/*
 * Copyright © 2024 Michael Smith <mikesmiffy128@gmail.com>
 * Copyright © 2022 Willian Henrique <wsimanbrazil@yahoo.com.br>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED “AS IS” AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
 * OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 * PERFORMANCE OF THIS SOFTWARE.
 */

#include <string.h>

#include "con_.h"
#include "intdefs.h"
#include "mem.h"
#include "os.h"
#include "x86.h"

// 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.

#if defined(__GNUC__) || defined(__clang__)
__attribute__((aligned(4096)))
#elif defined(_MSC_VER)
__declspec(align(4096))
#else
#error no way to align stuff!
#endif
static uchar trampolines[4096];
static uchar *nexttrampoline = trampolines;

bool hook_init(void) {
	// PE doesn't support rwx sections, not sure about ELF. Meh, just set it
	// here instead.
	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
#error no way to flush 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 += mem_loads32(func + 1) + 5;
	if (!os_mprot(func, 5, PAGE_EXECUTE_READWRITE)) return false;
	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) {
			con_warn("hook_inline: can't trampoline call instructions\n");
			return 0;
		}
		int ilen = x86_len(func + len);
		if (ilen == -1) {
			con_warn("hook_inline: unknown or invalid instruction\n");
			return 0;
		}
		len += ilen;
		if (len >= 5) break;
		if (func[len] == X86_JMPIW) {
			con_warn("hook_inline: can't trampoline jmp instructions\n");
			return 0;
		}
	}
	// for simplicity, just bump alloc the trampoline. no need to free anyway
	if (nexttrampoline - trampolines > sizeof(trampolines) - len - 6) {
		con_warn("hook_inline: 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;
	uint 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 = mem_loads32(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);
}

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