[Libguestfs] [PATCH v2] Go bindings: fix "C array of strings" -- char** -- allocation

[Richard W.M. Jones]

On Tue, Sep 21, 2021 at 09:29:39PM +0200, Laszlo Ersek wrote:
> The current "arg_string_list" and "free_string_list" implementations go
> back to commit b6f01f32606d ("Add Go (language) bindings.", 2013-07-03).
> There are two problems with them:
> 
> - "free_string_list" doesn't actually free anything,
> 
> - at the *first* such g.Internal_test() call site that passes an
>   Ostringlist member inside the Optargs argument, namely:
> 
> > g.Internal_test ("abc",
> >                  string_addr ("def"),
> >                  []string{},
> >                  false,
> >                  0,
> >                  0,
> >                  "123",
> >                  "456",
> >                  []byte{'a', 'b', 'c', '\000', 'a', 'b', 'c'},
> >                  &guestfs.OptargsInternal_test{Ostringlist_is_set: true,
> >                                                Ostringlist: []string{}
> >                                               }
> >                 )
> 
>   the "golang/run-bindtests" case crashes:
> 
> > panic: runtime error: cgo argument has Go pointer to Go pointer
> >
> > goroutine 1 [running]:
> > libguestfs.org/guestfs.(*Guestfs).Internal_test.func7(0xc000018180,
> > 0xadfb60, 0xadfb80, 0xc000010048, 0x0, 0x0, 0x0, 0xae3e10, 0xae3e30,
> > 0xade3a0, ...)
> >         golang/src/libguestfs.org/guestfs/guestfs.go:6729 +0xa9
> > libguestfs.org/guestfs.(*Guestfs).Internal_test(0xc000018180, 0x4ee3a5,
> > 0x3, 0xc000061be8, 0xc000061af8, 0x0, 0x0, 0xc000061a00, 0x0, 0x0, ...)
> >         golang/src/libguestfs.org/guestfs/guestfs.go:6729 +0x3c9
> > main.main()
> >         golang/bindtests/bindtests.go:77 +0x151e
> > exit status 2
> > FAIL run-bindtests (exit status: 1)
> 
> In Daniel P. Berrangé's words [1],
> 
> > You're allowed to pass a Go pointer to C via CGo, but the memory that
> > points to is not allowed to contained further Go pointers. So the struct
> > fields must strictly use a C pointer.
> 
> One pattern to solve the problem has been shown on stackoverflow [2].
> Thus, rewrite the "arg_string_list" and "free_string_list" functions
> almost entirely in C, following that example.
> 
> While this approach is not the most idiomatic Go, as a solution exists
> without C helper functions [3], it should still be acceptable, at least as
> an incremental improvement, for letting "golang/run-bindtests" pass.
> 
> [1] https://listman.redhat.com/archives/libguestfs/2021-September/msg00118.html
> [2] https://stackoverflow.com/questions/35924545/golang-cgo-panic-runtime-error-cgo-argument-has-go-pointer-to-go-pointer
> [3] https://listman.redhat.com/archives/libguestfs/2021-September/msg00106.html
> 
> Cc: "Daniel P. Berrangé" <berrange at redhat.com>
> Cc: "Richard W.M. Jones" <rjones at redhat.com>
> Signed-off-by: Laszlo Ersek <lersek at redhat.com>

This time I enabled the golang bindings, reproduced the bug you
reported, applied this patch, reran the tests and verified that it has
been fixed, so:

Tested-by: "Richard W.M. Jones" <rjones at redhat.com>
Acked-by: "Richard W.M. Jones" <rjones at redhat.com>

Rich.

> 
> Notes:
>     v2:
>     - update commit message [Daniel, Rich]
>     - update code comment [Daniel, Rich]
>     - no code changes
>     - retested
> 
>  generator/golang.ml | 47 ++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 38 insertions(+), 9 deletions(-)
> 
> diff --git a/generator/golang.ml b/generator/golang.ml
> index d328abe4ed08..0d6a92367f9b 100644
> --- a/generator/golang.ml
> +++ b/generator/golang.ml
> @@ -52,6 +52,40 @@ _go_guestfs_create_flags (unsigned flags)
>  {
>      return guestfs_create_flags (flags);
>  }
> +
> +// Passing a Go pointer to C via CGo is allowed, but the memory that points to
> +// is not allowed to contain further Go pointers. The helper functions below
> +// are one way to implement this, although the same can be achieved purely in
> +// Go as well. See the discussion here:
> +// <https://listman.redhat.com/archives/libguestfs/2021-September/msg00101.html>.
> +typedef char *pChar;
> +
> +pChar *allocStringArray (size_t nmemb)
> +{
> +    pChar *array;
> +
> +    array = malloc (sizeof (pChar) * (nmemb + 1));
> +    array[nmemb] = NULL;
> +    return array;
> +}
> +
> +void storeString (pChar *array, size_t idx, pChar string)
> +{
> +    array[idx] = string;
> +}
> +
> +void freeStringArray (pChar *array)
> +{
> +    pChar *position;
> +    pChar string;
> +
> +    position = array;
> +    while ((string = *position) != NULL) {
> +      free (string);
> +      position++;
> +    }
> +    free (array);
> +}
>  */
>  import \"C\"
>  
> @@ -148,12 +182,11 @@ func (g *Guestfs) Close () *GuestfsError {
>   * C strings and golang []string.
>   */
>  func arg_string_list (xs []string) **C.char {
> -    r := make ([]*C.char, 1 + len (xs))
> +    r := C.allocStringArray (C.size_t (len (xs)))
>      for i, x := range xs {
> -        r[i] = C.CString (x)
> +        C.storeString (r, C.size_t (i), C.CString (x));
>      }
> -    r[len (xs)] = nil
> -    return &r[0]
> +    return (**C.char) (r)
>  }
>  
>  func count_string_list (argv **C.char) int {
> @@ -167,11 +200,7 @@ func count_string_list (argv **C.char) int {
>  }
>  
>  func free_string_list (argv **C.char) {
> -    for *argv != nil {
> -        //C.free (*argv)
> -        argv = (**C.char) (unsafe.Pointer (uintptr (unsafe.Pointer (argv)) +
> -                                           unsafe.Sizeof (*argv)))
> -    }
> +    C.freeStringArray ((*C.pChar) (argv))
>  }
>  
>  func return_string_list (argv **C.char) []string {
> -- 
> 2.19.1.3.g30247aa5d201
> 

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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