[llvm] [Support] Fix UB in decodeULEB128 and decodeSLEB128 (NFC) (PR #114986)

[Krzysztof Pszeniczny via llvm-commits]

https://github.com/amharc created https://github.com/llvm/llvm-project/pull/114986

It currently might try to shift `Slice << Shift` for `Slice == 0` and `Shift > 63`. The behaviour is undefined in this case per [[expr.shift]/1](https://eel.is/c++draft/expr.shift#1).

>From 00769208fdae2f97177a6d2a2e68f88fc1693e97 Mon Sep 17 00:00:00 2001
From: Krzysztof Pszeniczny <kpszeniczny at google.com>
Date: Tue, 5 Nov 2024 14:43:07 +0100
Subject: [PATCH] [Support] Fix UB in decodeULEB128 and decodeSLEB128 (NFC)

It currently might try to shift `Slice << Shift` for `Slice == 0` and
`Shift > 63`. The behaviour is undefined in this case per
[expr.shift]/1.
---
 llvm/include/llvm/Support/LEB128.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/llvm/include/llvm/Support/LEB128.h b/llvm/include/llvm/Support/LEB128.h
index a15b73bc14dcde..19d113fec05d7d 100644
--- a/llvm/include/llvm/Support/LEB128.h
+++ b/llvm/include/llvm/Support/LEB128.h
@@ -150,7 +150,10 @@ inline uint64_t decodeULEB128(const uint8_t *p, unsigned *n = nullptr,
       Value = 0;
       break;
     }
-    Value += Slice << Shift;
+    // We might still have Shift > 63 && Slice == 0 here, so we need to protect
+    // against UB in <<.
+    if (LLVM_LIKELY(Shift <= 63))
+      Value += Slice << Shift;
     Shift += 7;
   } while (*p++ >= 128);
   if (n)
@@ -188,7 +191,10 @@ inline int64_t decodeSLEB128(const uint8_t *p, unsigned *n = nullptr,
         *n = (unsigned)(p - orig_p);
       return 0;
     }
-    Value |= Slice << Shift;
+    // We might still have Shift > 63 here, so we need to protect against UB
+    // in <<.
+    if (LLVM_LIKELY(Shift <= 63))
+      Value |= Slice << Shift;
     Shift += 7;
     ++p;
   } while (Byte >= 128);