[PATCH 8/8] x86: correct overflow checking for 16-bit PC-relative relocs

[Jan Beulich]

The only insn requiring a truly 16-bit PC-relative relocation outside of
16-bit mode is XBEGIN (with an operand size override). For it, the
relocation generated should behave similar to 8- and (for 64-bit) 32-bit
PC-relatives ones, i.e. be checked for a signed value to fit the field.
This same mode is also correct for 16-bit code. Outside of 16-bit code,
branches with operand size overrides act in a truly PC-relative way only
when living in the low 32k of address space, as they truncate rIP to 16
bits. This can't be expressed by a PC-relative relocation.

Putting in place a new testcase, I'd like to note that the two existing
ones (pcrel16 and pcrel16abs) appear to be pretty pointless: They don't
expect any error despite supposedly checking for overflow, and in fact
there can't possibly be any error for the
- former since gas doesn't emit any relocation in the first place there,
- latter because the way the relocation gets expressed by gas doesn't
  allow the linker to notice the overflow; it should be detected by gas
  if at all, but see above (an error would be reported here for x86-64
  afaict, but this test doesn't get re-used there).

bfd/
2021-04-XX  Jan Beulich  <jbeulich@suse.com>

	* elf32-i386.c (elf_howto_table): Switch R_386_PC16 to
	complain_overflow_signed.
	* elf64-x86-64.c (x86_64_elf_howto_table): Switch R_X86_64_PC16
	to complain_overflow_signed.

ld/
2021-04-XX  Jan Beulich  <jbeulich@suse.com>

	* testsuite/ld-i386/pcrel16-2.s, testsuite/ld-i386/pcrel16-2.d,
	testsuite/ld-x86-64/pcrel16-2.d: New.
	* testsuite/ld-i386/i386.exp: Run new test.
	* testsuite/ld-x86-64/x86-64.exp: Likewise.

--- a/bfd/elf32-i386.c
+++ b/bfd/elf32-i386.c
@@ -93,7 +93,7 @@ static reloc_howto_type elf_howto_table[
   HOWTO(R_386_16, 0, 1, 16, false, 0, complain_overflow_bitfield,
 	bfd_elf_generic_reloc, "R_386_16",
 	true, 0xffff, 0xffff, false),
-  HOWTO(R_386_PC16, 0, 1, 16, true, 0, complain_overflow_bitfield,
+  HOWTO(R_386_PC16, 0, 1, 16, true, 0, complain_overflow_signed,
 	bfd_elf_generic_reloc, "R_386_PC16",
 	true, 0xffff, 0xffff, true),
   HOWTO(R_386_8, 0, 0, 8, false, 0, complain_overflow_bitfield,
--- a/bfd/elf64-x86-64.c
+++ b/bfd/elf64-x86-64.c
@@ -82,7 +82,7 @@ static reloc_howto_type x86_64_elf_howto
 	false),
   HOWTO(R_X86_64_16, 0, 1, 16, false, 0, complain_overflow_bitfield,
 	bfd_elf_generic_reloc, "R_X86_64_16", false, 0xffff, 0xffff, false),
-  HOWTO(R_X86_64_PC16,0, 1, 16, true, 0, complain_overflow_bitfield,
+  HOWTO(R_X86_64_PC16, 0, 1, 16, true, 0, complain_overflow_signed,
 	bfd_elf_generic_reloc, "R_X86_64_PC16", false, 0xffff, 0xffff, true),
   HOWTO(R_X86_64_8, 0, 0, 8, false, 0, complain_overflow_bitfield,
 	bfd_elf_generic_reloc, "R_X86_64_8", false, 0xff, 0xff, false),
--- a/ld/testsuite/ld-i386/i386.exp
+++ b/ld/testsuite/ld-i386/i386.exp
@@ -297,6 +297,7 @@ run_dump_test "abs"
 run_dump_test "pcrel8"
 run_dump_test "pcrel16"
 run_dump_test "pcrel16abs"
+run_dump_test "pcrel16-2"
 run_dump_test "alloc"
 run_dump_test "warn1"
 run_dump_test "tlsgd2"
--- /dev/null
+++ b/ld/testsuite/ld-i386/pcrel16-2.d
@@ -0,0 +1,5 @@
+#name: PCREL16 overflow (2)
+#as: --32
+#ld: -melf_i386
+#error: .*relocation truncated to fit: R_386_PC16 .*t16.*
+#error: .*relocation truncated to fit: R_386_PC16 .*_start.*
--- /dev/null
+++ b/ld/testsuite/ld-i386/pcrel16-2.s
@@ -0,0 +1,12 @@
+	.text
+	.global _start
+_start:
+	data16 xbegin t16
+	ret
+
+	.fill 0x8000,1,0xcc
+
+	.global t16
+t16:
+	data16 xbegin _start
+	ret
--- /dev/null
+++ b/ld/testsuite/ld-x86-64/pcrel16-2.d
@@ -0,0 +1,5 @@
+#name: PCREL16 overflow (2)
+#source: ../ld-i386/pcrel16-2.s
+#ld:
+#error: .*relocation truncated to fit: R_X86_64_PC16 .*t16.*
+#error: .*relocation truncated to fit: R_X86_64_PC16 .*_start.*
--- a/ld/testsuite/ld-x86-64/x86-64.exp
+++ b/ld/testsuite/ld-x86-64/x86-64.exp
@@ -261,6 +261,7 @@ run_dump_test "abs-l1om"
 run_dump_test "apic"
 run_dump_test "pcrel8"
 run_dump_test "pcrel16"
+run_dump_test "pcrel16-2"
 run_dump_test "tlsgd2"
 run_dump_test "tlsgd3"
 run_dump_test "tlsgd12"