Browse zstd specs (6 of 44)

Proofs / zstd / Rejecting bad input

rj_any_dict

On any dictionary bytes and any input, ZSTD_decompress_usingDict returns an error or a size at most the capacity. The dictionary and the output are one object (the spec explains why).

spec
30 lines of C
with its headers
63 lines
status
Proved
6 of 44
On this page

The spec

The spec is the C program harness/rj_any_dict.c, 30 lines, shown whole as it is in the repository at 2e46017. It includes harness/laws_common.h (33 lines). Its first comment states the property in full, with every bound.

C
// Law rj_any_dict: For any dictionary and any input (any bytes, any lengths the allocator provides) and any output
// capacity, ZSTD_decompress_usingDict, called any number of times on one context, returns an error code or a size at
// most the capacity (RFC 8878 5: "If a dictionary is provided by an external source, it should be loaded with great
// care, its content considered untrusted"). The dictionary and the output are one object, with any gap between them:
// zstd compares the output's start with the dictionary's end, which CompCert's C defines only within one object.
#include "laws_common.h"

int main(void) {
	ZSTD_DCtx *dctx = ZSTD_createDCtx();
	if (dctx == NULL)
		return 0;
	do {
		// Each call: any input; any dictionary, gap and capacity, laid out as dictionary, gap, output
		size_t n = upto(PTRDIFF_MAX), dict_len = upto(PTRDIFF_MAX), gap = upto(PTRDIFF_MAX);
		size_t cap = upto(PTRDIFF_MAX);
		unsigned char *src = any_bytes(n), *both = NULL;
		if (dict_len <= PTRDIFF_MAX - gap && dict_len + gap <= PTRDIFF_MAX - cap)
			both = any_bytes(dict_len + gap + cap);
		if (src == NULL || both == NULL)
			return 0;
		size_t result = ZSTD_decompress_usingDict(dctx, both + dict_len + gap, cap, src, n, both, dict_len);
		// Check. zstd.h, ZSTD_decompress: "@return : the number of bytes decompressed into `dst` (<=
		// `dstCapacity`), or an errorCode if it fails"
		gp_assert(ZSTD_isError(result) || result <= cap);
		free(src);
		free(both);
	} while (nondet_u8() != 0);
	ZSTD_freeDCtx(dctx);
	return 0;
}
harness/laws_common.h, 33 lines
C
// laws_common.h: trivial helpers shared by the zstd laws (each law includes it once). No models, no algorithms.
#define ZSTD_STATIC_LINKING_ONLY                     // zstd.h's experimental part (ZSTD_WINDOWLOG_MAX, ...)
#include <stdint.h>
#include <stdlib.h>
#include "quickproof.h"
#include "zstd.h"
#include "zstd_errors.h"
uint64_t ZSTD_XXH64(const void *input, size_t length, uint64_t seed);   // zstd's XXH64 (law ck_xxh64 checks it)
static size_t upto(size_t max) {                    // any value from 0 to max
	size_t value = nondet_size();
	return value < max ? value : max;
}
static unsigned char *any_bytes(size_t n) {         // a new object of n arbitrary bytes (NULL if malloc fails)
	unsigned char *bytes = malloc(n);
	if (bytes != NULL && n != 0)
		nondet_bytes(bytes, n);
	return bytes;
}
static unsigned char *copy_of(const unsigned char *source, size_t n) {   // a new object holding source[0..n-1]
	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) {   // a[0..n-1] == b[0..n-1]
	for (size_t i = 0; i < n; i++)
		if (a[i] != b[i])
			return 0;
	return 1;
}
static int out_of_memory(size_t result) {           // zstd could not allocate (this ends a run: no claim)
	return ZSTD_getErrorCode(result) == ZSTD_error_memory_allocation;
}

The theorem

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

Rocq
Theorem rj_any_dict : 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 rj_any_dict : harness_safe Zstd.contracts.rj_any_dict.whole_prog.
Proof. exact (proj1 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 (proj2 Zstd.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
  • patch/decoder.c
  • quickproof/libc/libc.c
  • quickproof/libc/malloc_unit.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), one thread, and binaries built by a compiler (the theorem is about the code as CompCert defines C for x86_64, LP64).
  • Other configurations of zstd (flags, legacy formats, assembly) and the compressor.
  • The edits to zstd's code, listed in the repository, are checked against the original by testing, 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.