Re: [bpf PATCH] bpf: sockmap, decrement copied count correctly in redirect error case

[Daniel Borkmann <daniel@iogearbox.net>]

On 08/25/2018 02:37 AM, John Fastabend wrote:
> Currently, when a redirect occurs in sockmap and an error occurs in
> the redirect call we unwind the scatterlist once in the error path
> of bpf_tcp_sendmsg_do_redirect() and then again in sendmsg(). Then
> in the error path of sendmsg we decrement the copied count by the
> send size.
> 
> However, its possible we partially sent data before the error was
> generated. This can happen if do_tcp_sendpages() partially sends the
> scatterlist before encountering a memory pressure error. If this
> happens we need to decrement the copied value (the value tracking
> how many bytes were actually sent to TCP stack) by the number of
> remaining bytes _not_ the entire send size. Otherwise we risk
> confusing userspace.
> 
> Also we don't need two calls to free the scatterlist one is
> good enough. So remove the one in bpf_tcp_sendmsg_do_redirect() and
> then properly reduce copied by the number of remaining bytes which
> may in fact be the entire send size if no bytes were sent.
> 
> To do this use bool to indicate if free_start_sg() should do mem
> accounting or not.
> 
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>

Applied to bpf, thanks John!