[Libguestfs] [libnbd PATCH] common/utils/vector: Better overflow handling

[Nir Soffer]

On Mon, Nov 8, 2021 at 9:56 PM Eric Blake <eblake at redhat.com> wrote:
>
> Check newcap * itemsize for overflow prior to calling realloc, so that
> we don't accidentally truncate an existing array.  Set errno to ENOMEM
> on all failure paths, rather than leaving it indeterminate on
> overflow.  The patch works by assuming a prerequisite that
> v->cap*itemsize is always smaller than size_t.
>
> Fixes: 985dfa72ae (common/utils/vector.c: Optimize vector append)
> ---
>
> Unless you see problems in this, I'll push this and also port it to
> nbdkit.
>
>  common/utils/vector.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/common/utils/vector.c b/common/utils/vector.c
> index a4b43ce..c37f0c3 100644
> --- a/common/utils/vector.c
> +++ b/common/utils/vector.c
> @@ -1,5 +1,5 @@
>  /* nbdkit
> - * Copyright (C) 2018-2020 Red Hat Inc.
> + * Copyright (C) 2018-2021 Red Hat Inc.
>   *
>   * Redistribution and use in source and binary forms, with or without
>   * modification, are permitted provided that the following conditions are
> @@ -44,12 +44,14 @@ generic_vector_reserve (struct generic_vector *v, size_t n, size_t itemsize)
>    size_t reqcap, newcap;
>
>    reqcap = v->cap + n;
> -  if (reqcap < v->cap)
> +  if (reqcap * itemsize < v->cap * itemsize) {
> +    errno = ENOMEM;
>      return -1; /* overflow */
> +  }
>
>    newcap = (v->cap * 3 + 1) / 2;

Should we use:

    newcap = v->cap + (v->cap + 1) / 2

so we don't overflow too early?

And if we overflow - meaning that we cannot grow by factor of 1.5,
grow to maximum size:

    if (newcap * itemsize < v->cap * itemsize)
       newcap = SIZE_MAX / itemsize;

This makes a difference when itemsize is 1 or 2. The current
code will stop growing efficiently when allocation is 1.45g, and
then go back to realloc() on every call.

> -  if (newcap < reqcap)
> +  if (newcap * itemsize < reqcap * itemsize)
>      newcap = reqcap;

If we handle overflow before, we can keep the old check
comparing caps.

I can post a patch with this changes if you like.

Nir