[release-branch.go1.19] cmd/link/internal/ppc64: fix trampoline reuse distance calculation

If a compatible trampoline has been inserted by a previously laid
function in the same section, and is known to be sufficiently close,
it can be reused.

When testing if the trampoline can be reused, the addend of the direct
call should be ignored. It is already encoded in the trampoline. If the
addend is non-zero, and the target sufficiently far away, and just
beyond direct call reach, this may cause the trampoline to be
incorrectly reused.

This was observed on go1.17.13 and openshift-installer commit f3c53b382
building in release mode with the following error:

github.com/aliyun/alibaba-cloud-sdk-go/services/cms.(*Client).DescribeMonitoringAgentAccessKeyWithChan.func1: direct call too far: runtime.duffzero+1f0-tramp0-1 -2000078

Fixes #56834

Change-Id: I54af957302506d4e3cd5d3121542c83fe980e912
Reviewed-on: https://go-review.googlesource.com/c/go/+/451415
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/451417
Reviewed-by: Joedian Reid <joedian@golang.org>

2 files changed, 103 insertions, 2 deletions

diff --git a/src/cmd/go/testdata/script/trampoline_reuse_test.txt b/src/cmd/go/testdata/script/trampoline_reuse_test.txt
new file mode 100644
index 0000000000..bca897c16d
--- /dev/null
+++ b/src/cmd/go/testdata/script/trampoline_reuse_test.txt
@@ -0,0 +1,100 @@
+# Verify PPC64 does not reuse a trampoline which is too far away.
+# This tests an edge case where the direct call relocation addend should
+# be ignored when computing the distance from the direct call to the
+# already placed trampoline
+[short] skip
+[!ppc64] [!ppc64le] skip
+[aix] skip
+
+# Note, this program does not run. Presumably, 'DWORD $0' is simpler to
+# assembly 2^26 or so times.
+#
+# We build something which should be laid out as such:
+#
+# bar.Bar
+# main.Func1
+# bar.Bar+400-tramp0
+# main.BigAsm
+# main.Func2
+# bar.Bar+400-tramp1
+#
+# bar.Bar needs to be placed far enough away to generate relocations
+# from main package calls. and main.Func1 and main.Func2 are placed
+# a bit more than the direct call limit apart, but not more than 0x400
+# bytes beyond it (to verify the reloc calc).
+
+go build
+
+-- go.mod --
+
+module foo
+
+go 1.19
+
+-- main.go --
+
+package main
+
+import "foo/bar"
+
+func Func1()
+
+func main() {
+        Func1()
+        bar.Bar2()
+}
+
+-- foo.s --
+
+TEXT main·Func1(SB),0,$0-0
+        CALL bar·Bar+0x400(SB)
+        CALL main·BigAsm(SB)
+// A trampoline will be placed here to bar.Bar
+
+// This creates a gap sufficiently large to prevent trampoline reuse
+#define NOP64 DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0;
+#define NOP256 NOP64 NOP64 NOP64 NOP64
+#define NOP2S10 NOP256 NOP256 NOP256 NOP256
+#define NOP2S12 NOP2S10 NOP2S10 NOP2S10 NOP2S10
+#define NOP2S14 NOP2S12 NOP2S12 NOP2S12 NOP2S12
+#define NOP2S16 NOP2S14 NOP2S14 NOP2S14 NOP2S14
+#define NOP2S18 NOP2S16 NOP2S16 NOP2S16 NOP2S16
+#define NOP2S20 NOP2S18 NOP2S18 NOP2S18 NOP2S18
+#define NOP2S22 NOP2S20 NOP2S20 NOP2S20 NOP2S20
+#define NOP2S24 NOP2S22 NOP2S22 NOP2S22 NOP2S22
+#define BIGNOP NOP2S24 NOP2S24
+TEXT main·BigAsm(SB),0,$0-0
+        // Fill to the direct call limit so Func2 must generate a new trampoline.
+        // As the implicit trampoline above is just barely unreachable.
+        BIGNOP
+        MOVD $main·Func2(SB), R3
+
+TEXT main·Func2(SB),0,$0-0
+        CALL bar·Bar+0x400(SB)
+// Another trampoline should be placed here.
+
+-- bar/bar.s --
+
+#define NOP64 DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0;
+#define NOP256 NOP64 NOP64 NOP64 NOP64
+#define NOP2S10 NOP256 NOP256 NOP256 NOP256
+#define NOP2S12 NOP2S10 NOP2S10 NOP2S10 NOP2S10
+#define NOP2S14 NOP2S12 NOP2S12 NOP2S12 NOP2S12
+#define NOP2S16 NOP2S14 NOP2S14 NOP2S14 NOP2S14
+#define NOP2S18 NOP2S16 NOP2S16 NOP2S16 NOP2S16
+#define NOP2S20 NOP2S18 NOP2S18 NOP2S18 NOP2S18
+#define NOP2S22 NOP2S20 NOP2S20 NOP2S20 NOP2S20
+#define NOP2S24 NOP2S22 NOP2S22 NOP2S22 NOP2S22
+#define BIGNOP NOP2S24 NOP2S24 NOP2S10
+// A very big not very interesting function.
+TEXT bar·Bar(SB),0,$0-0
+        BIGNOP
+
+-- bar/bar.go --
+
+package bar
+
+func Bar()
+
+func Bar2() {
+}
diff --git a/src/cmd/link/internal/ppc64/asm.go b/src/cmd/link/internal/ppc64/asm.go
index 5d5fbe2a97..6313879da0 100644
--- a/src/cmd/link/internal/ppc64/asm.go
+++ b/src/cmd/link/internal/ppc64/asm.go
@@ -900,8 +900,9 @@ func trampoline(ctxt *ld.Link, ldr *loader.Loader, ri int, rs, s loader.Sym) {
 				if ldr.SymValue(tramp) == 0 {
 					break
 				}
-
-				t = ldr.SymValue(tramp) + r.Add() - (ldr.SymValue(s) + int64(r.Off()))
+				// Note, the trampoline is always called directly. The addend of the original relocation is accounted for in the
+				// trampoline itself.
+				t = ldr.SymValue(tramp) - (ldr.SymValue(s) + int64(r.Off()))
 
 				// With internal linking, the trampoline can be used if it is not too far.
 				// With external linking, the trampoline must be in this section for it to be reused.