total_out_lo32))
.try_into()
.unwrap(),
);
let total = (u64::from(strm.total_out_hi32) << 32) + u64::from(strm.total_out_lo32);
dest.set_len(usize::try_from(total).unwrap());

(ret, dest)
}
Expand Down Expand Up @@ -341,7 +343,8 @@ pub unsafe fn decompress_rs_with_capacity(
opaque: std::ptr::null_mut::<libc::c_void>(),
};

let mut dest = vec![0u8; capacity];
// Deliberately use uninitialized memory for the output.
let mut dest = Vec::<u8>::with_capacity(capacity);

strm.bzalloc = None;
strm.bzfree = None;
Expand All @@ -352,7 +355,7 @@ pub unsafe fn decompress_rs_with_capacity(
return (ret, vec![]);
}
strm.avail_in = source_len;
strm.avail_out = dest.len() as _;
strm.avail_out = dest.capacity() as _;
strm.next_in = source as *mut libc::c_char;
strm.next_out = dest.as_mut_ptr().cast::<core::ffi::c_char>();

Expand All @@ -363,10 +366,14 @@ pub unsafe fn decompress_rs_with_capacity(
BZ2_bzDecompressEnd(&mut strm);
break BZ_UNEXPECTED_EOF;
} else {
let used = dest.len() - strm.avail_out as usize;
let used = dest.capacity() - strm.avail_out as usize;

// We've written this many (initialized!) bytes to the output.
dest.set_len(used);

// The dest buffer is full.
let add_space: u32 = Ord::max(1024, dest.len().try_into().unwrap());
dest.resize(dest.len() + add_space as usize, 0);
let add_space: u32 = Ord::max(1024, dest.capacity().try_into().unwrap());
dest.reserve(add_space as usize);

// If resize() reallocates, it may have moved in memory.
strm.next_out = dest.as_mut_ptr().cast::<i8>().wrapping_add(used);
Expand All @@ -386,11 +393,8 @@ pub unsafe fn decompress_rs_with_capacity(
}
};

dest.truncate(
((u64::from(strm.total_out_hi32) << 32) + u64::from(strm.total_out_lo32))
.try_into()
.unwrap(),
);
let total = (u64::from(strm.total_out_hi32) << 32) + u64::from(strm.total_out_lo32);
dest.set_len(usize::try_from(total).unwrap());

(ret, dest)
}
Expand Down
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