[BOLT][AArch64] Detect veneers with missing data markers

[maksfb]

The linker may omit data markers for long absolute veneers causing BOLT to treat data as code. Detect such veneers and introduce data markers artificially before BOLT's disassembler kicks in.

[llvmbot]

@llvm/pr-subscribers-bolt

Author: Maksim Panchenko (maksfb)

Changes

The linker may omit data markers for long absolute veneers causing BOLT to treat data as code. Detect such veneers and introduce data markers artificially before BOLT's disassembler kicks in.

---

Full diff: https://github.com/llvm/llvm-project/pull/142069.diff

2 Files Affected:

• (modified) bolt/lib/Rewrite/RewriteInstance.cpp (+13)
• (modified) bolt/test/AArch64/veneer-lld-abs.s (+12-1)

diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index ad062ea3622d1..69c39cf5e5bac 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -1341,6 +1341,19 @@ void RewriteInstance::discoverFileObjects() {
         }
       }
     }
+
+    // The linker may omit data markers for absolute long veneers. Introduce
+    // those markers artificially to assist the disassembler.
+    for (BinaryFunction &BF :
+         llvm::make_second_range(BC->getBinaryFunctions())) {
+      if (BF.getOneName().starts_with("__AArch64AbsLongThunk_") &&
+          BF.getSize() == 16 && !BF.getSizeOfDataInCodeAt(8)) {
+        BC->errs() << "BOLT-WARNING: missing data marker detected in veneer "
+                   << BF << '\n';
+        BF.markDataAtOffset(8);
+        BC->AddressToConstantIslandMap[BF.getAddress() + 8] = &BF;
+      }
+    }
   }
 
   if (!BC->IsLinuxKernel) {
diff --git a/bolt/test/AArch64/veneer-lld-abs.s b/bolt/test/AArch64/veneer-lld-abs.s
index 7e6fe2d127060..b22301db66c54 100644
--- a/bolt/test/AArch64/veneer-lld-abs.s
+++ b/bolt/test/AArch64/veneer-lld-abs.s
@@ -6,10 +6,21 @@
 # RUN:    -fuse-ld=lld -Wl,-q
 # RUN: llvm-objdump -d %t.exe | FileCheck --check-prefix=CHECK-INPUT %s
 # RUN: llvm-objcopy --remove-section .rela.mytext %t.exe
-# RUN: llvm-bolt %t.exe -o %t.bolt --elim-link-veneers=true --lite=0
+# RUN: llvm-bolt %t.exe -o %t.bolt
 # RUN: llvm-objdump -d -j .text  %t.bolt | \
 # RUN:   FileCheck --check-prefix=CHECK-OUTPUT %s
 
+## Occasionally, we see the linker not generating $d symbols for long veneers
+## causing BOLT to fail veneer elimination.
+# RUN: llvm-objcopy --remove-symbol-prefix=\$d %t.exe %t.no-marker.exe
+# RUN: llvm-bolt %t.no-marker.exe -o %t.no-marker.bolt \
+# RUN:   2>&1 | FileCheck %s --check-prefix=CHECK-BOLT
+# RUN: llvm-objdump -d -j .text  %t.no-marker.bolt | \
+# RUN:   FileCheck --check-prefix=CHECK-OUTPUT %s
+
+# CHECK-BOLT-NOT: BOLT-WARNING: unable to disassemble instruction
+# CHECK-BOLT: BOLT-WARNING: missing data marker detected in veneer __AArch64AbsLongThunk_far_function
+
 .text
 .balign 4
 .global far_function

[maksfb]

@MaskRay, any idea why LLD will sometimes omit $d for long absolute thunks?

[aaupov]

LG

[aaupov]

Just curious where does the name come from? Is it in the symbol table, or a synthetic name created by us during disassembly?

[maksfb]

From the linker, i.e. the symbol table.

[aaupov]

So the linker knows these are veneers, and just omits data symbols even if they're mandated by ABI.

[maksfb]

The linker creates them, so - yes.

[efriedma-quic]

You might want to file an lld bug if it's dropping markers?

[MaskRay]

@MaskRay, any idea why LLD will sometimes omit $d for long absolute thunks?

This should not happen.... Was there a binary manipulation that stripped the mapping symbols?

[maksfb]

@MaskRay, any idea why LLD will sometimes omit $d for long absolute thunks?

This should not happen.... Was there a binary manipulation that stripped the mapping symbols?

No... This only happens for a few (4) thunks out of > 15K. As you can imagine, it's hard to repro on open-source binaries.

[maksfb]

You might want to file an lld bug if it's dropping markers?

#142326