[elfabi] Fix a bug when elfabi is supplied with a tbe file contains no non-local symbol.

[haowei]

Summary

Before this patch, elfabi wrote 0 to sh_info of the .dynsym section when it is supplied with a tbe file contains no non-local symbol, making the ELF stub file invalid. This patch fixed this issue.

[MaskRay]

an empty symbol table

It is not empty. It contains exactly one entry, a local symbol with index 0.

llvm/lib/InterfaceStub/ELFObjHandler.cpp

306

// The number of local symbols, which is exactly 1 (the undefined symbol at index 0)

[haowei]

In D96930#2570520, @MaskRay wrote:

an empty symbol table

It is not empty. It contains exactly one entry, a local symbol with index 0.

What I mean is that the Symbol section in the yaml tbe file is empty. But I think it is confusing to use the term empty symbol table here. I changed the description to make it more precise.

[phosek]

LGTM

llvm/lib/InterfaceStub/ELFObjHandler.cpp

void fillStrTabShdr(ContentSection<ELFStringTableBuilder, ELFT> &StrTab,
     StrTab.Shdr.sh_link = 0;
   }
   void fillSymTabShdr(ContentSection<ELFSymbolTableBuilder<ELFT>, ELFT> &SymTab,
                       uint32_t ShType) const {
     SymTab.Shdr.sh_type = ShType;
     SymTab.Shdr.sh_flags = SHF_ALLOC;
     SymTab.Shdr.sh_addr = SymTab.Addr;
     SymTab.Shdr.sh_offset = SymTab.Offset;
-    SymTab.Shdr.sh_info = SymTab.Size / sizeof(Elf_Sym) > 1 ? 1 : 0;
+    // Only non-local symbols are included, so sh_info should be always 1.
+    SymTab.Shdr.sh_info = 1;
     SymTab.Shdr.sh_size = SymTab.Size;
     SymTab.Shdr.sh_name = this->ShStrTab.Content.getOffset(SymTab.Name);
     SymTab.Shdr.sh_addralign = SymTab.Align;
     SymTab.Shdr.sh_entsize = sizeof(Elf_Sym);
     SymTab.Shdr.sh_link = this->DynStr.Index;
   }
   void fillDynTabShdr(
       ContentSection<ELFDynamicTableBuilder<ELFT>, ELFT> &DynTab) const {

llvm/test/tools/llvm-elfabi/write-stub-empty-dynsym.test

+## Test writing stub elf when symbol table is empty.
+
+# RUN: llvm-elfabi %s --output-target=elf64-little %t
+# RUN: llvm-readobj -S %t | FileCheck %s -DCLASS="64-bit (0x2)" -DDE="LittleEndian (0x1)" -DHS=64 -DPHES=56 -DSHES=64 -DDYNSYMAL=8 -DDYNSYMES=24 -DDYNAMICAL=8 -DDYNAMICES=16 -DDYNTABZ=000000000
+
+--- !tapi-tbe
+TbeVersion: 1.0
+Arch: x86_64
+NeededLibs:
+  - libc.so.6
+Symbols: {}
+...
+
+# CHECK:      Section {
+# CHECK-NEXT:   Index: 0
+# CHECK-NEXT:   Name:  (0)
+# CHECK-NEXT:   Type: SHT_NULL
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Address: 0x0
+# CHECK-NEXT:   Offset: 0x0
+# CHECK-NEXT:   Size: 0
+# CHECK-NEXT:   Link: 0
+# CHECK-NEXT:   Info: 0
+# CHECK-NEXT:   AddressAlignment: 0
+# CHECK-NEXT:   EntrySize: 0
+# CHECK-NEXT: }
+# CHECK-NEXT: Section {
+# CHECK-NEXT:   Index: 1
+# CHECK-NEXT:   Name: .dynsym
+# CHECK-NEXT:   Type: SHT_DYNSYM
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     SHF_ALLOC
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Address:
+# CHECK-NEXT:   Offset:
+# CHECK-NEXT:   Size:
+# CHECK-NEXT:   Link: 2
+# CHECK-NEXT:   Info: 1
+# CHECK-NEXT:   AddressAlignment: [[DYNSYMAL]]
+# CHECK-NEXT:   EntrySize: [[DYNSYMES]]
+# CHECK-NEXT: }
+# CHECK-NEXT: Section {
+# CHECK-NEXT:   Index: 2
+# CHECK-NEXT:   Name: .dynstr
+# CHECK-NEXT:   Type: SHT_STRTAB
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     SHF_ALLOC
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Address:
+# CHECK-NEXT:   Offset:
+# CHECK-NEXT:   Size:
+# CHECK-NEXT:   Link: 0
+# CHECK-NEXT:   Info: 0
+# CHECK-NEXT:   AddressAlignment: 1
+# CHECK-NEXT:   EntrySize: 0
+# CHECK-NEXT: }
+# CHECK-NEXT: Section {
+# CHECK-NEXT:   Index: 3
+# CHECK-NEXT:   Name: .dynamic
+# CHECK-NEXT:   Type: SHT_DYNAMIC
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     SHF_ALLOC
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Address:
+# CHECK-NEXT:   Offset:
+# CHECK-NEXT:   Size:
+# CHECK-NEXT:   Link: 2
+# CHECK-NEXT:   Info: 0
+# CHECK-NEXT:   AddressAlignment: [[DYNAMICAL]]
+# CHECK-NEXT:   EntrySize: [[DYNAMICES]]
+# CHECK-NEXT: }
+# CHECK-NEXT: Section {
+# CHECK-NEXT:   Index: 4
+# CHECK-NEXT:   Name: .shstrtab
+# CHECK-NEXT:   Type: SHT_STRTAB
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Address:
+# CHECK-NEXT:   Offset:
+# CHECK-NEXT:   Size:
+# CHECK-NEXT:   Link: 0
+# CHECK-NEXT:   Info: 0
+# CHECK-NEXT:   AddressAlignment: 1
+# CHECK-NEXT:   EntrySize: 0
+# CHECK-NEXT: }