/usr/share/texmf-texlive/metafont/misc/mode2dpi.mf is in texlive-base 2009-15.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | % Usage: mf '\mode=what_ever_mode; \input mode2dpi'
let myexit = primitive_end_;
mode_setup;
string dpi;
dpi := decimal round pixels_per_inch;
message "DPI = " & dpi;
fontmaking := 0;
myexit;
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