[Libguestfs] libnbd golang failure on RISC-V

Nir Soffer
Thu, 09 Jun 2022 16:22:45 -0000

On Thu, Jun 9, 2022 at 6:48 PM Richard W.M. Jones <rjones at redhat.com>
wrote:
>
> On Thu, Jun 09, 2022 at 04:24:12PM +0100, Richard W.M. Jones wrote:
> > On Thu, Jun 09, 2022 at 03:20:02PM +0100, Daniel P. Berrang? wrote:
> > > > + go test -count=1 -v
> > > > === RUN   Test010Load
> > > > --- PASS: Test010Load (0.00s)
> > > > === RUN   TestAioBuffer
> > > > --- PASS: TestAioBuffer (0.00s)
> > > > === RUN   TestAioBufferFree
> > > > --- PASS: TestAioBufferFree (0.00s)
> > > > === RUN   TestAioBufferBytesAfterFree
> > > > SIGABRT: abort
> > > > PC=0x3fdf6f9bac m=0 sigcode=18446744073709551610
> > >
> > > So suggesting TestAioBufferBytesAfterFree is as fault, but quite
> > > odd as that test case is trivial and whle it allocates some
> > > native memory it doesn't seem to write to it. Unless the
problem
> > > happened in an earlier test case and we have delayed detection ?
> > >
> > > I guess I'd try throwing darts at the wall by chopping out
bits
> > > of test code to see what makes it disappear.
> > >
> > > Perhaps also try swapping MakeAioBuffer with MakeAioBufferZero
> > > in case pre-existing data into the C.malloc()d block is confusing
> > > Go ?
> >
> > Interestingly if I remove libnbd_020_aio_buffer_test.go completely,
> > and disable GODEBUG, then the tests pass.  (Reproducer commands at end
> > of email).  So I guess at least one of the problems is confined to
> > this test and/or functions it calls in the main library.
> > Unfortunately this test is huge.
> >
> > At your suggestion, replacing every MakeAioBuffer with
> > MakeAioBufferZero in that test, but it didn't help.  Also tried
> > replacing malloc -> calloc in the aio_buffer.go implementation
which
> > didn't help.
> >
> > I'll try some more random things ...
>
> Adding a few printf's shows something interesting:
>
> === RUN   TestAioBufferBytesAfterFree
> calling Free on 0x3fbc1882b0
> calling C.GoBytes on 0x3fbc1882b0
> SIGABRT: abort
> PC=0x3fe6aaebac m=0 sigcode=18446744073709551610
>
> goroutine 21 [running]:
> gsignal
>         :0
> abort
>         :0
> runtime.throwException
>         ../../../libgo/runtime/go-unwind.c:128
> runtime.unwindStack
>         ../../../libgo/go/runtime/panic.go:535
> panic
>         ../../../libgo/go/runtime/panic.go:750
> runtime.panicmem
>         ../../../libgo/go/runtime/panic.go:210
> runtime.sigpanic
>         ../../../libgo/go/runtime/signal_unix.go:634
> _wordcopy_fwd_aligned
>         :0
> __GI_memmove
>         :0
> runtime.stringtoslicebyte
>         ../../../libgo/go/runtime/string.go:155
> __go_string_to_byte_array
>         ../../../libgo/go/runtime/string.go:509
> _cgo_23192bdcbd72_Cfunc_GoBytes
>         ./cgo-c-prolog-gccgo:46
>
> This is a simple use after free because the Free function in
> aio_buffer.go frees the array and then the Bytes function attempts to
> copy b.Size bytes from the NULL pointer.
>
> I didn't write this test so I'm not quite sure what it's trying
to
> achieve.

The test verifies that using the buffer in the wrong way fails in a clean
way (panic) and not silent double free like it was before
https://gitlab.com/nbdkit/libnbd/-/commit/3394f47556cac009fa7d39c9e2f7e5f2468bd65d

> It seems to be deliberately trying to cause a panic, but
> causes a segfault instead?  (And why only on RISC-V?)
>
>   func TestAioBufferBytesAfterFree(t *testing.T) {
>         buf := MakeAioBuffer(uint(32))
>         buf.Free()
>
>         defer func() {
>                 if r := recover(); r == nil {
>                         t.Fatal("Did not recover from panic calling
Bytes() after Free()")
>                 }
>         }()
>
>         buf.Bytes()
>   }
>
> Since this only happens on RISC-V I guess it might be something to do
> with the golang implementation on this architecture being unable to
> turn segfaults into panics.
>
> Removing all three *AfterFree tests fixes the tests.

But this hides the real issue - if users use Bytes() in the wrong way, we want
the panic, not the segfault - the tests are good!

> It seems a bit of an odd function however.  Wouldn't it be better to
> changes the Bytes function so that it tests if the pointer is NULL and
> panics?

I cannot find now any docs for GoBytes, maybe I tested that it panics
in this case,
but this does not work this arch (bug?). Panic with a good error message about
the wrong usage will be much better.

>
> NB: this _does not_ address the other problem where GODEBUG=cgocheck=2
> complains about "fatal error: Go pointer stored into non-Go
memory".

Do we keep go pointers in buffers allocated in C?

Nir