Re: [PATCH v2 1/4] osdep: Make qemu_madvise() to set errno in all cases

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi Michal,

On 31/5/24 09:28, Michal Privoznik wrote:
> The unspoken premise of qemu_madvise() is that errno is set on
> error. And it is mostly the case except for posix_madvise() which
> is documented to return either zero (on success) or a positive
> error number.

Watch out, Linux:

   RETURN VALUE

      On success, posix_madvise() returns 0.  On failure,
      it returns a positive error number.

but on Darwin:

   RETURN VALUES

      Upon successful completion, a value of 0 is returned.
      Otherwise, a value of -1 is returned and errno is set
      to indicate the error.

(Haven't checked other POSIX OSes).

So we likely need more #ifdef'ry here.

> This means, we must set errno ourselves. And while
> at it, make the function return a negative value on error, just
> like other error paths do.
> 
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> ---
>   util/osdep.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/util/osdep.c b/util/osdep.c
> index e996c4744a..e42f4e8121 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -57,7 +57,12 @@ int qemu_madvise(void *addr, size_t len, int advice)
>   #if defined(CONFIG_MADVISE)
>       return madvise(addr, len, advice);
>   #elif defined(CONFIG_POSIX_MADVISE)
> -    return posix_madvise(addr, len, advice);
> +    int rc = posix_madvise(addr, len, advice);
> +    if (rc) {
> +        errno = rc;
> +        return -1;
> +    }
> +    return 0;
>   #else
>       errno = EINVAL;
>       return -1;