fix decoding of rex-prefixed modrm+sib operands selecting index 0b100 and base 0b1010.1.4

for memory operands with a base, index, and displacement either
the wrong base would be selected (register number ignored, so only
`*ax` or `r8*` would be reported), or yaxpeax-x86 would report a
base register is present when it is not (`RegIndexBaseScaleDisp`
when the operand is actually `RegScaleDisp`)

thank you to Evan Johnson for catching and reporting this bug!

also bump crate version to 0.1.4 as this will be immediately tagged and
released.

5 files changed, 68 insertions, 5 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 5f0b9f7..8de4801 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,13 @@
+## 0.1.4
+* [long mode only]: fix decoding of rex-prefixed modrm+sib operands selecting index 0b100 and base 0b101
+  - for memory operands with a base, index, and displacement either
+    the wrong base would be selected (register number ignored, so only
+    `*ax` or `r8*` would be reported), or yaxpeax-x86 would report a
+    base register is present when it is not (`RegIndexBaseScaleDisp`
+    when the operand is actually `RegScaleDisp`)
+
+thank you to Evan Johnson for catching and reporting this bug!
+
 ## 0.1.3
 * fix 0x80-opcode instructions not having an opcode
   - this meant that for example `lock xorb [rax], 0` would decode as invalid
diff --git a/Cargo.toml b/Cargo.toml
index c6da196..dda7c14 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,7 +1,7 @@
 [package]
 
 name = "yaxpeax-x86"
-version = "0.1.3"
+version = "0.1.4"
 authors = [ "iximeow <me@iximeow.net>" ]
 license = "0BSD"
 repository = "http://git.iximeow.net/yaxpeax-x86/"
diff --git a/src/long_mode/mod.rs b/src/long_mode/mod.rs
index 363c605..d165e38 100644
--- a/src/long_mode/mod.rs
+++ b/src/long_mode/mod.rs
@@ -5665,20 +5665,28 @@ fn read_sib<T: Iterator<Item=u8>>(bytes_iter: &mut T, instr: &mut Instruction, m
 
                 OperandSpec::DispU32
             } else {
+                instr.modrm_mmm.num |= 0b101;
+
                 if instr.prefixes.rex().x() {
                     instr.sib_index.num = 0b1100;
                     let scale = 1u8 << (sibbyte >> 6);
                     instr.scale = scale;
 
                     if disp == 0 {
-                        OperandSpec::RegIndexBaseScale
+                        if modbits == 0 {
+                            OperandSpec::RegScale
+                        } else {
+                            OperandSpec::RegIndexBaseScale
+                        }
                     } else {
                         instr.disp = disp as i64 as u64;
-                        OperandSpec::RegIndexBaseScaleDisp
+                        if modbits == 0 {
+                            OperandSpec::RegScaleDisp
+                        } else {
+                            OperandSpec::RegIndexBaseScaleDisp
+                        }
                     }
                 } else {
-                    instr.modrm_mmm.num |= 0b101;
-
                     if disp == 0 {
                         OperandSpec::Deref
                     } else {
diff --git a/test/long_mode/mod.rs b/test/long_mode/mod.rs
index 3b31dbb..8489822 100644
--- a/test/long_mode/mod.rs
+++ b/test/long_mode/mod.rs
@@ -109,6 +109,17 @@ fn test_modrm_decode() {
     test_display(&[0x41, 0x33, 0x84, 0xa5, 0x11, 0x22, 0x33, 0x44], "xor eax, [r13 + 0x44332211]");
     test_display(&[0x33, 0x04, 0xe5, 0x11, 0x22, 0x33, 0x44], "xor eax, [0x44332211]");
     test_display(&[0x41, 0x33, 0x04, 0xe5, 0x11, 0x22, 0x33, 0x44], "xor eax, [0x44332211]");
+
+    // specifically sib with base == 0b101
+    // mod bits 00
+    test_display(&[0x42, 0x33, 0x34, 0x25, 0x20, 0x30, 0x40, 0x50], "xor esi, [r12 * 1 + 0x50403020]");
+    test_display(&[0x43, 0x33, 0x34, 0x25, 0x20, 0x30, 0x40, 0x50], "xor esi, [r12 * 1 + 0x50403020]");
+    // mod bits 01
+    test_display(&[0x42, 0x33, 0x74, 0x25, 0x20], "xor esi, [rbp + r12 * 1 + 0x20]");
+    test_display(&[0x43, 0x33, 0x74, 0x25, 0x20], "xor esi, [r13 + r12 * 1 + 0x20]");
+    // mod bits 10
+    test_display(&[0x42, 0x33, 0xb4, 0x25, 0x20, 0x30, 0x40, 0x50], "xor esi, [rbp + r12 * 1 + 0x50403020]");
+    test_display(&[0x43, 0x33, 0xb4, 0x25, 0x20, 0x30, 0x40, 0x50], "xor esi, [r13 + r12 * 1 + 0x50403020]");
 }
 
 #[test]
diff --git a/test/protected_mode/mod.rs b/test/protected_mode/mod.rs
index 84448ef..dd0a51f 100644
--- a/test/protected_mode/mod.rs
+++ b/test/protected_mode/mod.rs
@@ -50,6 +50,40 @@ fn test_display_under(decoder: &InstDecoder, data: &[u8], expected: &'static str
 }
 
 #[test]
+fn test_modrm_decode() {
+    // just modrm
+    test_display(&[0x33, 0x08], "xor ecx, [eax]");
+    test_display(&[0x33, 0x20], "xor esp, [eax]");
+    test_display(&[0x33, 0x05, 0x78, 0x56, 0x34, 0x12], "xor eax, [0x12345678]");
+    test_display(&[0x33, 0x41, 0x23], "xor eax, [ecx + 0x23]");
+    test_display(&[0x33, 0x81, 0x23, 0x01, 0x65, 0x43], "xor eax, [ecx + 0x43650123]");
+    test_display(&[0x33, 0xc1], "xor eax, ecx");
+
+    // sib
+    test_display(&[0x33, 0x04, 0x0a], "xor eax, [edx + ecx * 1]");
+    test_display(&[0x33, 0x04, 0x4a], "xor eax, [edx + ecx * 2]");
+    test_display(&[0x33, 0x04, 0x8a], "xor eax, [edx + ecx * 4]");
+    test_display(&[0x33, 0x04, 0xca], "xor eax, [edx + ecx * 8]");
+    test_display(&[0x33, 0x04, 0x20], "xor eax, [eax]");
+    test_display(&[0x33, 0x04, 0x60], "xor eax, [eax]");
+    test_display(&[0x33, 0x04, 0xa0], "xor eax, [eax]");
+    test_display(&[0x33, 0x04, 0xe0], "xor eax, [eax]");
+    test_display(&[0x33, 0x04, 0x25, 0x11, 0x22, 0x33, 0x44], "xor eax, [0x44332211]");
+
+    test_display(&[0x33, 0x44, 0x65, 0x11], "xor eax, [ebp + 0x11]");
+    test_display(&[0x33, 0x84, 0xa5, 0x11, 0x22, 0x33, 0x44], "xor eax, [ebp + 0x44332211]");
+    test_display(&[0x33, 0x04, 0xe5, 0x11, 0x22, 0x33, 0x44], "xor eax, [0x44332211]");
+
+    // specifically sib with base == 0b101
+    // mod bits 00
+    test_display(&[0x33, 0x34, 0x25, 0x20, 0x30, 0x40, 0x50], "xor esi, [0x50403020]");
+    // mod bits 01
+    test_display(&[0x33, 0x74, 0x25, 0x20], "xor esi, [ebp + 0x20]");
+    // mod bits 10
+    test_display(&[0x33, 0xb4, 0x25, 0x20, 0x30, 0x40, 0x50], "xor esi, [ebp + 0x50403020]");
+}
+
+#[test]
 fn test_mmx() {
     test_display(&[0x0f, 0xf7, 0xc1], "maskmovq mm0, mm1");
     test_invalid(&[0x0f, 0xf7, 0x01]);