[PATCH v5 1/2] json: Add function to unescape JSON-encoded strings

[Patrick Steinhardt]

JSON strings require certain characters to be encoded, either by using a
single reverse solidus character "\" for a set of popular characters, or
by using a Unicode representation of "\uXXXXX". The jsmn library doesn't
handle unescaping for us, so we must implement this functionality for
ourselves.

Add a new function `grub_json_unescape ()` that takes a potentially
escaped JSON string as input and returns a new unescaped string.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 grub-core/lib/json/json.c | 118 ++++++++++++++++++++++++++++++++++++++
 grub-core/lib/json/json.h |  12 ++++
 2 files changed, 130 insertions(+)

diff --git a/grub-core/lib/json/json.c b/grub-core/lib/json/json.c
index 1c20c75ea..1eadd1ce9 100644
--- a/grub-core/lib/json/json.c
+++ b/grub-core/lib/json/json.c
@@ -262,3 +262,121 @@ grub_json_getint64 (grub_int64_t *out, const grub_json_t 
*parent, const char *ke
 
   return GRUB_ERR_NONE;
 }
+
+grub_err_t
+grub_json_unescape (char **out, grub_size_t *outlen, const char *in, 
grub_size_t inlen)
+{
+  grub_err_t ret = GRUB_ERR_NONE;
+  grub_size_t inpos, resultpos;
+  char *result;
+
+  if (out == NULL || outlen == NULL)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("output parameters are not 
set"));
+
+  result = grub_calloc (1, inlen + 1);
+  if (result == NULL)
+    return GRUB_ERR_OUT_OF_MEMORY;
+
+  for (inpos = resultpos = 0; inpos < inlen; inpos++)
+    {
+      if (in[inpos] == '\\')
+       {
+         inpos++;
+         if (inpos >= inlen)
+           {
+             ret = grub_error (GRUB_ERR_BAD_ARGUMENT, N_("expected escaped 
character"));
+             goto err;
+           }
+
+         switch (in[inpos])
+           {
+             case '"':
+               result[resultpos++] = '"';
+               break;
+
+             case '/':
+               result[resultpos++] = '/';
+               break;
+
+             case '\\':
+               result[resultpos++] = '\\';
+               break;
+
+             case 'b':
+               result[resultpos++] = '\b';
+               break;
+
+             case 'f':
+               result[resultpos++] = '\f';
+               break;
+
+             case 'r':
+               result[resultpos++] = '\r';
+               break;
+
+             case 'n':
+               result[resultpos++] = '\n';
+               break;
+
+             case 't':
+               result[resultpos++] = '\t';
+               break;
+
+             case 'u':
+               {
+                 char values[4] = {0};
+                 unsigned i;
+
+                 inpos++;
+                 if (inpos + ARRAY_SIZE(values) > inlen)
+                   {
+                     ret = grub_error (GRUB_ERR_BAD_ARGUMENT, N_("unicode 
sequence too short"));
+                     goto err;
+                   }
+
+                 for (i = 0; i < ARRAY_SIZE(values); i++)
+                   {
+                     char c = in[inpos++];
+
+                     if (c >= '0' && c <= '9')
+                       values[i] = c - '0';
+                     else if (c >= 'A' && c <= 'F')
+                       values[i] = c - 'A' + 10;
+                     else if (c >= 'a' && c <= 'f')
+                       values[i] = c - 'a' + 10;
+                     else
+                       {
+                         ret = grub_error (GRUB_ERR_BAD_ARGUMENT,
+                                           N_("unicode sequence with invalid 
character '%c'"), c);
+                         goto err;
+                       }
+                   }
+
+                 if (values[0] != 0 || values[1] != 0)
+                   result[resultpos++] = values[0] << 4 | values[1];
+                 result[resultpos++] = values[2] << 4 | values[3];
+
+                 /* Offset the increment that's coming in via the loop 
increment. */
+                 inpos--;
+
+                 break;
+               }
+
+             default:
+               ret = grub_error (GRUB_ERR_BAD_ARGUMENT, N_("unrecognized 
escaped character '%c'"), in[inpos]);
+               goto err;
+           }
+       }
+      else
+         result[resultpos++] = in[inpos];
+    }
+
+  *out = result;
+  *outlen = resultpos;
+
+ err:
+  if (ret != GRUB_ERR_NONE)
+    grub_free (result);
+
+  return ret;
+}
diff --git a/grub-core/lib/json/json.h b/grub-core/lib/json/json.h
index 4ea2a22d8..626074c35 100644
--- a/grub-core/lib/json/json.h
+++ b/grub-core/lib/json/json.h
@@ -125,4 +125,16 @@ extern grub_err_t EXPORT_FUNC(grub_json_getint64) 
(grub_int64_t *out,
                                                   const grub_json_t *parent,
                                                   const char *key);
 
+/*
+ * Unescape escaped characters and Unicode sequences in the
+ * given JSON-encoded string. Returns a newly allocated string
+ * passed back via the `out` parameter that has a length of
+ * `*outlen`.
+ *
+ * See https://datatracker.ietf.org/doc/html/rfc8259#section-7 for more
+ * information on escaping in JSON.
+ */
+extern grub_err_t EXPORT_FUNC(grub_json_unescape) (char **out, grub_size_t 
*outlen,
+                                                  const char *in, grub_size_t 
inlen);
+
 #endif
-- 
2.37.0

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