[ELF][PPC32] Make R_PPC32_PLTREL retain .got

[kernigh]

Summary

PLT usage needs the first 12 bytes of the .got section. We need to keep .got and
DT_GOT_PPC even if .got/_GLOBAL_OFFSET_TABLE_ are not referenced (large PIC code
may only reference .got2), which is the case in OpenBSD's ld.so, leading
to a misleading error, "unsupported insecure BSS PLT object".

Fix this by adding R_PPC32_PLTREL to the list of hasGotOffRel.

[kernigh]

I don't have commit access to LLVM.

You might wonder why this diff adds R_PPC32_PLTREL but not R_PPC64_CALL_PLT. My problem (where .got is missing) happened only in 32-bit code. Code that calls the plt must set register r30 in 32-bit code, or r2 in 64-bit. Most 64-bit code sets r2 = .TOC. = address of .got; LLD might keep .got because it sees this .TOC. reference. 32-bit -fPIC code can set r30 = somewhere in .got2, so there is no reference to .got, and LLD can discard .got unless we add R_PPC32_PLTREL to the list.

[MaskRay]

Fix tests. Simplify new test.

[MaskRay]

In D114982#3167885, @kernigh wrote:

I don't have commit access to LLVM.

You might wonder why this diff adds R_PPC32_PLTREL but not R_PPC64_CALL_PLT. My problem (where .got is missing) happened only in 32-bit code. Code that calls the plt must set register r30 in 32-bit code, or r2 in 64-bit. Most 64-bit code sets r2 = .TOC. = address of .got; LLD might keep .got because it sees this .TOC. reference. 32-bit -fPIC code can set r30 = somewhere in .got2, so there is no reference to .got, and LLD can discard .got unless we add R_PPC32_PLTREL to the list.

Seems that OpenBSD ld.so doesnot reference .got / _GLOBAL_OFFSET_TABLE_ at all. Other ld.so implementations all reference .got/_GLOBAL_OFFSET_TABLE_ somehow.

[MaskRay]

LGTM.

lld/ELF/Relocations.cpp

static void scanReloc(InputSectionBase &sec, OffsetGetter &getOffset, RelTy *&i,
 
   // If the relocation does not emit a GOT or GOTPLT entry but its computation
   // uses their addresses, we need GOT or GOTPLT to be created.
   //
   // The 5 types that relative GOTPLT are all x86 and x86-64 specific.
   if (oneof<R_GOTPLTONLY_PC, R_GOTPLTREL, R_GOTPLT, R_PLT_GOTPLT,
             R_TLSDESC_GOTPLT, R_TLSGD_GOTPLT>(expr)) {
     in.gotPlt->hasGotPltOffRel = true;
-  } else if (oneof<R_GOTONLY_PC, R_GOTREL, R_PPC64_TOCBASE, R_PPC64_RELAX_TOC>(
-                 expr)) {
+  } else if (oneof<R_GOTONLY_PC, R_GOTREL, R_PPC32_PLTREL, R_PPC64_TOCBASE,
+                   R_PPC64_RELAX_TOC>(expr)) {
     in.got->hasGotOffRel = true;
   }
 
   // Process TLS relocations, including relaxing TLS relocations. Note that
   // R_TPREL and R_TPREL_NEG relocations are resolved in processRelocAux.
   if (expr == R_TPREL || expr == R_TPREL_NEG) {
     if (config->shared) {
       errorOrWarn("relocation " + toString(type) + " against " + toString(sym) +

lld/test/ELF/ppc32-plt-got.s

+# REQUIRES: ppc
+
+## Ensure that, if we call a function through the plt, then the linker
+## isn't discarding the .got section.
+
+# RUN: llvm-mc -filetype=obj -triple=powerpc %s -o %t.o
+# RUN: ld.lld -shared %t.o -o %t.so
+# RUN: llvm-readobj -Sdr %t.so | FileCheck %s
+
+## This -fPIC code sets r30 and calls the plt, but has no .got entries
+## and never references _GLOBAL_OFFSET_TABLE_.  The linker must not
+## discard .got, only because the plt needs the first 12 bytes of .got.
+
+.section .got2,"aw",@progbits
+.set .LTOC, .+0x8000
+
+.text
+.L0:
+addis 30,30,.LTOC-.L0@ha
+addi 30,30,.LTOC-.L0@l
+bl baz+0x8000@plt
+
+## DT_PPC_GOT must point to .got, which must have the 12-byte header.
+## The only relocation must be R_PPC_JMP_SLOT.
+
+# CHECK:      Sections [
+# CHECK:        Name: .got ({{[0-9]+}})
+# CHECK:        Address:
+# CHECK-SAME:   {{ }}[[GOT_ADDRESS:0x[0-9A-F]+]]{{$}}
+# CHECK:        Size:
+# CHECK-SAME:   {{ 12$}}
+# CHECK:      DynamicSection [
+# CHECK-NEXT:   Tag        Type     Name/Value
+# CHECK:        0x70000000 PPC_GOT  [[GOT_ADDRESS]]
+# CHECK:      Relocations [
+# CHECK-NEXT:   Section ({{[0-9]+}}) .rela.plt {
+# CHECK-NEXT:     0x{{[0-9A-F]+}} R_PPC_JMP_SLOT baz 0x0
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]