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

[Andrei Homescu via llvm-commits]

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@@ -122,7 +122,28 @@ bool mapSecondary(const Options &Options, uptr CommitBase, uptr CommitSize,
   Flags |= MAP_RESIZABLE;
   Flags |= MAP_ALLOWNOMEM;
 
-  const uptr MaxUnusedCacheBytes = MaxUnusedCachePages * getPageSizeCached();
+  const uptr PageSize = getPageSizeCached();
+  if (SCUDO_TRUSTY) {
+    /*
+     * On Trusty we need AllocPos to be usable for shared memory, 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 (useMemoryTagging<Config>(Options) && isAligned(TaggedSize, PageSize)) {
+      return MemMap.remap(CommitBase, TaggedSize, "scudo:secondary",
+                          MAP_MEMTAG | Flags) &&
+             MemMap.remap(AllocPos, CommitSize - TaggedSize, "scudo:secondary",
+                          Flags);
+    } else {
+      const uptr RemapFlags =
+          (useMemoryTagging<Config>(Options) ? MAP_MEMTAG : 0) | Flags;
+      return MemMap.remap(CommitBase, CommitSize, "scudo:secondary",
+                          RemapFlags);
+    }
+  }
----------------
ahomescu wrote:

> If you mean that Trusty can request more than 4 pages and we should still apply the original optimization there, I don't know exactly how that would work. On Trusty, we can't split the mapping past AllocPos.
>
> Maybe we could make this configurable at either build or run time?

Copied my response here to keep the entire discussion going in the same place.

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