Re: [PATCH] tcg: Remove null pointer arithmetic in tcg_malloc()

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Wed, 2025-06-18 at 12:35 +0200, Ilya Leoshkevich wrote:
> Clang 20.1.6 (Fedora 20.1.6-1.fc42)'s UBSAN complains:
> 
>     qemu/include/tcg/tcg.h:715:19: runtime error: applying non-zero
> offset 64 to null pointer
> 
> The code uses NULL as pool's initial start and end, with the
> intention
> that `pool_cur + size > pool_end` should trigger the allocation.
> Unfortunately C prohibits adding non-zero to NULL, even if the result
> is not dereferenced.
> 
> Fix by using a dummy pool.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  tcg/tcg.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index d714ae2889c..afcc7ec8849 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -1358,13 +1358,14 @@ void *tcg_malloc_internal(TCGContext *s, int
> size)
>  
>  void tcg_pool_reset(TCGContext *s)
>  {
> +    static uint8_t dummy_pool;
>      TCGPool *p, *t;
>      for (p = s->pool_first_large; p; p = t) {
>          t = p->next;
>          g_free(p);
>      }
>      s->pool_first_large = NULL;
> -    s->pool_cur = s->pool_end = NULL;
> +    s->pool_cur = s->pool_end = &dummy_pool;
>      s->pool_current = NULL;
>  }

Hmm, come to think of it, something like `&dummy_pool + 2` is not
allowed either, it's just that UBSAN doesn't complain about it. I will
send a v2.