Report branches to section offsets that need thunks

Assemblers emit a call to a static function as a relocation against a
section symbol plus an offset. A range extension thunk jumps to the
start of a symbol, so taking such a branch through a thunk landed at
the wrong address. Report it as an error instead.

src/thunks.rs

@@ -30,6 +30,7 @@ use crate::chunks::output_section::OutputSection;
 use crate::chunks::{ChunkId, OutputSectionId};
 use crate::context::Context;
 use crate::elf::*;
+use crate::error;
 use crate::input_sections::{InputSection, InputSectionId};
 use crate::symbol::{AddrFlags, Symbol, SymbolId};
 use crate::target::{Family, Target};
@@ -454,6 +455,24 @@ pub fn remove_redundant_thunks<E: Target>(ctx: &mut Context<E>) {
                         continue;
                     }
                     let sym = &ctx.symbols[file.base.symbols[rel.r_sym() as usize]];
+
+                    // A thunk jumps to the start of a symbol, so it can't serve a
+                    // branch to an offset from a section symbol, which assemblers
+                    // emit for calls to static functions.
+                    if sym.ty() == STT_SECTION
+                        && isec.rel_addend(rel) != 0
+                        && requires_thunk(ctx, isec, rel, sym, false)
+                    {
+                        error!(
+                            "{}: relocation {} against {} needs a range extension thunk, \
+                             which can't jump to an offset from a section; recompile \
+                             with -ffunction-sections",
+                            isec.display(file),
+                            rel.type_name::<E>(),
+                            sym
+                        );
+                    }
+
                     if !sym.is_marked() && requires_thunk(ctx, isec, rel, sym, false) {
                         sym.mark();
                     }

tests/arch-aarch64-thunk-section-addend.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+. $(dirname $0)/common.inc
+
+# Assemblers emit calls to static functions as relocations against a
+# section symbol plus an offset. A range extension thunk jumps to the
+# start of a symbol, so such a call is an error if it needs a thunk.
+
+cat <<EOF | $CC -c -o $t/a.o -xassembler -
+.text
+f1:
+  ret
+f2:
+  ret
+
+.section .far,"ax"
+.globl _start
+_start:
+  bl f1
+  bl f2
+EOF
+
+$CC -B. -nostdlib -static -o $t/exe1 $t/a.o
+
+not $CC -B. -nostdlib -static -o $t/exe2 $t/a.o \
+  -Wl,--section-start=.text=0x10000000,--section-start=.far=0x20000000 >& $t/log
+grep -F 'R_AARCH64_CALL26 against .text needs a range extension thunk' $t/log