x86: correct overflow checking for 16-bit PC-relative relocs

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).

6 files changed, 31 insertions, 0 deletions

diff --git a/ld/ChangeLog b/ld/ChangeLog
index b736aa8..9bb178e 100644
--- a/ld/ChangeLog
+++ b/ld/ChangeLog
@@ -1,3 +1,10 @@
+2021-04-26  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.
+
 2021-04-23  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR ld/27771
diff --git a/ld/testsuite/ld-i386/i386.exp b/ld/testsuite/ld-i386/i386.exp
index 6ad69a8..d0b3f69 100644
--- 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"
diff --git a/ld/testsuite/ld-i386/pcrel16-2.d b/ld/testsuite/ld-i386/pcrel16-2.d
new file mode 100644
index 0000000..c1c340f
--- /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.*
diff --git a/ld/testsuite/ld-i386/pcrel16-2.s b/ld/testsuite/ld-i386/pcrel16-2.s
new file mode 100644
index 0000000..2e97417
--- /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
diff --git a/ld/testsuite/ld-x86-64/pcrel16-2.d b/ld/testsuite/ld-x86-64/pcrel16-2.d
new file mode 100644
index 0000000..5346a5b
--- /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.*
diff --git a/ld/testsuite/ld-x86-64/x86-64.exp b/ld/testsuite/ld-x86-64/x86-64.exp
index 877d6ba..269f70d 100644
--- 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"