I alloted just 100 gb to Linux while dual booting. Now I need more space but can't extend the partition without messing up the boot sector and spending a lot of time :/

how did that happen
MBR is always at the start and a partition for efi should be too

@Kimmax now that I checked it, you're right. But still think it would take a lot of time because all of the ext4 partition will have to be copied?

@YourNemesis if sda3 is your windoze partition you should be good to go acutally.
Shrinkt the partition using the windows onboard tools, boot linux, USE THE TERMINAL, print layout using sector units, delete linux partition, create new partition with start = first sector after windoze partition, end = original end sector. Write changes to disk, resize filesystem using resize2fs.
???
Profit

@Kimmax also partprobe or reboot between changing the partition table

@Kimmax yes sda3 is Windows.
Why should I delete the Linux partition?
I planning to use gparted.

@YourNemesis learn to use the terminal, the GUI has way to much ways to fuck up. Using the terminal you have the exact commands and nothing more. You're deleting the linux partition, right. But you're recreating it at the same location + more space at the beginning. The filesystem is still intact and will know where to start. Using resize2fs you expand the filesystem to include the new extra space

@Kimmax cool, will try it next time. I already proceeded with gparted. Hopefully all went well.
Thanks!