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Here is the shape I want to achieve, sharp square hole in the cylinder: enter image description here

I did inset, extrude and beveled edges to keep square form.

The problem is when I am applying subsurf modifier, it distorts hole edges with weird overhangs: enter image description here

Here is my blender file:

And here is modifier settings: enter image description here

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As said by others, those corner triangles (probably bevel applied) are involved in this (edge split is not),

enter image description here

I tred adding some more loopcut, eg. like

enter image description here

with subsurf:

enter image description here

in object mode:

enter image description here

compared to your squared hole:

enter image description here

under another angle

enter image description here

here is visible the full cylinder, without subsurf

enter image description here

with subsurf (note that I also removed sharp edges but added loops at top and bottom)

enter image description here

and in object mode (here I disabled edge split: top/bottom loops gives the edge more definition even with the subsurf)

enter image description here

here a test file

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  • $\begingroup$ I wish there was a way to do this without cutting through all the cylinder. However this method gives exactly what I want to get. $\endgroup$ – Alexander Jan 13 '18 at 9:31
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The issue is cause by the beveling of the edges that you did. When you applied the bevel operation, a small triangle was created in each corner.

enter image description here

Triangles are notorious for having issues like this when being subdivided. In this case, it may be difficult (although not impossible) to remove the triangle completely though. If you restructure the mesh so that the triangle is located on a section that isn't a corner the smoothing will look much nicer. Here is a way you could do that.

enter image description here

Here is a comparison between the corners when subdivided:

enter image description here

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  • $\begingroup$ Thanks, I've also came to this solution earlier, but it still produces artifacts with thinner inset outline. $\endgroup$ – Alexander Jan 13 '18 at 9:23

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