cmd/link: don't use label symbol for absolute address relocations on ARM64 PE

On ARM64 PE, when external linking, the PE relocation does not
have an explicit addend, and instead has the addend encoded in
the instruction or data. An instruction (e.g. ADRP, ADD) has
limited width for the addend, so when the addend is large we use
a label symbol, which points to the middle of the original target
symbol, and a smaller addend. But for an absolute address
relocation in the data section, we have the full width to encode
the addend and we should not use the label symbol. Also, since we
do not adjust the addend in the data, using the label symbol will
actually make it point to the wrong address. E.g for an R_ADDR
relocation targeting x+0x123456, we should emit 0x123456 in the
data with an IMAGE_REL_ARM64_ADDR64 relocation pointing to x,
whereas the current code emits  0x123456 in the data with an
IMAGE_REL_ARM64_ADDR64 relocation pointing to the label symbol
x+1MB, so it will actually be resolved to x+0x223456. This CL
fixes this.

Fixes #47557.

Change-Id: I64e02b56f1d792f8c20ca61b78623ef5c3e34d7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/360895
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

2 files changed, 19 insertions, 1 deletions

diff --git a/src/cmd/link/internal/arm64/asm.go b/src/cmd/link/internal/arm64/asm.go
index f56c9c1222..abae0f87bc 100644
--- a/src/cmd/link/internal/arm64/asm.go
+++ b/src/cmd/link/internal/arm64/asm.go
@@ -602,7 +602,7 @@ func pereloc1(arch *sys.Arch, out *ld.OutBuf, ldr *loader.Loader, s loader.Sym,
 	rs := r.Xsym
 	rt := r.Type
 
-	if r.Xadd != signext21(r.Xadd) {
+	if rt == objabi.R_ADDRARM64 && r.Xadd != signext21(r.Xadd) {
 		// If the relocation target would overflow the addend, then target
 		// a linker-manufactured label symbol with a smaller addend instead.
 		label := ldr.Lookup(offsetLabelName(ldr, rs, r.Xadd/peRelocLimit*peRelocLimit), ldr.SymVersion(rs))
diff --git a/src/cmd/link/link_test.go b/src/cmd/link/link_test.go
index 2b0b2dc4a1..ad7658bb25 100644
--- a/src/cmd/link/link_test.go
+++ b/src/cmd/link/link_test.go
@@ -997,13 +997,31 @@ package main
 
 var x = [1<<25]byte{1<<23: 23, 1<<24: 24}
 
+var addr = [...]*byte{
+	&x[1<<23-1],
+	&x[1<<23],
+	&x[1<<23+1],
+	&x[1<<24-1],
+	&x[1<<24],
+	&x[1<<24+1],
+}
+
 func main() {
+	// check relocations in instructions
 	check(x[1<<23-1], 0)
 	check(x[1<<23], 23)
 	check(x[1<<23+1], 0)
 	check(x[1<<24-1], 0)
 	check(x[1<<24], 24)
 	check(x[1<<24+1], 0)
+
+	// check absolute address relocations in data
+	check(*addr[0], 0)
+	check(*addr[1], 23)
+	check(*addr[2], 0)
+	check(*addr[3], 0)
+	check(*addr[4], 24)
+	check(*addr[5], 0)
 }
 
 func check(x, y byte) {