On Sun, Jan 09, 2011 at 09:12:49AM +0100, Rogier Wolff wrote: > > No. The superblock nor its offset will never change. It's like the > > syscall ABI, only worse. If we changed it would break *everybody*. > > Fortunately there is a huge amount of space left over in the 1024 byte > > superblock. > > It's called defensive programming. It prevents bugs before they > happen. By your reasoning you could've written 2048 or 0x800 there. Defensive programming would be something like BUG_ON(sizeof(struct ext4_super_block) != 1024); (unfortunately #error sizeof(struct ext4_super_block) != 1024 won't work since #error is handled by the preprocessor, and I don't think we can trigger a compile-time warning for a structure size issue). We could add that, if people like. I do have regression tests (i.e., boot a system with ext4) which would die if anything like that changed, though. And yes, I have similar regression tests in e2fsprogs that would trigger if the superblock size were to ever change. - Ted P.S. The only way I can think of to do it at compile time would be to build a test .o file with -g, and then use a program like pahole that pulls the information out of the DWARF information. Might actually be a good thing to do that, since it could also be useful for automating searches for unoptimize structures. Unfortunately, many developers don't have the DWARF utilities installed, so that would add a dependency on the kernel build. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
