Bug 2002024

Description of problem:

See the bpf mailing list discussion here:
https://lore.kernel.org/bpf/095f116b-7399-25a5-dca2-145cbd093326@fb.com/T/#mf02b028b693bb45551faa6df99092ca3d2b4bec6

fedora packaged clang has the following private patch which sets default unwindtalbe true for all architectures:
https://git.centos.org/rpms/clang/blob/b99d8d4a38320329e10570f308c3e2d8cf295c78/f/SOURCES/0002-PATCH-clang-Make-funwind-tables-the-default-on-all-a.patch

But as discussed in the bpf mailing list, this cause annoying user experience for bpf developers. Without the above private patch, everything works fine and no annoying .eh_frame section for bpf programs.

So I proposed to amend the above patch to set default unwindtable to false to bpf targets. The following is the new patch I proposed:

[$ ~/work/llvm-project/clang/lib/Driver] git diff
diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index b2ddef141a75..6c074c4dc401 100644 
--- a/clang/lib/Driver/ToolChain.cpp    
+++ b/clang/lib/Driver/ToolChain.cpp    
@@ -257,7 +257,9 @@ std::string ToolChain::getInputFilename(const InputInfo &Input) const {
 }
 
 bool ToolChain::IsUnwindTablesDefault(const ArgList &Args) const {
-  return false;
+  if (getArch() == llvm::Triple::bpfel || getArch() == llvm::Triple::bpfeb)
+    return false;              
+  return true; 
 }
 
 Tool *ToolChain::getClang() const {
diff --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
index 1d8a3cdce92a..cbf6bc330936 100644
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -2705,6 +2705,9 @@ void Generic_GCC::printVerboseInfo(raw_ostream &OS) const {
 
 bool Generic_GCC::IsUnwindTablesDefault(const ArgList &Args) const {
   switch (getArch()) {
+  case llvm::Triple::bpfel:
+  case llvm::Triple::bpfeb:
+    return false;
   case llvm::Triple::aarch64:
   case llvm::Triple::ppc:
   case llvm::Triple::ppcle:
@@ -2713,7 +2716,7 @@ bool Generic_GCC::IsUnwindTablesDefault(const ArgList &Args) const {
   case llvm::Triple::x86_64:
     return true;
   default:
-    return false;
+    return true;
   }
 }



Version-Release number of selected component (if applicable):


How reproducible:
[$ ~/work/tests/llvm/eh_frame] cat t.c
int test() { return 0; }
[$ ~/work/tests/llvm/eh_frame] clang -target bpf -O2 -c t.c
[$ ~/work/tests/llvm/eh_frame] llvm-readelf -S t.o | grep eh_frame
  [ 3] .eh_frame         PROGBITS        0000000000000000 000050 000030 00   A  0   0  8
  [ 4] .rel.eh_frame     REL             0000000000000000 0000e0 000010 10      6   3  8

Steps to Reproduce:
1. compile t.c with clang target bpf to generate ELF file
2. check ELF section list for .eh_frame
3.

Actual results:
[$ ~/work/tests/llvm/eh_frame] llvm-readelf -S t.o | grep eh_frame
  [ 3] .eh_frame         PROGBITS        0000000000000000 000050 000030 00   A  0   0  8
  [ 4] .rel.eh_frame     REL             0000000000000000 0000e0 000010 10      6   3  8

Expected results:
[$ ~/work/tests/llvm/eh_frame] llvm-readelf -S t.o | grep eh_frame
[$ ~/work/tests/llvm/eh_frame]

Additional info: