Mercurial > hg > nginx

view src/http/v3/ngx_http_v3_filter_module.c @ 9282:acb8548c00e9

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
HTTP/3: fixed dynamic table overflow. While inserting a new entry into the dynamic table, first the entry is added, and then older entries are evicted until table size is within capacity. After the first step, the number of entries may temporarily exceed the maximum calculated from capacity by one entry, which previously caused table overflow. The easiest way to trigger the issue is to keep adding entries with empty names and values until first eviction. The issue was introduced by 987bee4363d1.
author Roman Arutyunyan <arut@nginx.com>
date Tue, 28 May 2024 17:18:50 +0400
parents 23f109f0facc
children
line wrap: on
line source


/*
 * Copyright (C) Roman Arutyunyan
 * Copyright (C) Nginx, Inc.
 */


#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_http.h>


/* static table indices */
#define NGX_HTTP_V3_HEADER_AUTHORITY                 0
#define NGX_HTTP_V3_HEADER_PATH_ROOT                 1
#define NGX_HTTP_V3_HEADER_CONTENT_LENGTH_ZERO       4
#define NGX_HTTP_V3_HEADER_DATE                      6
#define NGX_HTTP_V3_HEADER_LAST_MODIFIED             10
#define NGX_HTTP_V3_HEADER_LOCATION                  12
#define NGX_HTTP_V3_HEADER_METHOD_GET                17
#define NGX_HTTP_V3_HEADER_SCHEME_HTTP               22
#define NGX_HTTP_V3_HEADER_SCHEME_HTTPS              23
#define NGX_HTTP_V3_HEADER_STATUS_200                25
#define NGX_HTTP_V3_HEADER_ACCEPT_ENCODING           31
#define NGX_HTTP_V3_HEADER_CONTENT_TYPE_TEXT_PLAIN   53
#define NGX_HTTP_V3_HEADER_VARY_ACCEPT_ENCODING      59
#define NGX_HTTP_V3_HEADER_ACCEPT_LANGUAGE           72
#define NGX_HTTP_V3_HEADER_SERVER                    92
#define NGX_HTTP_V3_HEADER_USER_AGENT                95


typedef struct {
    ngx_chain_t         *free;
    ngx_chain_t         *busy;
} ngx_http_v3_filter_ctx_t;


static ngx_int_t ngx_http_v3_header_filter(ngx_http_request_t *r);
static ngx_int_t ngx_http_v3_body_filter(ngx_http_request_t *r,
    ngx_chain_t *in);
static ngx_chain_t *ngx_http_v3_create_trailers(ngx_http_request_t *r,
    ngx_http_v3_filter_ctx_t *ctx);
static ngx_int_t ngx_http_v3_filter_init(ngx_conf_t *cf);


static ngx_http_module_t  ngx_http_v3_filter_module_ctx = {
    NULL,                                  /* preconfiguration */
    ngx_http_v3_filter_init,               /* postconfiguration */

    NULL,                                  /* create main configuration */
    NULL,                                  /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    NULL,                                  /* create location configuration */
    NULL                                   /* merge location configuration */
};


ngx_module_t  ngx_http_v3_filter_module = {
    NGX_MODULE_V1,
    &ngx_http_v3_filter_module_ctx,        /* module context */
    NULL,                                  /* module directives */
    NGX_HTTP_MODULE,                       /* module type */
    NULL,                                  /* init master */
    NULL,                                  /* init module */
    NULL,                                  /* init process */
    NULL,                                  /* init thread */
    NULL,                                  /* exit thread */
    NULL,                                  /* exit process */
    NULL,                                  /* exit master */
    NGX_MODULE_V1_PADDING
};


static ngx_http_output_header_filter_pt  ngx_http_next_header_filter;
static ngx_http_output_body_filter_pt    ngx_http_next_body_filter;


static ngx_int_t
ngx_http_v3_header_filter(ngx_http_request_t *r)
{
    u_char                    *p;
    size_t                     len, n;
    ngx_buf_t                 *b;
    ngx_str_t                  host, location;
    ngx_uint_t                 i, port;
    ngx_chain_t               *out, *hl, *cl, **ll;
    ngx_list_part_t           *part;
    ngx_table_elt_t           *header;
    ngx_connection_t          *c;
    ngx_http_v3_session_t     *h3c;
    ngx_http_v3_filter_ctx_t  *ctx;
    ngx_http_core_loc_conf_t  *clcf;
    ngx_http_core_srv_conf_t  *cscf;
    u_char                     addr[NGX_SOCKADDR_STRLEN];

    if (r->http_version != NGX_HTTP_VERSION_30) {
        return ngx_http_next_header_filter(r);
    }

    if (r->header_sent) {
        return NGX_OK;
    }

    r->header_sent = 1;

    if (r != r->main) {
        return NGX_OK;
    }

    h3c = ngx_http_v3_get_session(r->connection);

    if (r->method == NGX_HTTP_HEAD) {
        r->header_only = 1;
    }

    if (r->headers_out.last_modified_time != -1) {
        if (r->headers_out.status != NGX_HTTP_OK
            && r->headers_out.status != NGX_HTTP_PARTIAL_CONTENT
            && r->headers_out.status != NGX_HTTP_NOT_MODIFIED)
        {
            r->headers_out.last_modified_time = -1;
            r->headers_out.last_modified = NULL;
        }
    }

    if (r->headers_out.status == NGX_HTTP_NO_CONTENT) {
        r->header_only = 1;
        ngx_str_null(&r->headers_out.content_type);
        r->headers_out.last_modified_time = -1;
        r->headers_out.last_modified = NULL;
        r->headers_out.content_length = NULL;
        r->headers_out.content_length_n = -1;
    }

    if (r->headers_out.status == NGX_HTTP_NOT_MODIFIED) {
        r->header_only = 1;
    }

    c = r->connection;

    out = NULL;
    ll = &out;

    len = ngx_http_v3_encode_field_section_prefix(NULL, 0, 0, 0);

    if (r->headers_out.status == NGX_HTTP_OK) {
        len += ngx_http_v3_encode_field_ri(NULL, 0,
                                           NGX_HTTP_V3_HEADER_STATUS_200);

    } else {
        len += ngx_http_v3_encode_field_lri(NULL, 0,
                                            NGX_HTTP_V3_HEADER_STATUS_200,
                                            NULL, 3);
    }

    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

    if (r->headers_out.server == NULL) {
        if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_ON) {
            n = sizeof(NGINX_VER) - 1;

        } else if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_BUILD) {
            n = sizeof(NGINX_VER_BUILD) - 1;

        } else {
            n = sizeof(NGINX_NAME) - 1;
        }

        len += ngx_http_v3_encode_field_lri(NULL, 0,
                                            NGX_HTTP_V3_HEADER_SERVER,
                                            NULL, n);
    }

    if (r->headers_out.date == NULL) {
        len += ngx_http_v3_encode_field_lri(NULL, 0, NGX_HTTP_V3_HEADER_DATE,
                                            NULL, ngx_cached_http_time.len);
    }

    if (r->headers_out.content_type.len) {
        n = r->headers_out.content_type.len;

        if (r->headers_out.content_type_len == r->headers_out.content_type.len
            && r->headers_out.charset.len)
        {
            n += sizeof("; charset=") - 1 + r->headers_out.charset.len;
        }

        len += ngx_http_v3_encode_field_lri(NULL, 0,
                                    NGX_HTTP_V3_HEADER_CONTENT_TYPE_TEXT_PLAIN,
                                    NULL, n);
    }

    if (r->headers_out.content_length == NULL) {
        if (r->headers_out.content_length_n > 0) {
            len += ngx_http_v3_encode_field_lri(NULL, 0,
                                        NGX_HTTP_V3_HEADER_CONTENT_LENGTH_ZERO,
                                        NULL, NGX_OFF_T_LEN);

        } else if (r->headers_out.content_length_n == 0) {
            len += ngx_http_v3_encode_field_ri(NULL, 0,
                                       NGX_HTTP_V3_HEADER_CONTENT_LENGTH_ZERO);
        }
    }

    if (r->headers_out.last_modified == NULL
        && r->headers_out.last_modified_time != -1)
    {
        len += ngx_http_v3_encode_field_lri(NULL, 0,
                                  NGX_HTTP_V3_HEADER_LAST_MODIFIED, NULL,
                                  sizeof("Mon, 28 Sep 1970 06:00:00 GMT") - 1);
    }

    if (r->headers_out.location && r->headers_out.location->value.len) {

        if (r->headers_out.location->value.data[0] == '/'
            && clcf->absolute_redirect)
        {
            if (clcf->server_name_in_redirect) {
                cscf = ngx_http_get_module_srv_conf(r, ngx_http_core_module);
                host = cscf->server_name;

            } else if (r->headers_in.server.len) {
                host = r->headers_in.server;

            } else {
                host.len = NGX_SOCKADDR_STRLEN;
                host.data = addr;

                if (ngx_connection_local_sockaddr(c, &host, 0) != NGX_OK) {
                    return NGX_ERROR;
                }
            }

            port = ngx_inet_get_port(c->local_sockaddr);

            location.len = sizeof("https://") - 1 + host.len
                           + r->headers_out.location->value.len;

            if (clcf->port_in_redirect) {
                port = (port == 443) ? 0 : port;

            } else {
                port = 0;
            }

            if (port) {
                location.len += sizeof(":65535") - 1;
            }

            location.data = ngx_pnalloc(r->pool, location.len);
            if (location.data == NULL) {
                return NGX_ERROR;
            }

            p = ngx_cpymem(location.data, "https://", sizeof("https://") - 1);
            p = ngx_cpymem(p, host.data, host.len);

            if (port) {
                p = ngx_sprintf(p, ":%ui", port);
            }

            p = ngx_cpymem(p, r->headers_out.location->value.data,
                              r->headers_out.location->value.len);

            /* update r->headers_out.location->value for possible logging */

            r->headers_out.location->value.len = p - location.data;
            r->headers_out.location->value.data = location.data;
            ngx_str_set(&r->headers_out.location->key, "Location");
        }

        r->headers_out.location->hash = 0;

        len += ngx_http_v3_encode_field_lri(NULL, 0,
                                           NGX_HTTP_V3_HEADER_LOCATION, NULL,
                                           r->headers_out.location->value.len);
    }

#if (NGX_HTTP_GZIP)
    if (r->gzip_vary) {
        if (clcf->gzip_vary) {
            len += ngx_http_v3_encode_field_ri(NULL, 0,
                                      NGX_HTTP_V3_HEADER_VARY_ACCEPT_ENCODING);

        } else {
            r->gzip_vary = 0;
        }
    }
#endif

    part = &r->headers_out.headers.part;
    header = part->elts;

    for (i = 0; /* void */; i++) {

        if (i >= part->nelts) {
            if (part->next == NULL) {
                break;
            }

            part = part->next;
            header = part->elts;
            i = 0;
        }

        if (header[i].hash == 0) {
            continue;
        }

        len += ngx_http_v3_encode_field_l(NULL, &header[i].key,
                                          &header[i].value);
    }

    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0, "http3 header len:%uz", len);

    b = ngx_create_temp_buf(r->pool, len);
    if (b == NULL) {
        return NGX_ERROR;
    }

    b->last = (u_char *) ngx_http_v3_encode_field_section_prefix(b->last,
                                                                 0, 0, 0);

    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http3 output header: \":status: %03ui\"",
                   r->headers_out.status);

    if (r->headers_out.status == NGX_HTTP_OK) {
        b->last = (u_char *) ngx_http_v3_encode_field_ri(b->last, 0,
                                                NGX_HTTP_V3_HEADER_STATUS_200);

    } else {
        b->last = (u_char *) ngx_http_v3_encode_field_lri(b->last, 0,
                                                 NGX_HTTP_V3_HEADER_STATUS_200,
                                                 NULL, 3);
        b->last = ngx_sprintf(b->last, "%03ui", r->headers_out.status);
    }

    if (r->headers_out.server == NULL) {
        if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_ON) {
            p = (u_char *) NGINX_VER;
            n = sizeof(NGINX_VER) - 1;

        } else if (clcf->server_tokens == NGX_HTTP_SERVER_TOKENS_BUILD) {
            p = (u_char *) NGINX_VER_BUILD;
            n = sizeof(NGINX_VER_BUILD) - 1;

        } else {
            p = (u_char *) NGINX_NAME;
            n = sizeof(NGINX_NAME) - 1;
        }

        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"server: %*s\"", n, p);

        b->last = (u_char *) ngx_http_v3_encode_field_lri(b->last, 0,
                                                     NGX_HTTP_V3_HEADER_SERVER,
                                                     p, n);
    }

    if (r->headers_out.date == NULL) {
        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"date: %V\"",
                       &ngx_cached_http_time);

        b->last = (u_char *) ngx_http_v3_encode_field_lri(b->last, 0,
                                                     NGX_HTTP_V3_HEADER_DATE,
                                                     ngx_cached_http_time.data,
                                                     ngx_cached_http_time.len);
    }

    if (r->headers_out.content_type.len) {
        if (r->headers_out.content_type_len == r->headers_out.content_type.len
            && r->headers_out.charset.len)
        {
            n = r->headers_out.content_type.len + sizeof("; charset=") - 1
                + r->headers_out.charset.len;

            p = ngx_pnalloc(r->pool, n);
            if (p == NULL) {
                return NGX_ERROR;
            }

            p = ngx_cpymem(p, r->headers_out.content_type.data,
                           r->headers_out.content_type.len);

            p = ngx_cpymem(p, "; charset=", sizeof("; charset=") - 1);

            p = ngx_cpymem(p, r->headers_out.charset.data,
                           r->headers_out.charset.len);

            /* updated r->headers_out.content_type is also needed for logging */

            r->headers_out.content_type.len = n;
            r->headers_out.content_type.data = p - n;
        }

        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"content-type: %V\"",
                       &r->headers_out.content_type);

        b->last = (u_char *) ngx_http_v3_encode_field_lri(b->last, 0,
                                    NGX_HTTP_V3_HEADER_CONTENT_TYPE_TEXT_PLAIN,
                                    r->headers_out.content_type.data,
                                    r->headers_out.content_type.len);
    }

    if (r->headers_out.content_length == NULL
        && r->headers_out.content_length_n >= 0)
    {
        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"content-length: %O\"",
                       r->headers_out.content_length_n);

        if (r->headers_out.content_length_n > 0) {
            p = ngx_sprintf(b->last, "%O", r->headers_out.content_length_n);
            n = p - b->last;

            b->last = (u_char *) ngx_http_v3_encode_field_lri(b->last, 0,
                                        NGX_HTTP_V3_HEADER_CONTENT_LENGTH_ZERO,
                                        NULL, n);

            b->last = ngx_sprintf(b->last, "%O",
                                  r->headers_out.content_length_n);

        } else {
            b->last = (u_char *) ngx_http_v3_encode_field_ri(b->last, 0,
                                       NGX_HTTP_V3_HEADER_CONTENT_LENGTH_ZERO);
        }
    }

    if (r->headers_out.last_modified == NULL
        && r->headers_out.last_modified_time != -1)
    {
        n = sizeof("Mon, 28 Sep 1970 06:00:00 GMT") - 1;

        p = ngx_pnalloc(r->pool, n);
        if (p == NULL) {
            return NGX_ERROR;
        }

        ngx_http_time(p, r->headers_out.last_modified_time);

        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"last-modified: %*s\"", n, p);

        b->last = (u_char *) ngx_http_v3_encode_field_lri(b->last, 0,
                                              NGX_HTTP_V3_HEADER_LAST_MODIFIED,
                                              p, n);
    }

    if (r->headers_out.location && r->headers_out.location->value.len) {
        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"location: %V\"",
                       &r->headers_out.location->value);

        b->last = (u_char *) ngx_http_v3_encode_field_lri(b->last, 0,
                                           NGX_HTTP_V3_HEADER_LOCATION,
                                           r->headers_out.location->value.data,
                                           r->headers_out.location->value.len);
    }

#if (NGX_HTTP_GZIP)
    if (r->gzip_vary) {
        ngx_log_debug0(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"vary: Accept-Encoding\"");

        b->last = (u_char *) ngx_http_v3_encode_field_ri(b->last, 0,
                                      NGX_HTTP_V3_HEADER_VARY_ACCEPT_ENCODING);
    }
#endif

    part = &r->headers_out.headers.part;
    header = part->elts;

    for (i = 0; /* void */; i++) {

        if (i >= part->nelts) {
            if (part->next == NULL) {
                break;
            }

            part = part->next;
            header = part->elts;
            i = 0;
        }

        if (header[i].hash == 0) {
            continue;
        }

        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, c->log, 0,
                       "http3 output header: \"%V: %V\"",
                       &header[i].key, &header[i].value);

        b->last = (u_char *) ngx_http_v3_encode_field_l(b->last,
                                                        &header[i].key,
                                                        &header[i].value);
    }

    if (r->header_only) {
        b->last_buf = 1;
    }

    cl = ngx_alloc_chain_link(r->pool);
    if (cl == NULL) {
        return NGX_ERROR;
    }

    cl->buf = b;
    cl->next = NULL;

    n = b->last - b->pos;

    h3c->payload_bytes += n;

    len = ngx_http_v3_encode_varlen_int(NULL, NGX_HTTP_V3_FRAME_HEADERS)
          + ngx_http_v3_encode_varlen_int(NULL, n);

    b = ngx_create_temp_buf(r->pool, len);
    if (b == NULL) {
        return NGX_ERROR;
    }

    b->last = (u_char *) ngx_http_v3_encode_varlen_int(b->last,
                                                    NGX_HTTP_V3_FRAME_HEADERS);
    b->last = (u_char *) ngx_http_v3_encode_varlen_int(b->last, n);

    hl = ngx_alloc_chain_link(r->pool);
    if (hl == NULL) {
        return NGX_ERROR;
    }

    hl->buf = b;
    hl->next = cl;

    *ll = hl;
    ll = &cl->next;

    if (r->headers_out.content_length_n >= 0
        && !r->header_only && !r->expect_trailers)
    {
        len = ngx_http_v3_encode_varlen_int(NULL, NGX_HTTP_V3_FRAME_DATA)
              + ngx_http_v3_encode_varlen_int(NULL,
                                              r->headers_out.content_length_n);

        b = ngx_create_temp_buf(r->pool, len);
        if (b == NULL) {
            return NGX_ERROR;
        }

        b->last = (u_char *) ngx_http_v3_encode_varlen_int(b->last,
                                                       NGX_HTTP_V3_FRAME_DATA);
        b->last = (u_char *) ngx_http_v3_encode_varlen_int(b->last,
                                              r->headers_out.content_length_n);

        h3c->payload_bytes += r->headers_out.content_length_n;
        h3c->total_bytes += r->headers_out.content_length_n;

        cl = ngx_alloc_chain_link(r->pool);
        if (cl == NULL) {
            return NGX_ERROR;
        }

        cl->buf = b;
        cl->next = NULL;

        *ll = cl;

    } else {
        ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_v3_filter_ctx_t));
        if (ctx == NULL) {
            return NGX_ERROR;
        }

        ngx_http_set_ctx(r, ctx, ngx_http_v3_filter_module);
    }

    for (cl = out; cl; cl = cl->next) {
        h3c->total_bytes += cl->buf->last - cl->buf->pos;
        r->header_size += cl->buf->last - cl->buf->pos;
    }

    return ngx_http_write_filter(r, out);
}


static ngx_int_t
ngx_http_v3_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    u_char                    *chunk;
    off_t                      size;
    ngx_int_t                  rc;
    ngx_buf_t                 *b;
    ngx_chain_t               *out, *cl, *tl, **ll;
    ngx_http_v3_session_t     *h3c;
    ngx_http_v3_filter_ctx_t  *ctx;

    if (in == NULL) {
        return ngx_http_next_body_filter(r, in);
    }

    ctx = ngx_http_get_module_ctx(r, ngx_http_v3_filter_module);
    if (ctx == NULL) {
        return ngx_http_next_body_filter(r, in);
    }

    h3c = ngx_http_v3_get_session(r->connection);

    out = NULL;
    ll = &out;

    size = 0;
    cl = in;

    for ( ;; ) {
        ngx_log_debug1(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                       "http3 chunk: %O", ngx_buf_size(cl->buf));

        size += ngx_buf_size(cl->buf);

        if (cl->buf->flush
            || cl->buf->sync
            || ngx_buf_in_memory(cl->buf)
            || cl->buf->in_file)
        {
            tl = ngx_alloc_chain_link(r->pool);
            if (tl == NULL) {
                return NGX_ERROR;
            }

            tl->buf = cl->buf;
            *ll = tl;
            ll = &tl->next;
        }

        if (cl->next == NULL) {
            break;
        }

        cl = cl->next;
    }

    if (size) {
        tl = ngx_chain_get_free_buf(r->pool, &ctx->free);
        if (tl == NULL) {
            return NGX_ERROR;
        }

        b = tl->buf;
        chunk = b->start;

        if (chunk == NULL) {
            chunk = ngx_palloc(r->pool, NGX_HTTP_V3_VARLEN_INT_LEN * 2);
            if (chunk == NULL) {
                return NGX_ERROR;
            }

            b->start = chunk;
            b->end = chunk + NGX_HTTP_V3_VARLEN_INT_LEN * 2;
        }

        b->tag = (ngx_buf_tag_t) &ngx_http_v3_filter_module;
        b->memory = 0;
        b->temporary = 1;
        b->pos = chunk;

        b->last = (u_char *) ngx_http_v3_encode_varlen_int(chunk,
                                                       NGX_HTTP_V3_FRAME_DATA);
        b->last = (u_char *) ngx_http_v3_encode_varlen_int(b->last, size);

        tl->next = out;
        out = tl;

        h3c->payload_bytes += size;
    }

    if (cl->buf->last_buf) {
        tl = ngx_http_v3_create_trailers(r, ctx);
        if (tl == NULL) {
            return NGX_ERROR;
        }

        cl->buf->last_buf = 0;

        *ll = tl;

    } else {
        *ll = NULL;
    }

    for (cl = out; cl; cl = cl->next) {
        h3c->total_bytes += cl->buf->last - cl->buf->pos;
    }

    rc = ngx_http_next_body_filter(r, out);

    ngx_chain_update_chains(r->pool, &ctx->free, &ctx->busy, &out,
                            (ngx_buf_tag_t) &ngx_http_v3_filter_module);

    return rc;
}


static ngx_chain_t *
ngx_http_v3_create_trailers(ngx_http_request_t *r,
    ngx_http_v3_filter_ctx_t *ctx)
{
    size_t                  len, n;
    u_char                 *p;
    ngx_buf_t              *b;
    ngx_uint_t              i;
    ngx_chain_t            *cl, *hl;
    ngx_list_part_t        *part;
    ngx_table_elt_t        *header;
    ngx_http_v3_session_t  *h3c;

    h3c = ngx_http_v3_get_session(r->connection);

    len = 0;

    part = &r->headers_out.trailers.part;
    header = part->elts;

    for (i = 0; /* void */; i++) {

        if (i >= part->nelts) {
            if (part->next == NULL) {
                break;
            }

            part = part->next;
            header = part->elts;
            i = 0;
        }

        if (header[i].hash == 0) {
            continue;
        }

        len += ngx_http_v3_encode_field_l(NULL, &header[i].key,
                                          &header[i].value);
    }

    cl = ngx_chain_get_free_buf(r->pool, &ctx->free);
    if (cl == NULL) {
        return NULL;
    }

    b = cl->buf;

    b->tag = (ngx_buf_tag_t) &ngx_http_v3_filter_module;
    b->memory = 0;
    b->last_buf = 1;

    if (len == 0) {
        b->temporary = 0;
        b->pos = b->last = NULL;
        return cl;
    }

    b->temporary = 1;

    len += ngx_http_v3_encode_field_section_prefix(NULL, 0, 0, 0);

    b->pos = ngx_palloc(r->pool, len);
    if (b->pos == NULL) {
        return NULL;
    }

    b->last = (u_char *) ngx_http_v3_encode_field_section_prefix(b->pos,
                                                                 0, 0, 0);

    part = &r->headers_out.trailers.part;
    header = part->elts;

    for (i = 0; /* void */; i++) {

        if (i >= part->nelts) {
            if (part->next == NULL) {
                break;
            }

            part = part->next;
            header = part->elts;
            i = 0;
        }

        if (header[i].hash == 0) {
            continue;
        }

        ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                       "http3 output trailer: \"%V: %V\"",
                       &header[i].key, &header[i].value);

        b->last = (u_char *) ngx_http_v3_encode_field_l(b->last,
                                                        &header[i].key,
                                                        &header[i].value);
    }

    n = b->last - b->pos;

    h3c->payload_bytes += n;

    hl = ngx_chain_get_free_buf(r->pool, &ctx->free);
    if (hl == NULL) {
        return NULL;
    }

    b = hl->buf;
    p = b->start;

    if (p == NULL) {
        p = ngx_palloc(r->pool, NGX_HTTP_V3_VARLEN_INT_LEN * 2);
        if (p == NULL) {
            return NULL;
        }

        b->start = p;
        b->end = p + NGX_HTTP_V3_VARLEN_INT_LEN * 2;
    }

    b->tag = (ngx_buf_tag_t) &ngx_http_v3_filter_module;
    b->memory = 0;
    b->temporary = 1;
    b->pos = p;

    b->last = (u_char *) ngx_http_v3_encode_varlen_int(p,
                                                    NGX_HTTP_V3_FRAME_HEADERS);
    b->last = (u_char *) ngx_http_v3_encode_varlen_int(b->last, n);

    hl->next = cl;

    return hl;
}


static ngx_int_t
ngx_http_v3_filter_init(ngx_conf_t *cf)
{
    ngx_http_next_header_filter = ngx_http_top_header_filter;
    ngx_http_top_header_filter = ngx_http_v3_header_filter;

    ngx_http_next_body_filter = ngx_http_top_body_filter;
    ngx_http_top_body_filter = ngx_http_v3_body_filter;

    return NGX_OK;
}