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...including for catalyst. The usecase for this is to put certain security-critical variables into a special segment/section that's mapped as read-only most of the time, and that temporary gets remapped as writeable when these variables are written to be the program. This protects against them being written to by heap spraying attacks. This special section should be mapped as read-only at program start, so using `-segprot MY_PROTECTED_MEMORY_THINGER rw r` to mark that segment as rw maxprot and r initprot is exactly what we want. lld has so far rejected mismatching initprot and maxprot. ld64 doesn't reject this, but silently writes initprot into both fields (!) It looks like this might not be fully intentional, see https://crbug.com/41495919#comment5 and http://crbug.com/41495919#comment8. In any case, when postprocessing ld64's output to have different values for initprot and maxprot, the dynamic loader seems to do the right thing (see also the previous two links). The same technique also works on Windows, using both link.exe and lld-link.exe using `/SECTION:myprotsect,R`. So, since this is useful, allow it when targeting macOS, and make it do what you'd expect. Since loader support for this on iOS is less clear, keep disallowing it there for now. See the PR for the program I used to check that this seems to work. (I only checked on arm64 macOS 14.5 so far; will run this on many more systems on bots once this is merged and rolled in.)
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