asciiPercent computation: another potential multiplication overflow (#307)

* build: resolve a -Wconversion compiler warning

wordEncoder.cpp:312:91: warning: conversion from
‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned
int’} to ‘double’ may change value [-Wconversion]
  312 | buffer.length() == 0 ? 1 : static_cast<double>(asciiCount) / buffer.length();
      |                                                              ~~~~~~~~~~~~~^~

* wordEncoder: replace value 100 for asciiPercent

asciiPercent is a ratio, and not counting in units of hundredths
anymore. The maximum value therefore should be 1 not 100.

* vmime: avoid integer multiply wraparound in text::createFromString

The change from commit v0.9.2-194-gb447adbe needs to be applied to
one more function that replicates the same code.

(If the input string is 42949673 characters long or larger, there will
be integer overflow on 32-bit platforms when multiplying by 100.
Switch that one computation to floating point.)

2 files changed, 5 insertions, 7 deletions

diff --git a/src/vmime/text.cpp b/src/vmime/text.cpp
index 08d27bb9..2c14110b 100644
--- a/src/vmime/text.cpp
+++ b/src/vmime/text.cpp
@@ -269,7 +269,7 @@ shared_ptr <text> text::newFromString(const string& in, const charset& ch) {
 
 void text::createFromString(const string& in, const charset& ch) {
 
-	size_t asciiPercent = 0;
+	double asciiPercent = 0;
 
 	removeAllWords();
 
@@ -282,14 +282,12 @@ void text::createFromString(const string& in, const charset& ch) {
 
 	if (!alwaysEncode) {
 		const auto asciiCount = utility::stringUtils::countASCIIchars(in.begin(), in.end());
-		asciiPercent = (in.length() == 0 ? 100 : (100 * asciiCount) / in.length());
+		asciiPercent = in.length() == 0 ? 1 : static_cast<double>(asciiCount) / static_cast<double>(in.length());
 	}
 
-	// If there are "too much" non-ASCII chars, produce just one
-	// vmime::word. Because encoding happens word-wise, all of the input
-	// gets encoded.
+	// Cf. wordEncoder::guessBestEncoding for details
 
-	if (alwaysEncode || asciiPercent < 60) {  // less than 60% ASCII chars
+	if (alwaysEncode || asciiPercent < 0.60) {  // less than 60% ASCII chars
 
 		appendWord(make_shared <word>(in, ch));
 		return;
diff --git a/src/vmime/wordEncoder.cpp b/src/vmime/wordEncoder.cpp
index d1632022..404a57b4 100644
--- a/src/vmime/wordEncoder.cpp
+++ b/src/vmime/wordEncoder.cpp
@@ -309,7 +309,7 @@ wordEncoder::Encoding wordEncoder::guessBestEncoding(
 		utility::stringUtils::countASCIIchars(buffer.begin(), buffer.end());
 
 	const double asciiPercent =
-		buffer.length() == 0 ? 100 : static_cast<double>(asciiCount) / buffer.length();
+		buffer.length() == 0 ? 1 : static_cast<double>(asciiCount) / static_cast<double>(buffer.length());
 
 	if (asciiPercent < 0.60) {
 		return ENCODING_B64;