Re: [PATCH 4/6] tap: Don't risk truncating frames on full buffer in tap_pasta_input()

[Stefano Brivio <sbrivio@redhat.com>]

On Tue,  3 Sep 2024 22:02:33 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> tap_pasta_input() keeps reading frames from the tap device until the
> buffer is full.  However, this has an ugly edge case, when we get close
> to buffer full, we will provide just the remaining space as a read()
> buffer.  If this is shorter than the next frame to read, the tap device
> will truncate the frame and discard the remainder.
> 
> Adjust the code to make sure we always have room for a maximum size frame.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  tap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index 71742748..2fbcef04 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1076,8 +1076,8 @@ static void tap_pasta_input(struct ctx *c, const struct timespec *now)
>  
>  	tap_flush_pools();
>  
> -	for (n = 0; n < (ssize_t)TAP_BUF_BYTES; n += len) {
> -		len = read(c->fd_tap, pkt_buf + n, TAP_BUF_BYTES - n);
> +	for (n = 0; n < (ssize_t)TAP_BUF_BYTES - ETH_MAX_MTU; n += len) {

Logically speaking, this should be TAP_BUF_BYTES - ETH_MAX_MTU + 1, I
guess, because if we have ETH_MAX_MTU bytes left, we can read another
frame. Only if we have strictly less than that we might truncate one.

In any case, not a strong preference, and perhaps this version is
actually more readable.

> +		len = read(c->fd_tap, pkt_buf + n, ETH_MAX_MTU);
>  
>  		if (len <= 0) {
>  			if (errno == EINTR) {

-- 
Stefano