shithub: rgbds

Fix critical oversight in lexer buffer refilling Since the lexer buffer wraps, the refilling gets handled in two steps: First, iff the buffer would wrap, the buffer is refilled until its end. Then, if more characters are requested, that amount is refilled too. An important detail is that `read()` may not return as many characters as requested; for this reason, the first step checks if its `read()` was "full", and skips the second step otherwise. This is also where a bug lied. After a *lot* of trying, I eventually managed to reproduce the bug on an OpenBSD VM, and after adding a couple of `assert`s in `peekInternal`, this is what happened, starting at line 724: 0. `lexerState->nbChars` is 0, `lexerState->index` is 19; 1. We end up with `target` = 42, and `writeIndex` = 19; 2. 42 + 19 is greater than `LEXER_BUF_SIZE` (= 42), so the `if` is entered; 3. Within the first `readChars`, **`read` only returns 16 bytes**, advancing `writeIndex` to 35 and `target` to 26; 4. Within the second `readChars`, a `read(26)` is issued, overflowing the buffer. The bug should be clear now: **the check at line 750 failed to work!** Why? Because `readChars` modifies `writeIndex`. The fix is simply to cache the number of characters expected, and use that.

--- a/src/asm/lexer.c

+++ b/src/asm/lexer.c

@@ -729,6 +729,8 @@

 		ssize_t nbCharsRead = 0, totalCharsRead = 0;

 #define readChars(size) do { \

+	/* This buffer overflow made me lose WEEKS of my life. Never again. */ \

+	assert(writeIndex + (size) <= LEXER_BUF_SIZE); \

 	nbCharsRead = read(lexerState->fd, &lexerState->buf[writeIndex], (size)); \

 	if (nbCharsRead == -1) \

 		fatalerror("Error while reading \"%s\": %s\n", lexerState->path, errno); \

@@ -741,9 +743,11 @@

 		/* If the range to fill passes over the buffer wrapping point, we need two reads */

 		if (writeIndex + target > LEXER_BUF_SIZE) {

-			readChars(LEXER_BUF_SIZE - writeIndex);

+			size_t nbExpectedChars = LEXER_BUF_SIZE - writeIndex;

+			readChars(nbExpectedChars);

 			/* If the read was incomplete, don't perform a second read */

-			if (nbCharsRead < LEXER_BUF_SIZE - writeIndex)

+			if (nbCharsRead < nbExpectedChars)

 				target = 0;

 		if (target != 0)