[Qemu-devel] [Bug 1839325] Re: Go programs crash on qemu-sh4 due to issues with atomics

[Peter Maydell]

I just did an objdump -x of the /usr/lib/sh4-linux-gnu/libgo.so.14,
which will be the shipped version from the Debian package, and in the
section header it has:

 24 .bss          000191f8  00fe74ec  00fe74ec  00fd74ec  2**2
                  ALLOC

and in the symbol table it has:

00ff98f4 l     O .bss   00000104              runtime.work

So the compiler has put the 'runtime.work' struct at a non-multiple-of-8
offset into the bss, and it's given the BSS alignment requirements that
are only 4-aligned, not 8-aligned. That means it's random luck whether
the struct gets 8-aligned or not.

This looks to me like it's a bug in the sh4 gccgo --
https://go101.org/article/memory-layout.html says that the first word in
a struct or variable is supposed to be guaranteed to be 8-aligned, so
the compiler needs to align things more strictly than it is currently
doing.

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1839325

Title:
  Go programs crash on qemu-sh4 due to issues with atomics

Status in QEMU:
  New

Bug description:
  After #1738545 [1] was fixed, Go applications work fine on qemu-arm
  but still crash on qemu-sh4. From the backtrace, it looks like an
  issue with the atomics in qemu-sh4:

  (sid-sh4-sbuild)root@epyc:/# cat hello.go
  package main

  import "fmt"

  func main() {
        fmt.Println("hello world")
  }

  (sid-sh4-sbuild)root@epyc:/# gccgo-9 hello.go -o hello
  (sid-sh4-sbuild)root@epyc:/# ./hello 
  panic: (        runtime runtime.errorString) (0x7f74527c,0x80a038)
  fatal error: panic on system stack
  panic: (        runtime runtime.errorString) (0x7f74527c,0x80a038)
  fatal error: panic on system stack

  runtime stack:
  runtime..z2finternal..z2fatomic.Load64
          ../../../src/libgo/go/runtime/internal/atomic/atomic.c:37
  runtime_mstart
          ../../../src/libgo/runtime/proc.c:596

  goroutine 1 [running]:
          goroutine running on other thread; stack unavailable

  runtime stack:
  runtime..z2finternal..z2fatomic.Load64
          ../../../src/libgo/go/runtime/internal/atomic/atomic.c:37
  runtime_mstart
          ../../../src/libgo/runtime/proc.c:596
  (sid-sh4-sbuild)root@epyc:/#

  The same sample Go program runs fine on my SH7785LCR SH4 evaluation
  board:

  root@tirpitz:~> uname -a
  Linux tirpitz 3.16.7-ckt7 #8 PREEMPT Fri Oct 21 18:47:41 CEST 2016 sh4a 
GNU/Linux
  root@tirpitz:~> cat hello.go
  package main

  import "fmt"

  func main() {
        fmt.Println("hello world")
  }

  root@tirpitz:~> gccgo-9 hello.go -o hello
  root@tirpitz:~> ./hello 
  hello world
  root@tirpitz:~>

  Please note: In order to be able to reproduce this, one also needs to
  revert commit 61dedf2af7 [2], otherwise the Go application crashes
  differently:

  (sid-sh4-sbuild)root@epyc:/# ./hello        
  Unhandled trap: 0x180
  pc=0x7e5f7f9e sr=0x00000000 pr=0x7ee3d582 fpscr=0x00080004
  spc=0x00000000 ssr=0x00000000 gbr=0x7e590480 vbr=0x00000000
  sgr=0x00000000 dbr=0x00000000 delayed_pc=0x7e5f7f60 fpul=0x00034f3b
  r0=0x008007d4 r1=0x00000000 r2=0xfffe0b2a r3=0x00000002
  r4=0x008006e4 r5=0x00872000 r6=0x00200000 r7=0x00000000
  r8=0x7f7bca7c r9=0x7fffebd4 r10=0x00800480 r11=0x7f7bc0f0
  r12=0x7f7a3fa4 r13=0x008004c0 r14=0x7f7b2238 r15=0x7fffebd0
  r16=0x00000000 r17=0x00000000 r18=0x00000000 r19=0x00000000
  r20=0x00000000 r21=0x00000000 r22=0x00000000 r23=0x00000000
  (sid-sh4-sbuild)root@epyc:/#

  > [1] https://bugs.launchpad.net/bugs/1738545
  > [2] https://bugs.launchpad.net/bugs/1796520

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1839325/+subscriptions