Check for buffer underruns in checkType

[rkta]

This fixes Issue #282.

[N-R-K]

Strictly speaking, a pointer going out of bound (with one exception of 1 past the end) is already UB ¹. Instead of trying to check for UB and then correcting it, it's better to check plen before subtracting.

if (prop - prop_buffer < plen) // would underflow
    prop = prop_buffer;
else
    prop -= plen;

Footnotes

1. https://port70.net/~nsz/c/c99/n1256.html#6.5.6p8 ↩

[rkta]

On Fri, Nov 17, 2023 at 09:02:50AM -0800, NRK wrote: @N-R-K requested changes on this pull request. > @@ -389,6 +389,7 @@ checkType(Str s, Lineprop **oprop, Linecolor **ocolor) else { Strshrink(s, plen); prop -= plen; + prop = prop < prop_buffer ? prop_buffer : prop; Strictly speaking, a pointer going out of bound (with one exception of 1 past the end) is already UB [^0]. Instead of trying to check for UB and then correcting it, it's better to check `plen` before subtracting.

hm, I guess this is why I wrote edc6026 as I did. :-D I even thought about this, but came to the wrong conclusion that it's fine as long as I don't dereference. Thanks for the review. I'm not happy with this whole function, but need to understand it better. Will update this commit in the coming days.