From f729f51e475ff98d0caf529f0723ef810b1c88ef Mon Sep 17 00:00:00 2001 From: Simon Tatham Date: Sun, 23 May 2021 08:45:55 +0100 Subject: [PATCH] WASM: move save file encoding from JS into C. The previous fix worked OK, but it was conceptually wrong. Puzzles save files are better regarded as binary, not text: the length fields are measured in bytes, so translating the file into a different multibyte character encoding would invalidate them. So it was wrong to fetch a C byte string containing the exactly right binary data, then translate it into a Javascript string as if decoding from UTF-8, then retranslate to a representation of a bytewise encoding via encodeURIComponent, and then label the result as application/octet-stream. This probably wouldn't have caused any problems in practice, because I don't remember any situation in which my save files use characters outside printable ASCII (plus newline). But it's not actually forbidden, so a save file might choose to do that some day, so that UTF-8 decode/reencode hidden in the JS was a latent bug. Now the URI-encoding is done on the C side, while we still know exactly what the binary data ought to look like and can be sure we're translating it byte for byte into the output encoding for the data: URI. By the time the JS receives a string pointer from get_save_file, it's already URI-encoded, which _guarantees_ that it's in ASCII and won't be messed about with by Emscripten's UTF8ToString. --- emcc.c | 49 +++++++++++++++++++++++++++++++++++++++++++++---- emccpre.js | 3 +-- 2 files changed, 46 insertions(+), 6 deletions(-) diff --git a/emcc.c b/emcc.c index 9a6c742..ff715a4 100644 --- a/emcc.c +++ b/emcc.c @@ -787,12 +787,53 @@ struct savefile_write_ctx { size_t pos; }; -static void savefile_write(void *vctx, const void *buf, int len) +static void savefile_write(void *vctx, const void *vbuf, int len) { + static const unsigned char length[256] = { + /* + * Assign a length of 1 to any printable ASCII character that + * can be written literally in URI-encoding, i.e. + * + * A-Z a-z 0-9 - _ . ! ~ * ' ( ) + * + * Assign length 3 (for % and two hex digits) to all other + * byte values. + */ + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 1, 3, 3, 3, 3, 3, 1, 1, 1, 1, 3, 3, 1, 1, 3, + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 3, 3, 3, 3, + 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 3, 3, 1, + 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, + 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 3, 3, 1, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, + }; + static const char hexdigits[] = "0123456789ABCDEF"; + struct savefile_write_ctx *ctx = (struct savefile_write_ctx *)vctx; - if (ctx->buffer) - memcpy(ctx->buffer + ctx->pos, buf, len); - ctx->pos += len; + const unsigned char *buf = (const unsigned char *)vbuf; + for (int i = 0; i < len; i++) { + unsigned char c = buf[i]; + int clen = length[c]; + if (ctx->buffer) { + if (clen == 1) { + ctx->buffer[ctx->pos] = c; + } else { + ctx->buffer[ctx->pos] = '%'; + ctx->buffer[ctx->pos+1] = hexdigits[c >> 4]; + ctx->buffer[ctx->pos+2] = hexdigits[c & 0xF]; + } + } + ctx->pos += clen; + } } char *get_save_file(void) diff --git a/emccpre.js b/emccpre.js index 0a498de..caa3250 100644 --- a/emccpre.js +++ b/emccpre.js @@ -379,8 +379,7 @@ function initPuzzle() { "Click to download the ")); var a = document.createElement("a"); a.download = "puzzle.sav"; - a.href = "data:application/octet-stream," + - encodeURIComponent(savefile_text); + a.href = "data:application/octet-stream," + savefile_text; a.appendChild(document.createTextNode("saved-game file")); dlg_form.appendChild(a); dlg_form.appendChild(document.createTextNode("."));