--- /dev/null
+
+/*
+ * Copyright (C) Nginx, Inc.
+ */
+
+
+/*
+ * JSON string unescape. Decodes escape sequences defined by
+ * RFC 8259, Section 7, including \uXXXX and UTF-16 surrogate pairs.
+ */
+
+
+#include <ngx_config.h>
+#include <ngx_core.h>
+#include <ngx_json_unescape.h>
+
+
+static ngx_inline u_char *ngx_json_utf8_encode(u_char *dst,
+ uint32_t codepoint);
+
+
+ngx_int_t
+ngx_json_unescape_string(ngx_str_t *str)
+{
+ size_t size;
+ u_char *d, *s, *start, ch;
+ uint32_t codepoint, high_surrogate;
+ ngx_int_t n;
+ ngx_uint_t hex_left;
+
+ enum {
+ sw_usual = 0,
+ sw_quoted,
+ sw_hex,
+ sw_surrogate_start, /* expect '\' of \uDCxx after high surrogate */
+ sw_surrogate_u /* expect 'u' */
+ } state;
+
+ /*
+ * RFC 8259, Section 7:
+ *
+ * string = quotation-mark *char quotation-mark
+ *
+ * char = unescaped /
+ * escape (
+ * %x22 / ; " quotation mark U+0022
+ * %x5C / ; \ reverse solidus U+005C
+ * %x2F / ; / solidus U+002F
+ * %x62 / ; b backspace U+0008
+ * %x66 / ; f form feed U+000C
+ * %x6E / ; n line feed U+000A
+ * %x72 / ; r carriage return U+000D
+ * %x74 / ; t tab U+0009
+ * %x75 4HEXDIG ) ; uXXXX U+XXXX
+ *
+ * Non-BMP code points are encoded as a UTF-16 surrogate pair:
+ * \uD800..\uDBFF high surrogate
+ * \uDC00..\uDFFF low surrogate
+ * The pair decodes to U+10000 + (high - 0xD800) * 0x400
+ * + (low - 0xDC00).
+ * The input is an unquoted JSON string body (the bytes between the
+ * surrounding double quotes); it is decoded in place.
+ */
+
+ if (str->len == 0) {
+ return NGX_OK;
+ }
+
+ if (str->data == NULL) {
+ return NGX_ERROR;
+ }
+
+ start = str->data;
+ size = str->len;
+
+ d = s = start;
+
+ state = sw_usual;
+ codepoint = 0;
+ high_surrogate = 0;
+ hex_left = 0;
+
+ while (size--) {
+
+ ch = *s++;
+
+ switch (state) {
+ case sw_usual:
+
+ if (ch == '"') {
+ return NGX_ERROR;
+ }
+
+ if (ch == '\\') {
+ state = sw_quoted;
+ break;
+ }
+
+ if (ch < 0x20) {
+ /* RFC 8259: control characters must be escaped */
+ return NGX_ERROR;
+ }
+
+ *d++ = ch;
+ break;
+
+ case sw_quoted:
+
+ switch (ch) {
+
+ case 'u':
+ codepoint = 0;
+ hex_left = 4;
+ state = sw_hex;
+ break;
+
+ case '"':
+ case '/':
+ case '\\':
+ *d++ = ch;
+ state = sw_usual;
+ break;
+
+ case 'b':
+ *d++ = '\b';
+ state = sw_usual;
+ break;
+
+ case 'f':
+ *d++ = '\f';
+ state = sw_usual;
+ break;
+
+ case 'n':
+ *d++ = '\n';
+ state = sw_usual;
+ break;
+
+ case 'r':
+ *d++ = '\r';
+ state = sw_usual;
+ break;
+
+ case 't':
+ *d++ = '\t';
+ state = sw_usual;
+ break;
+
+ default:
+ return NGX_ERROR;
+ }
+
+ break;
+
+ case sw_hex:
+
+ n = ngx_json_hex_digit(ch);
+ if (n == NGX_ERROR) {
+ return NGX_ERROR;
+ }
+
+ codepoint = (codepoint << 4) | (uint32_t) n;
+
+ if (--hex_left > 0) {
+ break;
+ }
+
+ if (high_surrogate) {
+ if (codepoint < 0xDC00 || codepoint > 0xDFFF) {
+ return NGX_ERROR;
+ }
+
+ codepoint = 0x10000
+ + ((high_surrogate - 0xD800) << 10)
+ + (codepoint - 0xDC00);
+ d = ngx_json_utf8_encode(d, codepoint);
+ high_surrogate = 0;
+ state = sw_usual;
+ break;
+ }
+
+ if (codepoint >= 0xD800 && codepoint <= 0xDBFF) {
+ /* high surrogate - wait for the low surrogate */
+ high_surrogate = codepoint;
+ state = sw_surrogate_start;
+ break;
+ }
+
+ if (codepoint >= 0xDC00 && codepoint <= 0xDFFF) {
+ /* lone low surrogate */
+ return NGX_ERROR;
+ }
+
+ d = ngx_json_utf8_encode(d, codepoint);
+ state = sw_usual;
+ break;
+
+ case sw_surrogate_start:
+ /*
+ * Expect '\' to begin the low-surrogate escape.
+ * Lone surrogates are not valid Unicode scalar values
+ * (RFC 8259, Section 8.2).
+ */
+ if (ch == '\\') {
+ state = sw_surrogate_u;
+ break;
+ }
+
+ /* lone high surrogate */
+ return NGX_ERROR;
+
+ case sw_surrogate_u:
+ if (ch == 'u') {
+ codepoint = 0;
+ hex_left = 4;
+ state = sw_hex;
+ break;
+ }
+
+ /* lone high surrogate */
+ return NGX_ERROR;
+ }
+ }
+
+ if (state != sw_usual) {
+ /* truncated escape sequence or unpaired high surrogate */
+ return NGX_ERROR;
+ }
+
+ str->data = start;
+ str->len = d - start;
+
+ return NGX_OK;
+}
+
+
+static ngx_inline u_char *
+ngx_json_utf8_encode(u_char *dst, uint32_t codepoint)
+{
+ if (codepoint <= 0x7F) {
+ *dst++ = (u_char) codepoint;
+
+ } else if (codepoint <= 0x7FF) {
+ *dst++ = (u_char) (0xC0 | (codepoint >> 6));
+ *dst++ = (u_char) (0x80 | (codepoint & 0x3F));
+
+ } else if (codepoint <= 0xFFFF) {
+ *dst++ = (u_char) (0xE0 | (codepoint >> 12));
+ *dst++ = (u_char) (0x80 | ((codepoint >> 6) & 0x3F));
+ *dst++ = (u_char) (0x80 | (codepoint & 0x3F));
+
+ } else {
+ *dst++ = (u_char) (0xF0 | (codepoint >> 18));
+ *dst++ = (u_char) (0x80 | ((codepoint >> 12) & 0x3F));
+ *dst++ = (u_char) (0x80 | ((codepoint >> 6) & 0x3F));
+ *dst++ = (u_char) (0x80 | (codepoint & 0x3F));
+ }
+
+ return dst;
+}
+
+
+ngx_int_t
+ngx_json_hex_digit(u_char ch)
+{
+ if (ch >= '0' && ch <= '9') {
+ return ch - '0';
+ }
+
+ ch = (u_char) (ch | 0x20);
+
+ if (ch >= 'a' && ch <= 'f') {
+ return ch - 'a' + 10;
+ }
+
+ return NGX_ERROR;
+}
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