SymbolReader.java

import sun.misc.Unsafe;

import java.io.IOException;
import java.lang.reflect.Field;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.FileChannel;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.NoSuchElementException;

public class SymbolReader {
    final ByteBuffer buf;
    final long baseAddress;
    final int symtab;
    final int strings;

    SymbolReader(String libName) throws IOException {
        List<String> maps = Files.readAllLines(Paths.get("/proc/self/maps"));
        String map = maps.stream().filter(s -> s.endsWith(libName)).findFirst().get();

        try (FileChannel ch = FileChannel.open(Paths.get(map.substring(map.indexOf('/'))))) {
            this.buf = ch.map(FileChannel.MapMode.READ_ONLY, 0, ch.size()).order(ByteOrder.nativeOrder());
        }
        this.baseAddress = Long.parseUnsignedLong(map.substring(0, map.indexOf('-')), 16);
        this.symtab = findSection(2);
        this.strings = buf.getInt(section(buf.getInt(symtab + 40)) + 24);
    }

    int section(int n) {
        return buf.getInt(40) + (buf.getShort(58) & 0xffff) * n;
    }

    int findSection(int type) {
        int sections = buf.getShort(60) & 0xffff;
        for (int i = 0; i < sections; i++) {
            int section = section(i);
            if (buf.getInt(section + 4) == type) {
                return section;
            }
        }
        throw new NoSuchElementException("Section not found: " + type);
    }

    long findSymbol(String symbolName) {
        int symbols = buf.getInt(symtab + 24);
        int symbolsEnd = symbols + buf.getInt(symtab + 32);
        int entrySize = buf.getInt(symtab + 56);

        for (; symbols < symbolsEnd; symbols += entrySize) {
            int name = buf.getInt(symbols);
            long value = buf.getLong(symbols + 8);
            if (name != 0 && value != 0 && strcmp(strings + name, symbolName)) {
                return baseAddress + value;
            }
        }

        throw new NoSuchElementException("Symbol not found: " + symbolName);
    }

    boolean strcmp(int pos, String str) {
        int length = str.length();
        for (int i = 0; i < length; i++) {
            if (buf.get(pos++) != str.charAt(i)) {
                return false;
            }
        }
        return buf.get(pos) == 0;
    }

    static Unsafe getUnsafe() throws ReflectiveOperationException {
        Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        return (Unsafe) f.get(null);
    }

    static void eatAllMemory() {
        ArrayList<Object> list = new ArrayList();
        try {
            while (true) {
                list.add(new byte[1024 * 1024]);
            }
        } catch (OutOfMemoryError e) {
            list = null;
        }
    }

    public static void main(String[] args) throws Exception {
        Unsafe unsafe = getUnsafe();
        long addr = new SymbolReader("/libjvm.so")
                .findSymbol("_ZZ25report_java_out_of_memoryPKcE22out_of_memory_reported");

        System.out.println("OOM reported = " + unsafe.getInt(addr));
        eatAllMemory();
        System.out.println("OOM reported = " + unsafe.getInt(addr));
    }
}