cmd/link: linux/arm external linking broken

[minux]

$ go build -ldflags="-linkmode=external -v" cmd/objdump/testdata/fmthello.go 
# command-line-arguments
HEADER = -H5 -T0x10c00 -D0x0 -R0x1000
searching for runtime.a in $WORK/runtime.a
searching for runtime.a in $GOROOT/pkg/linux_arm/runtime.a
searching for math.a in $WORK/math.a
searching for math.a in $GOROOT/pkg/linux_arm/math.a
searching for runtime/cgo.a in $WORK/runtime/cgo.a
searching for runtime/cgo.a in $GOROOT/pkg/linux_arm/runtime/cgo.a
 0.00 deadcode
 0.46 pclntab=242507 bytes, funcdata total 87728 bytes
 0.52 dodata
 0.61 reloc
 0.66 asmb
 0.81 datblk
 0.81 sym
 0.81 elfsym
 0.84 symsize = 44432
 0.86 symsize = 44624
 0.86 dwarf
 0.93 symsize = 44624
runtime.save_g: missing xsym in relocation
runtime.load_g: missing xsym in relocation
runtime._initcgo: missing xsym in relocation
 2.22 dwarf pass 2.
 2.98 header

This is reason for recent linux/arm builder failures.

[minux]

The change the breaks this is
https://go-review.googlesource.com/#/c/9107/5/src/cmd/internal/obj/arm/asm5.go

[davecheney]

Can we revert that change to fix the issue ?

On Thu, Apr 23, 2015 at 4:04 PM, Minux Ma notifications@github.com wrote:

The change the breaks this is

https://go-review.googlesource.com/#/c/9107/5/src/cmd/internal/obj/arm/asm5.go

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Reply to this email directly or view it on GitHub
#10557 (comment).

[minux]

We can revert it to fix the build but it's not very necessary as we know exactly what's broken, so fixing it should not be hard. Besides, we still have time.

[mwhudson]

Whoops, sorry. Can we fix this by making tls stuff less fragile please?

[minux]

The special handling of runtime.tlsg is indeed very convoluted and fragile. I meant to clean it up for a long time. My last attempt (CL 2118), however, failed to eliminate all the special casing in the linker (actually you can argue that it made it worse because subsequent CLs added more special case code to fix the android/arm builder). The situation is so bad that in the darwin/arm64 port, I have to avoid the runtime.tlsg name altogether. I filed #10560 to remember to do the clean up and fix the arm64 problem.

[mwhudson]

This fixes it:

diff --git a/src/cmd/internal/ld/data.go b/src/cmd/internal/ld/data.go
index 13a4019..7de0b70 100644
--- a/src/cmd/internal/ld/data.go
+++ b/src/cmd/internal/ld/data.go
@@ -372,6 +372,17 @@ func relocsym(s *LSym) {
                        }

                case obj.R_TLS:
+                       if Linkmode == LinkExternal && Iself && HEADTYPE != obj.Hopenbsd {
+                               r.Done = 0
+                               r.Sym = Ctxt.Tlsg
+                               r.Xsym = Ctxt.Tlsg
+                               r.Xadd = r.Add
+                               if Thearch.Thechar == '6' {
+                                       panic("oh no you don't")
+                               }
+                               o = r.Add
+                               break
+                       }
                        if Linkmode == LinkInternal && Iself && Thearch.Thechar == '5' {
                                // On ELF ARM, the thread pointer is 8 bytes before
                                // the start of the thread-local data block, so add 8

I just want to set everything on fire though. Bit late in my TZ for that :)

[mwhudson]

This is fixed isn't it?

[rsc]

Trusting @mwhudson that this is fixed. If not, please reopen.