[lld] [LLD][RISCV] Warn on PCREL_LO referencing other Section (PR #107558)

[Sam Elliott via llvm-commits]

https://github.com/lenary created https://github.com/llvm/llvm-project/pull/107558

The RISC-V psABI states that "The `R_RISCV_PCREL_LO12_I` or `R_RISCV_PCREL_LO12_S` relocations contain a label pointing to an instruction in the same section with an `R_RISCV_PCREL_HI20` relocation entry that points to the target symbol."

In this case, GNU ld errors, but LLD does not -- I think because it is doing the right thing, certainly in the testcase provided.

Nonetheless, it would be good for LLD to at least warn the user, so they know the objects they are using are not following the psABI as written.

Fixes #107304

>From e6346f69fad36a1e2b2a7ec03251a5c738d189b7 Mon Sep 17 00:00:00 2001
From: Sam Elliott <quic_aelliott at quicinc.com>
Date: Fri, 6 Sep 2024 03:10:57 -0700
Subject: [PATCH] [LLD][RISCV] Warn on PCREL_LO referencing other Section

The RISC-V psABI states that "The `R_RISCV_PCREL_LO12_I` or
`R_RISCV_PCREL_LO12_S` relocations contain a label pointing to an
instruction in the same section with an `R_RISCV_PCREL_HI20` relocation
entry that points to the target symbol."

In this case, GNU ld errors, but LLD does not -- I think because it is
doing the right thing, certainly in the testcase provided.

Nonetheless, it would be good for LLD to at least warn the user, so they
know the objects they are using are not following the psABI as written.

Fixes #107304
---
 lld/ELF/Arch/RISCV.cpp                        |  9 +++++
 .../ELF/riscv-pcrel-hilo-error-sections.s     | 35 +++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 lld/test/ELF/riscv-pcrel-hilo-error-sections.s

diff --git a/lld/ELF/Arch/RISCV.cpp b/lld/ELF/Arch/RISCV.cpp
index 2435864ce5a7f0..80a7909c42f2e5 100644
--- a/lld/ELF/Arch/RISCV.cpp
+++ b/lld/ELF/Arch/RISCV.cpp
@@ -671,6 +671,15 @@ void RISCV::relocateAlloc(InputSectionBase &sec, uint8_t *buf) const {
       errorOrWarn(sec.getLocation(rel.offset) +
                   ": R_RISCV_SET_ULEB128 not paired with R_RISCV_SUB_SET128");
       return;
+    case R_RISCV_PC_INDIRECT:
+      if (Defined *def = dyn_cast<Defined>(rel.sym);
+          def->section && def->section != &sec) {
+        errorOrWarn(sec.getLocation(rel.offset) +
+              ": R_RISCV_PCREL_LO12 relocation points to a symbol '" +
+              rel.sym->getName() + "' in a different section '" +
+              def->section->name + "'");
+      }
+      break;
     default:
       break;
     }
diff --git a/lld/test/ELF/riscv-pcrel-hilo-error-sections.s b/lld/test/ELF/riscv-pcrel-hilo-error-sections.s
new file mode 100644
index 00000000000000..deffbd91b0ff61
--- /dev/null
+++ b/lld/test/ELF/riscv-pcrel-hilo-error-sections.s
@@ -0,0 +1,35 @@
+# REQUIRES: riscv
+
+# RUN: llvm-mc -filetype=obj -triple=riscv64 %s -o %t.o
+# RUN: not ld.lld %t.o 2>&1 | FileCheck %s
+
+# CHECK: error: {{.*}}:(.text.sec_one+0x0): R_RISCV_PCREL_LO12 relocation points to a symbol '.Lpcrel_hi0' in a different section '.text.sec_two'
+# CHECK: error: {{.*}}:(.text.sec_one+0x4): R_RISCV_PCREL_LO12 relocation points to a symbol '.Lpcrel_hi1' in a different section '.text.sec_two'
+# CHECK-NOT: R_RISCV_PCREL_LO12 relocation points to a symbol '.Lpcrel_hi2'
+
+## This test is checking that we warn the user when the relocations in their
+## object don't follow the RISC-V psABI. In particular, the psABI requires
+## that PCREL_LO12 relocations are in the same section as the pcrel_hi
+## instruction they point to.
+
+  .section .text.sec_one
+  .globl sec_one
+sec_one:
+  addi a0, a0, %pcrel_lo(.Lpcrel_hi0)
+  sw a0, %pcrel_lo(.Lpcrel_hi1)(a1)
+
+  .section .text.sec_two
+sec_two:
+.Lpcrel_hi0:
+  auipc a0, %pcrel_hi(a)
+.Lpcrel_hi1:
+  auipc a1, %pcrel_hi(a)
+
+.Lpcrel_hi2:
+  auipc a2, %pcrel_hi(a)
+  addi a2, a2, %pcrel_lo(.Lpcrel_hi2)
+
+  .data
+  .global a
+a:
+  .word 50