Browse zlib specs (7 of 62)

Proofs / zlib / Round trips

rt_partial

Given any unfinished prefix of a compressed stream, decompression only ever writes the start of the data.

spec
68 lines of C
with its headers
108 lines
status
Proved
7 of 62
On this page

The spec

The spec is the C program harness/rt_partial.c, 68 lines, shown whole as it is in the repository at 37189f4. It includes harness/laws_common.h (40 lines). Its first comment states the property in full, with every bound.

C
// Law rt_partial: For any data (any length the allocator provides) compressed with deflateInit2 (any level, windowBits,
// memLevel and strategy it accepts) and deflate (all of it, in pieces of at most 2^32 - 1 bytes, Z_FINISH with the
// last, into deflateBound_z bytes; the run ends unless it reaches Z_STREAM_END), inflate() fed any proper prefix of the
// compressed bytes and room for the data, in pieces of at most 2^32 - 1 bytes, with any flush, until it has read all of
// the prefix, writes only the start of the data: after every call, the bytes it has written so far are the first bytes
// of the data, as a copy made before compression holds them (Z_MEM_ERROR ends the run).
#include "laws_common.h"

int main(void) {
	size_t data_len = upto(PTRDIFF_MAX);
	unsigned char *data = alloc_any_bytes(data_len);
	if (data == NULL)
		return 0;
	unsigned char *expected = copy_of(data, data_len);
	unsigned char *out = malloc(data_len);
	if (expected == NULL || out == NULL)
		return 0;
	int level = (int) nondet_u32();
	int window_bits = (int) nondet_u32();
	int mem_level = (int) nondet_u32();
	int strategy = (int) nondet_u32();
	z_stream compressor;
	new_stream(&compressor);
	if (deflateInit2(&compressor, level, Z_DEFLATED, window_bits, mem_level, strategy) != Z_OK)
		return 0;
	size_t out_size = deflateBound_z(&compressor, data_len);
	unsigned char *compressed = malloc(out_size);
	if (compressed == NULL)
		return 0;
	compressor.next_in = data;
	compressor.next_out = compressed;
	int code;
	do {
		size_t rest = data_len - (size_t) (compressor.next_in - data);
		int flush = Z_NO_FLUSH;
		if (rest <= UINT_MAX)
			flush = Z_FINISH;
		compressor.avail_in = clip(rest);
		compressor.avail_out = clip(out_size - (size_t) (compressor.next_out - compressed));

		code = deflate(&compressor, flush);
	} while (code == Z_OK);
	if (code != Z_STREAM_END)
		return 0;
	size_t compressed_len = (size_t) (compressor.next_out - compressed);
	size_t prefix_len = upto(compressed_len - 1);
	z_stream decompressor;
	new_stream(&decompressor);
	// zlib.h, deflateInit2: "a request for 8 will result in 9"; "use 9 with inflateInit2()"
	if (window_bits == 8)
		window_bits = 9;
	if (inflateInit2(&decompressor, window_bits) != Z_OK)
		return 0;
	decompressor.next_in = compressed;
	decompressor.next_out = out;
	while (decompressor.next_in != compressed + prefix_len) {
		decompressor.avail_in = clip(prefix_len - (size_t) (decompressor.next_in - compressed));
		decompressor.avail_out = clip(data_len - (size_t) (decompressor.next_out - out));

		if (inflate(&decompressor, (int) nondet_u32()) == Z_MEM_ERROR)
			return 0;

		// zlib.h, inflate: "inflate decompresses as much data as possible, and stops when the input buffer
		// becomes empty or the output buffer becomes full."
		gp_assert(bytes_eq(out, expected, (size_t) (decompressor.next_out - out)));
	}
	return 0;
}
harness/laws_common.h, 40 lines
C
// laws_common.h: trivial helpers shared by the zlib laws (each law includes it once). No models, no algorithms.
#include <limits.h>
#include <stdlib.h>
#include "quickproof.h"
#include "zlib.h"
static size_t upto(size_t max) {
	size_t value = nondet_size();
	return value < max ? value : max;
}
static uInt clip(size_t n) { return n < UINT_MAX ? (uInt) n : UINT_MAX; }
static unsigned char *alloc_any_bytes(size_t n) {
	unsigned char *bytes = malloc(n);
	if (bytes != NULL)
		nondet_bytes(bytes, n);
	return bytes;
}
static unsigned char *copy_of(const unsigned char *source, size_t n) {
	unsigned char *copy = malloc(n);
	for (size_t i = 0; copy != NULL && i < n; i++)
		copy[i] = source[i];
	return copy;
}
static int bytes_eq(const unsigned char *a, const unsigned char *b, size_t n) {
	for (size_t i = 0; i < n; i++)
		if (a[i] != b[i])
			return 0;
	return 1;
}
static void set_buffer(z_stream *strm, unsigned char *in, size_t in_len, unsigned char *out, size_t out_len) {
	strm->next_in = in;
	strm->avail_in = (uInt) in_len;
	strm->next_out = out;
	strm->avail_out = (uInt) out_len;
}
static void new_stream(z_stream *strm) {
	strm->zalloc = Z_NULL;
	strm->zfree = Z_NULL;
	strm->opaque = Z_NULL;
	set_buffer(strm, Z_NULL, 0, Z_NULL, 0);
}

The theorem

QuickProof generated this statement from the spec, in src/contracts/rt_partial.v:

Rocq
Theorem rt_partial : harness_safe whole_prog.

src/contracts/laws.v states it on the spec's own program, whole_prog (the spec linked with the code under test), with its proof:

Rocq
Theorem rt_partial : harness_safe Zlib.contracts.rt_partial.whole_prog.
Proof. exact (proj1 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 Zlib.contracts.suite.suite))))))))))))))))))))))))))))))))))))))))). Qed.

Read as a statement about runs, it says that no run of the program has undefined behaviour or a failing check, for every value the spec can draw. That reading rests on VST's adequacy theorem, which is not yet instantiated in Rocq (formerly Coq) for QuickProof's environment.

The C files (units) its program links
  • adler32.c
  • crc32.c
  • deflate.c
  • trees.c
  • inflate.c
  • inffast.c
  • inftrees.c
  • zutil.c
  • quickproof/libc/libc.c

What it leaves out

  • What the first comment of the spec excludes, and its bounds.
  • Like every spec of this proof: termination (a run that never returns is not a counterexample), and binaries built by a compiler (the theorem is about the code as CompCert defines C for x86_64, LP64), one thread.
  • Where zlib reports Z_MEM_ERROR, most specs end the run and claim nothing about it; the first comment of the spec says so.
  • The edits to zlib's code, listed in the repository, are tested, not proved.

The summary page says what the proof as a whole covers.

How it was proved

AI agents wrote the proof, and Rocq checks it. qp verify passes on this repository: this statement is the one generated from the spec at the contract commit, and its proof uses only Rocq's standard axioms and VST's abstract malloc library.