fix disassembling 'backwards'

(e.g. pd -5) producing wrong output

fixes #5105

libr/core/cmd_print.c

@@ -1017,11 +1017,19 @@ static int pdi(RCore *core, int nb_opcodes, int nb_bytes, int fmt) {
 		}
 	} else if (!nb_bytes) {
 		if (nb_opcodes < 0) {
+			ut64 start;
 			/* Backward disassembly of `ilen` opcodes
 			 * - We compute the new starting offset
 			 * - Read at the new offset */
 			nb_opcodes = -nb_opcodes;
-			r_core_asm_bwdis_len (core, &nb_bytes, &core->offset, nb_opcodes);
+			if (r_core_prevop_addr (core, core->offset, nb_opcodes, &start)) {
+				// We have some anal_info.
+				nb_bytes = core->offset - start;
+			} else {
+				// anal ignorance.
+				r_core_asm_bwdis_len (core, &nb_bytes, &core->offset,
+						nb_opcodes);
+			}
 			r_core_read_at (core, core->offset, core->block, nb_bytes);
 		} else {
 			// workaround for the `for` loop below
@@ -2755,6 +2763,7 @@ static int cmd_print(void *data, const char *input) {
 		if (!processed_cmd) {
 			ut64 addr = core->offset;
 			ut8 *block = NULL;
+			ut64 start;
 
 			if (bw_disassemble) {
 				block = malloc (core->blocksize);
@@ -2769,7 +2778,13 @@ static int cmd_print(void *data, const char *input) {
 					} else { //pd
 						const int bs = core->blocksize;
 						int instr_len;
-						r_core_asm_bwdis_len (core, &instr_len, &addr, l);
+						if (r_core_prevop_addr (core, core->offset, l, &start)) {
+							// We have some anal_info.
+							instr_len = core->offset - start;
+						} else {
+							// anal ignorance.
+							r_core_asm_bwdis_len (core, &instr_len, &addr, l);
+						}
 						ut64 prevaddr = core->offset;
 						r_core_seek(core, prevaddr - instr_len, true);
 						block = realloc (block, R_MAX(instr_len, bs));

libr/core/visual.c

@@ -631,6 +631,30 @@ static ut64 prevop_addr (RCore *core, ut64 addr) {
 	return target - 4;
 }
 
+//  Returns true if we can use analysis to find the previous operation address,
+//  sets prev_addr to the value of the instruction numinstrs back.
+//  If we can't use the anal, then set prev_addr to UT64_MAX and return false;
+R_API bool r_core_prevop_addr (RCore* core, ut64 start_addr, int numinstrs,
+		ut64* prev_addr) {
+	RAnalBlock* bb;
+	int i;
+	// Check that we're in a bb, otherwise this prevop stuff won't work.
+	bb = r_anal_bb_from_offset (core->anal, start_addr);
+	if (bb) {
+		if (r_anal_bb_opaddr_at (bb, start_addr) != UT64_MAX) {
+			// Do some anal looping.
+			for (i = 0; i < numinstrs; ++i) {
+				*prev_addr = prevop_addr (core, start_addr);
+				start_addr = *prev_addr;
+			}
+			return true;
+		}
+	}
+	// Dang! not in a bb, return false and fallback to other methods.
+	*prev_addr = UT64_MAX;
+	return false;
+}
+
 static void reset_print_cur(RPrint *p) {
 	p->cur = 0;
 	p->ocur = -1;

libr/include/r_core.h

@@ -227,6 +227,7 @@ R_API int r_core_read_at(RCore *core, ut64 addr, ut8 *buf, int size);
 R_API int r_core_is_valid_offset (RCore *core, ut64 offset);
 R_API int r_core_shift_block(RCore *core, ut64 addr, ut64 b_size, st64 dist);
 R_API void r_core_visual_prompt_input (RCore *core);
+R_API bool r_core_prevop_addr (RCore* core, ut64 start_addr, int numinstrs, ut64* prev_addr);
 R_API bool r_core_visual_hudstuff(RCore *core);
 R_API int r_core_visual_classes(RCore *core);
 R_API int r_core_visual_types(RCore *core);