From a3b69a0215fdface0fd5730872a4b3242d979dca Mon Sep 17 00:00:00 2001
From: Uli Schlachter <psychon@znc.in>
Date: Tue, 9 Feb 2021 16:54:35 +0100
Subject: [PATCH] pdf font subset: Generate valid font names

A hash value is encoded in base 26 with upper case letters for font
names.

Commit ed984146 replaced "numerator = abs (hash);" with "numerator =
hash;" in this code, because hash has type uint32_t and the compiler
warned about taking the absolute value of an unsigned value.  However,
abs() is actually defined to take an int argument. Thus, there was some
implicit cast.

Since numerator has type long, i.e. is signed, it is now actually
possible to get an overflow in the implicit cast and then have a
negative number. The following code is not prepared for this and
produces non-letters when encoding the hash.

This commit fixes that problem by not using ldiv() and instead using /
and % to directly compute the needed values. This gets rid of the need
to convert to type long. Since now everything works with uint32_t, there
is no more chance for negative numbers messing things up.

Fixes: https://gitlab.freedesktop.org/cairo/cairo/-/issues/449
Signed-off-by: Uli Schlachter <psychon@znc.in>
---
 src/cairo-pdf-surface.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/src/cairo-pdf-surface.c b/src/cairo-pdf-surface.c
index 6da460878..52c49b6d2 100644
--- a/src/cairo-pdf-surface.c
+++ b/src/cairo-pdf-surface.c
@@ -5310,18 +5310,14 @@ _create_font_subset_tag (cairo_scaled_font_subset_t	*font_subset,
 {
     uint32_t hash;
     int i;
-    long numerator;
-    ldiv_t d;
 
     hash = _hash_data ((unsigned char *) font_name, strlen(font_name), 0);
     hash = _hash_data ((unsigned char *) (font_subset->glyphs),
 		       font_subset->num_glyphs * sizeof(unsigned long), hash);
 
-    numerator = hash;
     for (i = 0; i < 6; i++) {
-	d = ldiv (numerator, 26);
-	numerator = d.quot;
-        tag[i] = 'A' + d.rem;
+	tag[i] = 'A' + (hash % 26);
+	hash /= 26;
     }
     tag[i] = 0;
 }
-- 
GitLab