Make hb_sanitize_context_t::max_ops unsigned

[zauguin]

Currently max_ops in hb_sanitize_context_t is signed, but in ...::start_processing an unsigned value is assigned:

    this->max_ops = hb_max ((unsigned int) (this->end - this->start) * HB_SANITIZE_MAX_OPS_FACTOR,
			 (unsigned) HB_SANITIZE_MAX_OPS_MIN);

If this->end - this->start is big enough, then (this->end - this->start) * HB_SANITIZE_MAX_OPS_FACTOR can not be represented in a signed int and the assignment overflows, leading to negative values in max_ops. Of course, then a font will no longer load correctly. There are at least two possible fixes for this: Just make max_ops unsigned (negative values don't make sense there anyway and the comparisons with 0 are written such that they still work with unsigned values) or use hb_min((unsigned) HB_SANITIVE_MAX_OPS_MAX, ...) before assigning to ->max_ops to provide an upper bound for the value.

I used the first approach here because I wasn't sure if max_ops should be limited to HB_SANITIVE_MAX_OPS_MAX or if that constant only has meaning for special fonts.

The original issue had appeared in latex3/luaotfload#126 but can be reproduced without LuaTeX:

Download SourceHanNotoCJK.ttc from https://github.com/adobe-fonts/source-han-super-otc/releases (~390MiB) and try

hb-shape --font-file /path/to/SourceHanNotoCJK.ttc --face-index 17 "中國哲學書電子化計劃"

On my system this gives only a collection of GID 0 glyphs:

[gid0=0+1000|gid0=1+1000|gid0=2+1000|gid0=3+1000|gid0=4+1000|gid0=5+1000|gid0=6+1000|gid0=7+1000|gid0=8+1000|gid0=9+1000]

the same font in hb-view gives hb-view: FT_New_Memory_Face fail.

[behdad]

It's signed such that if we intentionally or accidentally decrement it multiple times before checking, it doesn't just wrap around. So, I want to keep it that way.

If font actually overflows that, breaking the font is also a good idea IMO. Anyway, just saturate it instead of overflow sounds best to me.

[zauguin]

@behdad OK, I added a PR for the hb_min((unsigned) HB_SANITIVE_MAX_OPS_MAX, ...) approach as #2080 then.

[behdad]

Thanks.