Re: [Qemu-devel] [PATCH] qemu-char: reset errno before qemu char write or read action

[Eric Blake <eblake@redhat.com>]

On 07/01/2018 07:49 PM, xinhua.Cao wrote:
> In the tcp_chr_write function, we checked errno,
> but errno was not reset before a read or write operation.
> Therefore, this check of errno's actions is often
> incorrect after EAGAIN has occurred.
> We reset errno before reading and writing to
> ensure the correctness of errno's judgment
> 
> Signed-off-by: xinhua.Cao <caoxinhua@huawei.com>
> ---
>   chardev/char-fe.c | 1 +
>   chardev/char.c    | 2 ++
>   2 files changed, 3 insertions(+)
> 
> diff --git a/chardev/char-fe.c b/chardev/char-fe.c
> index b1f228e..d96ca6f 100644
> --- a/chardev/char-fe.c
> +++ b/chardev/char-fe.c
> @@ -69,6 +69,7 @@ int qemu_chr_fe_read_all(CharBackend *be, uint8_t *buf, int len)
>   
>       while (offset < len) {
>       retry:
> +        errno = 0;
>           res = CHARDEV_GET_CLASS(s)->chr_sync_read(s, buf + offset,
>                                                     len - offset);
>           if (res == -1 && errno == EAGAIN) {

NACK.  read() is guaranteed to have set errno if res is -1, and we don't 
check errno unless we first checked that res was -1.  Thus, the EAGAIN 
check is not confused by a stale errno value.


-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org