[PATCH] HTTP/2: fixed buffer management with HTTP/2 auto-detection

[Sergey Kandaurov]

> On 20 Oct 2023, at 17:23, Sergey Kandaurov <pluknet at nginx.com> wrote:
> 
> # HG changeset patch
> # User Sergey Kandaurov <pluknet at nginx.com>
> # Date 1697808142 -14400
> #      Fri Oct 20 17:22:22 2023 +0400
> # Node ID 318c8ace6aa24506004bfbb7d52674f61a3716a5
> # Parent  3038bd4d78169a5e8a2624d79cf76f45f0805ddc
> HTTP/2: fixed buffer management with HTTP/2 auto-detection.
> 
> As part of normal HTTP/2 processing, incomplete frames are saved in the
> control state using a fixed size memcpy of NGX_HTTP_V2_STATE_BUFFER_SIZE.
> For this matter, two state buffers are reserved in the HTTP/2 recv buffer.
> 
> As part of HTTP/2 auto-detection on plain TCP connections, initial data
> is first read into a buffer specified by the client_header_buffer_size
> directive that doesn't have state reservation.  Previously, this made it
> possible to over-read the buffer as part of saving the state.
> 
> The fix is to read the available buffer size rather than a fixed size.
> Although memcpy of a fixed size can produce a better optimized code,

From my limited testing, replacing a fixed size with an available size
degrades "-O" optimized memcpy from SSE instructions over XMM registers
to simple MOVs.

> handling of incomplete frames isn't a common execution path, so it was
> sacrificed for the sake of simplicity of the fix.

Another approach is to displace initial data into the recv buffer
for subsequent processing, which would require additional handling
in ngx_http_v2_init().  After some pondering I declined it due to
added complexity without a good reason.

> 
> diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
> --- a/src/http/v2/ngx_http_v2.c
> +++ b/src/http/v2/ngx_http_v2.c
> @@ -386,13 +386,11 @@ ngx_http_v2_read_handler(ngx_event_t *re
>     h2mcf = ngx_http_get_module_main_conf(h2c->http_connection->conf_ctx,
>                                           ngx_http_v2_module);
> 
> -    available = h2mcf->recv_buffer_size - 2 * NGX_HTTP_V2_STATE_BUFFER_SIZE;
> +    available = h2mcf->recv_buffer_size - NGX_HTTP_V2_STATE_BUFFER_SIZE;
> 
>     do {
>         p = h2mcf->recv_buffer;
> -
> -        ngx_memcpy(p, h2c->state.buffer, NGX_HTTP_V2_STATE_BUFFER_SIZE);
> -        end = p + h2c->state.buffer_used;
> +        end = ngx_cpymem(p, h2c->state.buffer, h2c->state.buffer_used);
> 
>         n = c->recv(c, end, available);
> 
> @@ -2592,7 +2590,7 @@ ngx_http_v2_state_save(ngx_http_v2_conne
>         return ngx_http_v2_connection_error(h2c, NGX_HTTP_V2_INTERNAL_ERROR);
>     }
> 
> -    ngx_memcpy(h2c->state.buffer, pos, NGX_HTTP_V2_STATE_BUFFER_SIZE);
> +    ngx_memcpy(h2c->state.buffer, pos, size);
> 
>     h2c->state.buffer_used = size;
>     h2c->state.handler = handler;
> diff --git a/src/http/v2/ngx_http_v2_module.c b/src/http/v2/ngx_http_v2_module.c
> --- a/src/http/v2/ngx_http_v2_module.c
> +++ b/src/http/v2/ngx_http_v2_module.c
> @@ -388,7 +388,7 @@ ngx_http_v2_recv_buffer_size(ngx_conf_t 
> {
>     size_t *sp = data;
> 
> -    if (*sp <= 2 * NGX_HTTP_V2_STATE_BUFFER_SIZE) {
> +    if (*sp <= NGX_HTTP_V2_STATE_BUFFER_SIZE) {
>         return "value is too small";
>     }
> 

-- 
Sergey Kandaurov