[PATCH] D114982: [ELF][PowerPC] Don't discard .got if 32-bit plt needs it

[George Koehler via Phabricator via llvm-commits]

kernigh created this revision.
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Add R_PPC32_PLTREL to the list of GOT-offset relocations. The plt in 32-bit PowerPC code needs the first 12 bytes of the .got section, so LLD must create the GOT.

lld had discarded .got in code from clang -fPIC; the missing .got caused OpenBSD's ld.so to give a misleading error, "unsupported insecure BSS PLT object". Code from clang -fPIC has no entries in .got (because it uses .got2); lld would add a 12-byte header to .got, but since commit 818b508 <https://reviews.llvm.org/rG818b508953c7684a6c104b89c6f6ce84e961d82e>, lld doesn't count the header as entries. So, lld counted 0 entries in .got, discarded .got, and set DT_GOT_PPC to zero. This broke ld.so, because DT_GOT_PPC must point to the 12-byte header.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D114982

Files:
  lld/ELF/Relocations.cpp
  lld/test/ELF/ppc32-plt-got.s


Index: lld/test/ELF/ppc32-plt-got.s
===================================================================
--- /dev/null
+++ lld/test/ELF/ppc32-plt-got.s
@@ -0,0 +1,39 @@
+# 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", at progbits
+.set .LTOC, .+0x8000
+
+.text
+.L0:
+addis 30,30,.LTOC-.L0 at ha
+addi 30,30,.LTOC-.L0 at l
+bl baz+0x8000 at 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: ]
Index: lld/ELF/Relocations.cpp
===================================================================
--- lld/ELF/Relocations.cpp
+++ lld/ELF/Relocations.cpp
@@ -1365,8 +1365,8 @@
   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;
   }
 


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