Re: [PATCH v2 4/9] util: Return valid allocation for qemu_try_memalign() with zero size

["Philippe Mathieu-Daudé" <philippe.mathieu.daude@gmail.com>]

On 4/3/22 12:21, Peter Maydell wrote:
> Currently qemu_try_memalign()'s behaviour if asked to allocate
> 0 bytes is rather variable:
>   * on Windows, we will assert
>   * on POSIX platforms, we get the underlying behaviour of
>     the posix_memalign() or equivalent function, which may be
>     either "return a valid non-NULL pointer" or "return NULL"
> 
> Explictly check for 0 byte allocations, so we get consistent
> behaviour across platforms.  We handle them by incrementing the size
> so that we return a valid non-NULL pointer that can later be passed
> to qemu_vfree().  This is permitted behaviour for the
> posix_memalign() API and is the most usual way that underlying
> malloc() etc implementations handle a zero-sized allocation request,
> because it won't trip up calling code that assumes NULL means an
> error.  (This includes our own qemu_memalign(), which will abort on
> NULL.)
> 
> This change is a preparation for sharing the qemu_try_memalign() code
> between Windows and POSIX.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   util/oslib-posix.c | 3 +++
>   util/oslib-win32.c | 4 +++-
>   2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index 0278902ee79..f7e22f4ff9b 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -209,6 +209,9 @@ void *qemu_try_memalign(size_t alignment, size_t size)
>           g_assert(is_power_of_2(alignment));
>       }
>   
> +    if (size == 0) {
> +        size++;
> +    }
>   #if defined(CONFIG_POSIX_MEMALIGN)
>       int ret;
>       ret = posix_memalign(&ptr, alignment, size);
> diff --git a/util/oslib-win32.c b/util/oslib-win32.c
> index 05857414695..8c28d70904d 100644
> --- a/util/oslib-win32.c
> +++ b/util/oslib-win32.c
> @@ -48,12 +48,14 @@ void *qemu_try_memalign(size_t alignment, size_t size)
>   {
>       void *ptr;
>   
> -    g_assert(size != 0);

Better, X_try_Y() functions are not supposed to fail.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>