Is it possible to use existing Canon EF lenses with a Leica body with an M mount?
I've seen various discussions in the Internet on this combination, but usually the other way round (mounting a Leica lens on a Canon SLR body).
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Is it possible? Yes. Will you be able to focus it... not really. You won't be able to step it down either. Really... this is probably the worst technically possible combo I can think of.
This site lists the flange distances for most lenses. Any lens system that has a flange distance greater than yours should be able to be adapted without extra glass.
You can see that the Leica M mount is only 27.95mm and should be able to adapt almost any glass given the proper adapter. If that adapter exists, I can't say.