[lld] [lld][ARM] Fix assertion when mixing ARM and Thumb objects (PR #101985)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-elf

Author: Oliver Stannard (ostannard)

<details>
<summary>Changes</summary>

Previously, we selected the Thumb2 PLT sequences if any input object is marked as not supporting the ARM ISA, which then causes assertion failures when calls from ARM code in other objects are seen. I think the intention here was to only use Thumb PLTs when the target does not have the ARM ISA available, signalled by no objects being marked as having it available. To do that we need to track which ISAs we have seen as we parse the build attributes, and defer the decision  about PLTs until all input objects have been parsed.

This bug was triggered by real code in picolibc, which have some versions of string.h functions built with Thumb2-only build attributes, so that they are compatible with v7-A, v7-R and v7-M.

Fixes #<!-- -->99008.

---
Full diff: https://github.com/llvm/llvm-project/pull/101985.diff


4 Files Affected:

- (modified) lld/ELF/Config.h (+2) 
- (modified) lld/ELF/Driver.cpp (+11) 
- (modified) lld/ELF/InputFiles.cpp (+2-4) 
- (added) lld/test/ELF/arm-mixed-plts.s (+44) 


``````````diff
diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 183dc88a93e2f..d3ea095940cd8 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -218,6 +218,8 @@ struct Config {
   bool allowMultipleDefinition;
   bool fatLTOObjects;
   bool androidPackDynRelocs = false;
+  bool armHasArmISA = false;
+  bool armHasThumb2ISA = false;
   bool armThumbPLTs = false;
   bool armHasBlx = false;
   bool armHasMovtMovw = false;
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index a8c52e8c2b8e1..1928648a7c115 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -82,6 +82,7 @@ ConfigWrapper elf::config;
 Ctx elf::ctx;
 
 static void setConfigs(opt::InputArgList &args);
+static void setConfigsPostParse();
 static void readConfigs(opt::InputArgList &args);
 
 void elf::errorOrWarn(const Twine &msg) {
@@ -1886,6 +1887,14 @@ static void setConfigs(opt::InputArgList &args) {
   }
 }
 
+// Initialise config options which depend on information previously parsed from
+// input files, such as Arm build attributes.
+static void setConfigsPostParse() {
+  // Currently, Thumb PLTs require Thumb2, and are only used for targets which
+  // do not have the ARM ISA.
+  config->armThumbPLTs = config->armHasThumb2ISA && !config->armHasArmISA;
+}
+
 static bool isFormatBinary(StringRef s) {
   if (s == "binary")
     return true;
@@ -2897,6 +2906,8 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
 
   // No more lazy bitcode can be extracted at this point. Do post parse work
   // like checking duplicate symbols.
+  setConfigsPostParse();
+
   parallelForEach(ctx.objectFiles, [](ELFFileBase *file) {
     initSectionsAndLocalSyms(file, /*ignoreComdats=*/false);
   });
diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 3bc39848d6035..b87a3ab429219 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -200,10 +200,8 @@ static void updateSupportedARMFeatures(const ARMAttributeParser &attributes) {
       attributes.getAttributeValue(ARMBuildAttrs::ARM_ISA_use);
   std::optional<unsigned> thumb =
       attributes.getAttributeValue(ARMBuildAttrs::THUMB_ISA_use);
-  bool noArmISA = !armISA || *armISA == ARMBuildAttrs::Not_Allowed;
-  bool hasThumb2 = thumb && *thumb >= ARMBuildAttrs::AllowThumb32;
-  if (noArmISA && hasThumb2)
-    config->armThumbPLTs = true;
+  config->armHasArmISA |= armISA && *armISA >= ARMBuildAttrs::Allowed;
+  config->armHasThumb2ISA |= thumb && *thumb >= ARMBuildAttrs::AllowThumb32;
 }
 
 InputFile::InputFile(Kind k, MemoryBufferRef m)
diff --git a/lld/test/ELF/arm-mixed-plts.s b/lld/test/ELF/arm-mixed-plts.s
new file mode 100644
index 0000000000000..5699371449532
--- /dev/null
+++ b/lld/test/ELF/arm-mixed-plts.s
@@ -0,0 +1,44 @@
+# REQUIRES: arm
+
+# RUN: rm -rf %t && split-file %s %t
+# RUN: llvm-mc -filetype=obj -arm-add-build-attributes -triple=armv7a-none-linux-gnueabi %t/a.s -o %t1.o
+# RUN: llvm-mc -filetype=obj -arm-add-build-attributes -triple=armv7a-none-linux-gnueabi %t/b.s -o %t2.o
+# RUN: ld.lld -shared %t1.o %t2.o -o %t.so
+# RUN: llvm-objdump -d %t.so | FileCheck %s
+
+# Check that, when the input is a mixture of objects which can and cannot use
+# the ARM ISA, we use the default ARM PLT sequences.
+
+# CHECK:      000101d0 <.plt>:
+# CHECK-NEXT: 101d0: e52de004      str     lr, [sp, #-0x4]!
+# CHECK-NEXT: 101d4: e28fe600      add     lr, pc, #0, #12
+# CHECK-NEXT: 101d8: e28eea20      add     lr, lr, #32, #20
+# CHECK-NEXT: 101dc: e5bef084      ldr     pc, [lr, #0x84]!
+# CHECK-NEXT: 101e0: d4 d4 d4 d4   .word   0xd4d4d4d4
+# CHECK-NEXT: 101e4: d4 d4 d4 d4   .word   0xd4d4d4d4
+# CHECK-NEXT: 101e8: d4 d4 d4 d4   .word   0xd4d4d4d4
+# CHECK-NEXT: 101ec: d4 d4 d4 d4   .word   0xd4d4d4d4
+# CHECK-NEXT: 101f0: e28fc600      add     r12, pc, #0, #12
+# CHECK-NEXT: 101f4: e28cca20      add     r12, r12, #32, #20
+# CHECK-NEXT: 101f8: e5bcf06c      ldr     pc, [r12, #0x6c]!
+# CHECK-NEXT: 101fc: d4 d4 d4 d4   .word   0xd4d4d4d4
+
+#--- a.s
+  .globl foo
+  .type foo, %function
+  .globl bar
+  .type bar, %function
+
+  .thumb
+foo:
+  bl bar
+  bx lr
+
+#--- b.s
+  .eabi_attribute Tag_ARM_ISA_use, 0
+
+  .arm
+  .globl bar
+  .type bar, %function
+bar:
+  bx lr

``````````

</details>


https://github.com/llvm/llvm-project/pull/101985