// SPDX-License-Identifier: ISC
// SPDX-FileCopyrightText: 2024 Michael Smith <mikesmiffy128@gmail.com>
// SPDX-FileCopyrightText: 2022 Willian Henrique <wsimanbrazil@yahoo.com.br>
// SPDX-FileCopyrightText: 2024 Matthew Wozniak <me@woz.blue>

#include <string.h>

#include "3p/sst/x86.h"

#include "intdef.h"
#include "log.h"
#include "os.h"

#ifdef _WIN32
// try to avoid pulling in all of Windows.h for this... (redundant dllimport
// avoids warnings in hook.test.c where Windows.h is included via test.h)
__declspec(dllimport) int __stdcall FlushInstructionCache(
		void *, const void *, usize);
#endif

// Warning: half-arsed hacky implementation (because that's all we really need)
// Almost certainly breaks in some weird cases. Oh well! Most of the time,
// vtable hooking is more reliable, this is only for, uh, emergencies.

__attribute__((aligned(4096))) static uchar trampolines[4096];
static uchar *nexttrampoline = trampolines;

bool hook_init(void) {
	return os_mprot(trampolines, sizeof(trampolines), PAGE_EXECUTE_READWRITE);
}

static inline void iflush(void *p, int len) {
#if defined(_WIN32)
	// -1 is the current process, and it's a constant in the WDK, so it's
	// assumed we can safely avoid the useless GetCurrentProcess call
	FlushInstructionCache((void *)-1, p, len);
#elif defined(__GNUC__)
	__builtin___clear_cache((char *)p, (char *)p + len);
#else
	warn("no way to clear instruction cache!");
#endif
}

void *hook_inline(void *func_, void *target) {
	uchar *func = func_;
	// dumb hack: if we hit some thunk that immediately jumps elsewhere (which
	// seems common for win32 API functions), hook the underlying thing instead.
	while (*func == X86_JMPIW) func += *(i32 *)(func + 1) + 5;
	if (!os_mprot(func, 5, PAGE_EXECUTE_READWRITE)) return 0;
	int len = 0;
	for (;;) {
		// FIXME: these cases may result in somewhat dodgy error messaging. They
		// shouldn't happen anyway though. Maybe if we're confident we just
		// compile 'em out of release builds some day, but that sounds a little
		// scary. For now preferring confusing messages over crashes, I guess.
		if (func[len] == X86_CALL) {
			warn("can't trampoline call instructions\n");
			return 0;
		}
		int ilen = x86_len(func + len);
		if (ilen == -1) {
			warn("unknown or invalid instruction\n");
			return 0;
		}
		len += ilen;
		if (len >= 5) break;
		if (func[len] == X86_JMPIW) {
			warn("can't trampoline jmp instructions\n");
			return 0;
		}
	}
	// for simplicity, just bump alloc the trampoline. no need to free anyway
	if (nexttrampoline - trampolines > (int)sizeof(trampolines) - len - 6) {
		warn("out of trampoline space\n");
		return 0;
	}
	uchar *trampoline = nexttrampoline;
	nexttrampoline += len + 6; // NOT thread-safe. we don't need that anyway!
	*trampoline++ = len; // stick length in front for quicker unhooking
	memcpy(trampoline, func, len);
	trampoline[len] = X86_JMPIW;
	u32 diff = func - (trampoline + 5); // goto the continuation
	memcpy(trampoline + len + 1, &diff, 4);
	diff = (uchar *)target - (func + 5); // goto the hook target
	func[0] = X86_JMPIW;
	memcpy(func + 1, &diff, 4);
	iflush(func, 5);
	return trampoline;
}

void unhook_inline(void *orig) {
	uchar *p = orig;
	int len = p[-1];
	int off = *(i32 *)(p + len + 1);
	uchar *q = p + off + 5;
	memcpy(q, p, 5); // XXX: not atomic atm! (does any of it even need to be?)
	iflush(q, 5);
}

void *hook_dllapi(const char *module, const char *name, void *target) {
	void *func = os_dlsym(os_dlopen(module), name);
	debug("%s = %p", name, func);
	if (!func) warn("couldn't find function %s in %s", name, module);
	else return hook_inline(func, target);
	return NULL;
}

// vi: sw=4 ts=4 noet tw=80 cc=80