[LLD][ELF][AArch64] Add R_AARCH64_PLT_PAGE_PC to isRelExpr

[peter.smith]

Summary

As a follow on to D56666 (r351186) there is a case when taking the address of an ifunc when compiling -fpie and linking -pie that can generate a spurious can't create dynamic relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol in readonly segment. Specifically the case is where the ifunc is in the same translation unit as the address taker, so given -fpie the compiler knows that the ifunc is going to be defined in the executable so it can use a non-got-generating relocation.

The error message is due to R_AARCH64_PLT_PAGE_PC not being added to isRelExpr, its non PLT equivalent R_AARCH64_PAGE_PC is already in isRelExpr. I've checked that the output is comparable to ld.bfd and ld.gold.

I found this while checking combinations of compilation and link options involving taking the address of an ifunc; for ld.gold, ld.bfd and ld.lld.

[ruiu]

LGTM

ELF/Relocations.cpp

return isRelExprOneOf<R_GOT, R_GOT_OFF, R_HEXAGON_GOT, R_MIPS_GOT_LOCAL_PAGE,
                         R_GOT_PLT>(Expr);
 }
 
 // True if this expression is of the form Sym - X, where X is a position in the
 // file (PC, or GOT for example).
 static bool isRelExpr(RelExpr Expr) {
   return isRelExprOneOf<R_PC, R_GOTREL, R_GOTREL_FROM_END, R_MIPS_GOTREL,
                         R_PPC_CALL, R_PPC_CALL_PLT, R_AARCH64_PAGE_PC,
-                        R_RELAX_GOT_PC>(Expr);
+                        R_AARCH64_PLT_PAGE_PC, R_RELAX_GOT_PC>(Expr);
 }
 
 // Returns true if a given relocation can be computed at link-time.
 //
 // For instance, we know the offset from a relocation to its target at
 // link-time if the relocation is PC-relative and refers a
 // non-interposable function in the same executable. This function
 // will return true for such relocation.

test/ELF/aarch64-gnu-ifunc-address-pie.s

+# REQUIRES: aarch64
+# RUN: llvm-mc -filetype=obj -triple=aarch64-none-linux-gnu %s -o %t.o
+# RUN: ld.lld -pie %t.o -o %tout
+# RUN: llvm-objdump -D %tout | FileCheck %s
+# RUN: llvm-readobj -r %tout | FileCheck %s -check-prefix=CHECK-RELOCS
+
+# Test that when we take the address of a preemptible ifunc using -fpie, we can
+# handle the case when the ifunc is in the same translation unit as the address
+# taker. In this case the compiler knows that ifunc is not defined in a shared
+# library so it can use a non got generating relative reference.
+.text
+.globl myfunc
+.type myfunc,@gnu_indirect_function
+myfunc:
+ ret
+
+.text
+.globl main
+.type main,@function
+main:
+ adrp x8, myfunc
+ add  x8, x8, :lo12: myfunc
+ ret
+
+# CHECK: 0000000000010000 myfunc:
+# CHECK-NEXT:    10000: c0 03 5f d6   ret
+# CHECK: 0000000000010004 main:
+# CHECK-NEXT:    10004: 08 00 00 90   adrp  x8, #0
+# x8 = 0x10000
+# CHECK-NEXT:    10008: 08 41 00 91   add x8, x8, #16
+# x8 = 0x10010 = .plt for myfunc
+# CHECK-NEXT:    1000c: c0 03 5f d6   ret
+# CHECK-NEXT: Disassembly of section .plt:
+# CHECK-NEXT: 0000000000010010 .plt:
+# CHECK-NEXT:    10010: 90 00 00 90   adrp  x16, #65536
+# CHECK-NEXT:    10014: 11 02 40 f9   ldr x17, [x16]
+# CHECK-NEXT:    10018: 10 02 00 91   add x16, x16, #0
+# CHECK-NEXT:    1001c: 20 02 1f d6   br  x17
+
+# CHECK-RELOCS: Relocations [
+# CHECK-RELOCS-NEXT:   Section {{.*}} .rela.plt {
+# CHECK-RELOCS-NEXT:     0x20000 R_AARCH64_IRELATIVE - 0x10000
+# CHECK-RELOCS-NEXT:   }
+# CHECK-RELOCS-NEXT: ]