cmd/compile: SIGILL when cross-compiling from netbsd/arm64 to linux/mips

[bcmills]

            # internal/abi
            SIGILL: illegal instruction
            PC=0x273d40 m=4 sigcode=4
            instruction bytes: 0xa8 0x74 0x41 0x79 0x8 0x1 0x7f 0xb2 0xa8 0x74 0x1 0x79 0xd2 0xf9 0xff 0x17
            
            goroutine 9 [running]:
            cmd/internal/obj/mips.preprocess(0x4000176400, 0x4000443080, 0x40000a81a0)
            	/var/gobuilder/buildlet/go/src/cmd/internal/obj/mips/obj0.go:247 +0x1bd0 fp=0x4000637d50 sp=0x4000637bd0 pc=0x273d40
            cmd/internal/obj.Flushplist(0x4000176400, 0x4000637f28, 0x40000a81a0, {0xffffffffe6e7, 0xc})
            	/var/gobuilder/buildlet/go/src/cmd/internal/obj/plist.go:150 +0x630 fp=0x4000637e80 sp=0x4000637d50 pc=0x135610
            cmd/compile/internal/objw.(*Progs).Flush(...)
            	/var/gobuilder/buildlet/go/src/cmd/compile/internal/objw/prog.go:124
            cmd/compile/internal/ssagen.Compile(0x40003fdb80, 0x778704?)
            	/var/gobuilder/buildlet/go/src/cmd/compile/internal/ssagen/pgen.go:208 +0x574 fp=0x4000637f50 sp=0x4000637e80 pc=0x5d48b4
            cmd/compile/internal/gc.compileFunctions.func4.1(0x0?)
            	/var/gobuilder/buildlet/go/src/cmd/compile/internal/gc/compile.go:153 +0x3c fp=0x4000637f90 sp=0x4000637f50 pc=0x7785cc
            cmd/compile/internal/gc.compileFunctions.func3.1()
            	/var/gobuilder/buildlet/go/src/cmd/compile/internal/gc/compile.go:140 +0x48 fp=0x4000637fd0 sp=0x4000637f90 pc=0x778718
            runtime.goexit()
            	/var/gobuilder/buildlet/go/src/runtime/asm_arm64.s:1155 +0x4 fp=0x4000637fd0 sp=0x4000637fd0 pc=0x74814
            created by cmd/compile/internal/gc.compileFunctions.func3
            	/var/gobuilder/buildlet/go/src/cmd/compile/internal/gc/compile.go:138 +0x7c

greplogs --dashboard -md -l -e \\A.\*-arm\(.\*\\n\)\*\\s\*SIGILL --since=2022-01-01

2022-04-28T18:02:40-c15d0a9/netbsd-arm64-bsiegert

Looks like some kind of memory corruption to me, although at N=1 it isn't clear to me whether this is bug is specific to netbsd or arm64, or even some kind of weird bug in the mips compiler backend. 😅

(CC @golang/netbsd @golang/arm)

[cherrymui]

        cmd/internal/obj/mips.preprocess(0x4000176400, 0x4000443080, 0x40000a81a0)
       	/var/gobuilder/buildlet/go/src/cmd/internal/obj/mips/obj0.go:247 +0x1bd0 fp=0x4000637d50 sp=0x4000637bd0 pc=0x273d40

At this PC the instructions are

	MOVHU   186(R5), R8
	ORR     $2, R8, R8
	MOVH    R8, 186(R5)
	JMP     804

which indeed have those bytes and are valid instructions. So it is not bad codegen. Perhaps some kernel weirdness? Or some other process sending a SIGILL? (why?) Or bad I-cache? (The printed bytes are presumably loaded through D-cache. The content of the I-cache might be different.)

[randall77]

That instruction looks fine.

Here's the section of the compile binary at that git hash on netbsd/arm64:

  obj0.go:246           0x273d3c                b4ff3ac5                CBZ R5, -1578(PC)                                       
  obj0.go:247           0x273d40                794174a8                MOVHU 186(R5), R8                   <- this one                    
  obj0.go:247           0x273d44                b27f0108                ORR $2, R8, R8                                          

Looks like the instruction decoder and actual memory don't agree on the instruction involved. Definitely smells like corruption of some sort.