Initial public snapshot

With code from Bill. Thanks Bill!

1 files changed, 99 insertions, 0 deletions

diff --git a/src/hook.c b/src/hook.c
new file mode 100644
index 0000000..410e23e
--- /dev/null
+++ b/src/hook.c
@@ -0,0 +1,99 @@
+/*
+ * Copyright © 2021 Michael Smith <mikesmiffy128@gmail.com>
+ *
+ * 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 "udis86.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(_WIN32) && !defined(_WIN64)
+
+__attribute__((aligned(4096)))
+static uchar trampolines[4096];
+static uchar *nexttrampoline = trampolines;
+__attribute__((constructor))
+static void setrwx(void) {
+	// PE doesn't support rwx sections, not sure about ELF. Eh, just hack it in
+	// a constructor instead. If this fails and we segfault later, too bad!
+	os_mprot(trampolines, sizeof(trampolines), PAGE_EXECUTE_READWRITE);
+}
+
+#define RELJMP 0xE9 // the first byte of a 5-byte jmp
+
+void *hook_inline(void *func_, void *target) {
+	uchar *func = func_;
+	if (!os_mprot(func, 5, PAGE_EXECUTE_READWRITE)) return false;
+	struct ud udis;
+	ud_init(&udis);
+	ud_set_mode(&udis, 32);
+	// max insn length is 15, we overwrite 5, so max to copy is 4 + 15 = 19
+	ud_set_input_buffer(&udis, func, 19);
+	int len = 0;
+	while (ud_disassemble(&udis) && len < 5) {
+		if (ud_insn_mnemonic(&udis) == UD_Ijmp ||
+				ud_insn_mnemonic(&udis) == UD_Icall) {
+			con_warn("hook_inline: jmp adjustment NYI\n");
+			return 0;
+		}
+		len += ud_insn_len(&udis);
+	}
+	// for simplicity, just bump alloc the trampoline. no need to free anyway
+	if (nexttrampoline - trampolines > len + 6) goto nospc;
+	uchar *trampoline = (uchar *)InterlockedExchangeAdd(
+			(volatile long *)&nexttrampoline, len + 6);
+	if (trampoline - trampolines > len + 6) { // avoid TOCTOU
+nospc:	con_warn("hook_inline: out of trampoline space\n");
+		return 0;
+	}
+	*trampoline++ = len; // stick length in front for quicker unhooking
+	memcpy(trampoline, func, len);
+	trampoline[len] = RELJMP;
+	uint diff = func - (trampoline + 5); // goto the continuation
+	memcpy(trampoline + len + 1, &diff, 4);
+	uchar jmp[8];
+	jmp[0] = RELJMP;
+	diff = (uchar *)target - (func + 5); // goto the hook target
+	memcpy(jmp + 1, &diff, 4);
+	// pad with original bytes so we can do an 8-byte atomic write
+	memcpy(jmp + 5, func + 5, 3);
+	*(volatile uvlong *)func = *(uvlong *)jmp; // (assuming function is aligned)
+	FlushInstructionCache(GetCurrentProcess(), func, len);
+	return trampoline;
+}
+
+void unhook_inline(void *orig) {
+	uchar *p = (uchar *)orig;
+	int len = p[-1];
+	uint off = mem_load32(p + len + 1);
+	uchar *q = p + off + 5;
+	memcpy(q, p, 5); // XXX not atomic atm! (does any of it even need to be?)
+	FlushInstructionCache(GetCurrentProcess(), q, 5);
+}
+
+#else
+
+// TODO(linux): Implement for Linux and/or x86_64 when needed...
+
+#endif
+
+// vi: sw=4 ts=4 noet tw=80 cc=80