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commit a3812bb81e729648ad1ea72646051aa1a896154b
parent b371006ea3832673feed5f07c264ea5ceb518c5b
Author: magras <dr.magras@gmail.com>
Date:   Thu, 28 Feb 2019 04:56:01 +0300

fix use after free in font caching algorithm

Current font caching algorithm contains a use after free error. A font
removed from `frc` might be still listed in `wx.specbuf`. It will lead
to a crash inside `XftDrawGlyphFontSpec()`.

Steps to reproduce:
$ st -f 'Misc Tamsyn:scalable=false'
$ curl https://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-demo.txt

Of course, result depends on fonts installed on a system and fontconfig.
In my case, I'm getting consistent segfaults with different fonts.

I replaced a fixed array with a simple unbounded buffer with a constant
growth rate. Cache starts with a capacity of 0, gets increments by 16,
and never shrinks. On my machine after `cat UTF-8-demo.txt` buffer
reaches a capacity of 192. During casual use capacity stays at 0.

Diffstat:
Mx.c | 15+++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/x.c b/x.c @@ -241,8 +241,9 @@ typedef struct { } Fontcache; /* Fontcache is an array now. A new font will be appended to the array. */ -static Fontcache frc[16]; +static Fontcache *frc = NULL; static int frclen = 0; +static int frccap = 0; static char *usedfont = NULL; static double usedfontsize = 0; static double defaultfontsize = 0; @@ -1273,12 +1274,14 @@ xmakeglyphfontspecs(XftGlyphFontSpec *specs, const Glyph *glyphs, int len, int x fcpattern, &fcres); /* - * Overwrite or create the new cache entry. + * Allocate memory for the new cache entry. */ - if (frclen >= LEN(frc)) { - frclen = LEN(frc) - 1; - XftFontClose(xw.dpy, frc[frclen].font); - frc[frclen].unicodep = 0; + if (frclen >= frccap) { + frccap += 16; + if (!frc) + frc = xmalloc(frccap * sizeof(Fontcache)); + else + frc = xrealloc(frc, frccap * sizeof(Fontcache)); } frc[frclen].font = XftFontOpenPattern(xw.dpy,