[compiler-rt] [scudo] Avoid splitting aligned allocations on Trusty (PR #69281)

[Andrei Homescu via llvm-commits]

================
@@ -122,18 +122,35 @@ bool mapSecondary(const Options &Options, uptr CommitBase, uptr CommitSize,
   Flags |= MAP_RESIZABLE;
   Flags |= MAP_ALLOWNOMEM;
 
-  const uptr MaxUnusedCacheBytes = MaxUnusedCachePages * getPageSizeCached();
+  const uptr PageSize = getPageSizeCached();
+  const uptr MaxUnusedCacheBytes = MaxUnusedCachePages * PageSize;
   if (useMemoryTagging<Config>(Options) && CommitSize > MaxUnusedCacheBytes) {
-    const uptr UntaggedPos = Max(AllocPos, CommitBase + MaxUnusedCacheBytes);
-    return MemMap.remap(CommitBase, UntaggedPos - CommitBase, "scudo:secondary",
-                        MAP_MEMTAG | Flags) &&
-           MemMap.remap(UntaggedPos, CommitBase + CommitSize - UntaggedPos,
-                        "scudo:secondary", Flags);
-  } else {
-    const uptr RemapFlags =
-        (useMemoryTagging<Config>(Options) ? MAP_MEMTAG : 0) | Flags;
-    return MemMap.remap(CommitBase, CommitSize, "scudo:secondary", RemapFlags);
+    if (SCUDO_TRUSTY) {
+      /*
+       * On Trusty we need AllocPos to be usable for memrefs, which cannot
+       * cross multiple mappings. This means we need to split around AllocPos
+       * and not over it. We can only do this if the address is page-aligned.
+       */
+      const uptr TaggedSize = AllocPos - CommitBase;
+      if (TaggedSize != 0 && isAligned(TaggedSize, PageSize)) {
----------------
ahomescu wrote:

`mapSecondary` is called with `AllocPos == CommitBase` on line 215:
```
        mapSecondary<Config>(Options, Entry.CommitBase, Entry.CommitSize,
                             Entry.CommitBase, MAP_NOACCESS, Entry.MemMap);
```
but I'm wondering if that code path is every used on Trusty. It's also disabled explicitly on Fuchsia, wondering if Trusty should do that too.

https://github.com/llvm/llvm-project/pull/69281