dont free

dont-free.c

//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftNIO open source project
//
// Copyright (c) 2021 Apple Inc. and the SwiftNIO project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//

#include <string.h>
#include <stdlib.h>
#include <malloc/malloc.h>
#include <stdio.h>

#ifndef __APPLE__
#error Sorry, currently only Apple platforms supported. This can be made to work on Linux/other UNIX too though.
#endif

#define DYLD_INTERPOSE(_replacement,_replacee) \
   __attribute__((used)) static struct { const void *replacement; const void *replacee; } _interpose_##_replacee \
            __attribute__ ((section("__DATA,__interpose"))) = { (const void *)(unsigned long)&_replacement, (const void *)(unsigned long)&_replacee };

__attribute__((constructor))
static void go(void) {
    fprintf(stderr, "[[ DONT FREE is active, free() doesn't free anymore ]]\n");
}

void replacement_free(void *ptr) {
    if (ptr) {
        size_t size = malloc_size(ptr);
        memset(ptr, 0x41, size);
    }
}

void replacement_malloc_zone_free(malloc_zone_t *zone, void *ptr) {
    replacement_free(ptr);
}

DYLD_INTERPOSE(replacement_free, free)
DYLD_INTERPOSE(replacement_malloc_zone_free, malloc_zone_free)

Makefile

CFLAGS	:= -O2 -fPIC

test: libdontfree.dylib test.swift
	swiftc -L. -ldontfree test.swift

%.o: %.c
	$(CC) $(CFLAGS) -c -o $@ $^

libdontfree.dylib: dont-free.o
	$(CC) $(LDFLAGS) -shared -o $@ $^

clean:
	rm *.o *.dylib test

test.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftNIO open source project
//
// Copyright (c) 2021 Apple Inc. and the SwiftNIO project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//

class Foo {}

var allAddrs: Set<UInt> = []
var iterations = 0
while true {
    iterations += 1
    let foo = Foo()
    let addr = UInt(bitPattern: ObjectIdentifier(foo))
    if !allAddrs.contains(addr) {
        if CommandLine.arguments.dropFirst().first == "debug" {
            print("0x", String(addr, radix: 16), separator: "")
        }
        allAddrs.formUnion([addr])
    }
    if iterations % 100000 == 0 {
        let reused = iterations - allAddrs.count
        print("After \(iterations) iterations, we saw \(reused) reused and \(iterations - reused) fresh addresses.")
    }
}