dfhack/library/VTableInterpose.cpp

/*
https://github.com/peterix/dfhack
Copyright (c) 2009-2011 Petr Mrázek (peterix@gmail.com)

This software is provided 'as-is', without any express or implied
warranty. In no event will the authors be held liable for any
damages arising from the use of this software.

Permission is granted to anyone to use this software for any
purpose, including commercial applications, and to alter it and
redistribute it freely, subject to the following restrictions:

1. The origin of this software must not be misrepresented; you must
not claim that you wrote the original software. If you use this
software in a product, an acknowledgment in the product documentation
would be appreciated but is not required.

2. Altered source versions must be plainly marked as such, and
must not be misrepresented as being the original software.

3. This notice may not be removed or altered from any source
distribution.
*/

#include "Internal.h"

#include <string>
#include <vector>
#include <map>

#include "MemAccess.h"
#include "Core.h"
#include "VersionInfo.h"
#include "VTableInterpose.h"

#include "MiscUtils.h"

using namespace DFHack;

/*
 *  Code for accessing method pointers directly. Very compiler-specific.
 */

#if defined(_MSC_VER)

struct MSVC_MPTR {
    void *method;
    intptr_t this_shift;
};

bool DFHack::is_vmethod_pointer_(void *pptr)
{
    auto pobj = (MSVC_MPTR*)pptr;
    if (!pobj->method) return false;

    // MSVC implements pointers to vmethods via thunks.
    // This expects that they all follow a very specific pattern.
    auto pval = (unsigned*)pobj->method;
    switch (pval[0]) {
    case 0x20FF018BU: // mov eax, [ecx]; jmp [eax]
    case 0x60FF018BU: // mov eax, [ecx]; jmp [eax+0x??]
    case 0xA0FF018BU: // mov eax, [ecx]; jmp [eax+0x????????]
        return true;
    default:
        return false;
    }
}

int DFHack::vmethod_pointer_to_idx_(void *pptr)
{
    auto pobj = (MSVC_MPTR*)pptr;
    if (!pobj->method || pobj->this_shift != 0) return -1;

    auto pval = (unsigned*)pobj->method;
    switch (pval[0]) {
    case 0x20FF018BU: // mov eax, [ecx]; jmp [eax]
        return 0;
    case 0x60FF018BU: // mov eax, [ecx]; jmp [eax+0x??]
        return ((int8_t)pval[1])/sizeof(void*);
    case 0xA0FF018BU: // mov eax, [ecx]; jmp [eax+0x????????]
        return ((int32_t)pval[1])/sizeof(void*);
    default:
        return -1;
    }
}

void* DFHack::method_pointer_to_addr_(void *pptr)
{
    if (is_vmethod_pointer_(pptr)) return NULL;
    auto pobj = (MSVC_MPTR*)pptr;
    return pobj->method;
}

void DFHack::addr_to_method_pointer_(void *pptr, void *addr)
{
    auto pobj = (MSVC_MPTR*)pptr;
    pobj->method = addr;
    pobj->this_shift = 0;
}

#elif defined(__GXX_ABI_VERSION)

struct GCC_MPTR {
    intptr_t method;
    intptr_t this_shift;
};

bool DFHack::is_vmethod_pointer_(void *pptr)
{
    auto pobj = (GCC_MPTR*)pptr;
    return (pobj->method & 1) != 0;
}

int DFHack::vmethod_pointer_to_idx_(void *pptr)
{
    auto pobj = (GCC_MPTR*)pptr;
    if ((pobj->method & 1) == 0 || pobj->this_shift != 0)
        return -1;
    return (pobj->method-1)/sizeof(void*);
}

void* DFHack::method_pointer_to_addr_(void *pptr)
{
    auto pobj = (GCC_MPTR*)pptr;
    if ((pobj->method & 1) != 0 || pobj->this_shift != 0)
        return NULL;
    return (void*)pobj->method;
}

void DFHack::addr_to_method_pointer_(void *pptr, void *addr)
{
    auto pobj = (GCC_MPTR*)pptr;
    pobj->method = (intptr_t)addr;
    pobj->this_shift = 0;
}

#else
#error Unknown compiler type
#endif

void *virtual_identity::get_vmethod_ptr(int idx)
{
    assert(idx >= 0);
    void **vtable = (void**)vtable_ptr;
    if (!vtable) return NULL;
    return vtable[idx];
}

bool virtual_identity::set_vmethod_ptr(int idx, void *ptr)
{
    assert(idx >= 0);
    void **vtable = (void**)vtable_ptr;
    if (!vtable) return NULL;
    return Core::getInstance().p->patchMemory(&vtable[idx], &ptr, sizeof(void*));
}

/*
   VMethod interposing data structures.

   In order to properly support adding and removing hooks,
   it is necessary to track them. This is what this class
   is for. The task is further complicated by propagating
   hooks to child classes that use exactly the same original
   vmethod implementation.

   Every applied link contains in the saved_chain field a
   pointer to the next vmethod body that should be called
   by the hook the link represents. This is the actual
   control flow structure that needs to be maintained.

   There also are connections between link objects themselves,
   which constitute the bookkeeping for doing that. Finally,
   every link is associated with a fixed virtual_identity host,
   which represents the point in the class hierarchy where
   the hook is applied.

   When there are no subclasses (i.e. only one host), the
   structures look like this:

   +--------------+             +------------+
   | link1        |-next------->| link2      |-next=NULL
   |s_c: original |<-------prev-|s_c: $link1 |<--+
   +--------------+             +------------+   |
                                                 |
         host->interpose_list[vmethod_idx] ------+
         vtable: $link2

   The original vtable entry is stored in the saved_chain of the
   first link. The interpose_list map points to the last one.
   The hooks are called in order: link2 -> link1 -> original.

   When there are subclasses that use the same vmethod, but don't
   hook it, the topmost link gets a set of the child_hosts, and
   the hosts have the link added to their interpose_list:

   +--------------+                    +----------------+
   | link0 @host0 |<--+-interpose_list-| host1          |
   |              |-child_hosts-+----->| vtable: $link  |
   +--------------+   |         |      +----------------+
                      |         |
                      |         |      +----------------+
                      +-interpose_list-| host2          |
                                +----->| vtable: $link  |
                                       +----------------+

   When a child defines its own hook, the child_hosts link is
   severed and replaced with a child_next pointer to the new
   hook. The hook still points back the chain with prev.
   All child links to subclasses of host2 are migrated from
   link1 to link2.

   +--------------+-next=NULL         +--------------+-next=NULL
   | link1 @host1 |-child_next------->| link2 @host2 |-child_*--->subclasses
   |              |<-------------prev-|s_c: $link1   |
   +--------------+<-------+          +--------------+<-------+
                           |                                  |
   +--------------+        |          +--------------+        |
   | host1        |-i_list-+          | host2        |-i_list-+
   |vtable: $link1|                   |vtable: $link2|
   +--------------+                   +--------------+

 */

void VMethodInterposeLinkBase::set_chain(void *chain)
{
    saved_chain = chain;
    addr_to_method_pointer_(chain_mptr, chain);
}

VMethodInterposeLinkBase::VMethodInterposeLinkBase(virtual_identity *host, int vmethod_idx, void *interpose_method, void *chain_mptr)
    : host(host), vmethod_idx(vmethod_idx), interpose_method(interpose_method), chain_mptr(chain_mptr),
      applied(false), saved_chain(NULL), next(NULL), prev(NULL)
{
    if (vmethod_idx < 0 || interpose_method == NULL)
    {
        fprintf(stderr, "Bad VMethodInterposeLinkBase arguments: %d %08x\n",
                vmethod_idx, unsigned(interpose_method));
        fflush(stderr);
        abort();
    }
}

VMethodInterposeLinkBase::~VMethodInterposeLinkBase()
{
    if (is_applied())
        remove();
}

VMethodInterposeLinkBase *VMethodInterposeLinkBase::get_first_interpose(virtual_identity *id)
{
    auto item = id->interpose_list[vmethod_idx];
    if (!item)
        return NULL;

    if (item->host != id)
        return NULL;
    while (item->prev && item->prev->host == id)
        item = item->prev;

    return item;
}

void VMethodInterposeLinkBase::find_child_hosts(virtual_identity *cur, void *vmptr)
{
    auto &children = cur->getChildren();

    for (size_t i = 0; i < children.size(); i++)
    {
        auto child = static_cast<virtual_identity*>(children[i]);
        auto base = get_first_interpose(child);

        if (base)
        {
            assert(base->prev == NULL);

            if (base->saved_chain != vmptr)
                continue;

            child_next.insert(base);
        }
        else
        {
            void *cptr = child->get_vmethod_ptr(vmethod_idx);
            if (cptr != vmptr)
                continue;

            child_hosts.insert(child);
            find_child_hosts(child, vmptr);
        }
    }
}

void VMethodInterposeLinkBase::on_host_delete(virtual_identity *from)
{
    if (from == host)
    {
        // When in own host, fully delete
        remove();
    }
    else
    {
        // Otherwise, drop the link to that child:
        assert(child_hosts.count(from) != 0 &&
               from->interpose_list[vmethod_idx] == this);

        // Find and restore the original vmethod ptr
        auto last = this;
        while (last->prev) last = last->prev;

        from->set_vmethod_ptr(vmethod_idx, last->saved_chain);

        // Unlink the chains
        child_hosts.erase(from);
        from->interpose_list.erase(vmethod_idx);
    }
}

bool VMethodInterposeLinkBase::apply()
{
    if (is_applied())
        return true;
    if (!host->vtable_ptr)
        return false;

    // Retrieve the current vtable entry
    void *old_ptr = host->get_vmethod_ptr(vmethod_idx);
    VMethodInterposeLinkBase *old_link = host->interpose_list[vmethod_idx];

    assert(old_ptr != NULL && (!old_link || old_link->interpose_method == old_ptr));

    // Apply the new method ptr
    set_chain(old_ptr);

    if (!host->set_vmethod_ptr(vmethod_idx, interpose_method))
    {
        set_chain(NULL);
        return false;
    }

    // Push the current link into the home host
    applied = true;
    host->interpose_list[vmethod_idx] = this;
    prev = old_link;

    child_hosts.clear();
    child_next.clear();

    if (old_link && old_link->host == host)
    {
        // If the old link is home, just push into the plain chain
        assert(old_link->next == NULL);
        old_link->next = this;

        // Child links belong to the topmost local entry
        child_hosts.swap(old_link->child_hosts);
        child_next.swap(old_link->child_next);
    }
    else
    {
        // If creating a new local chain, find children with same vmethod
        find_child_hosts(host, old_ptr);

        if (old_link)
        {
            // Enter the child chain set
            assert(old_link->child_hosts.count(host));
            old_link->child_hosts.erase(host);
            old_link->child_next.insert(this);

            // Subtract our own children from the parent's sets
            for (auto it = child_next.begin(); it != child_next.end(); ++it)
                old_link->child_next.erase(*it);
            for (auto it = child_hosts.begin(); it != child_hosts.end(); ++it)
                old_link->child_hosts.erase(*it);
        }
    }

    // Chain subclass hooks
    for (auto it = child_next.begin(); it != child_next.end(); ++it)
    {
        auto nlink = *it;
        assert(nlink->saved_chain == old_ptr && nlink->prev == old_link);
        nlink->set_chain(interpose_method);
        nlink->prev = this;
    }

    // Chain passive subclass hosts
    for (auto it = child_hosts.begin(); it != child_hosts.end(); ++it)
    {
        auto nhost = *it;
        assert(nhost->interpose_list[vmethod_idx] == old_link);
        nhost->set_vmethod_ptr(vmethod_idx, interpose_method);
        nhost->interpose_list[vmethod_idx] = this;
    }

    return true;
}

void VMethodInterposeLinkBase::remove()
{
    if (!is_applied())
        return;

    // Remove the link from prev to this
    if (prev)
    {
        if (prev->host == host)
            prev->next = next;
        else
        {
            prev->child_next.erase(this);

            if (next)
                prev->child_next.insert(next);
        }
    }

    if (next)
    {
        next->set_chain(saved_chain);
        next->prev = prev;

        assert(child_next.empty() && child_hosts.empty());
    }
    else
    {
        // Remove from the list in the identity and vtable
        host->interpose_list[vmethod_idx] = prev;
        host->set_vmethod_ptr(vmethod_idx, saved_chain);

        for (auto it = child_next.begin(); it != child_next.end(); ++it)
        {
            auto nlink = *it;
            assert(nlink->saved_chain == interpose_method && nlink->prev == this);
            nlink->set_chain(saved_chain);
            nlink->prev = prev;
            if (prev)
                prev->child_next.insert(nlink);
        }

        for (auto it = child_hosts.begin(); it != child_hosts.end(); ++it)
        {
            auto nhost = *it;
            assert(nhost->interpose_list[vmethod_idx] == this);
            nhost->interpose_list[vmethod_idx] = prev;
            nhost->set_vmethod_ptr(vmethod_idx, saved_chain);
            if (prev)
                prev->child_hosts.insert(nhost);
        }
    }

    applied = false;
    prev = next = NULL;
    child_next.clear();
    child_hosts.clear();
    set_chain(NULL);
}