On Sun, Jul 19, 2020 at 12:34:11PM -0700, Linus Torvalds wrote: > On Sun, Jul 19, 2020 at 12:28 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > I think we should try to get rid of the exact semantics. > > Side note: I think one of the historical reasons for the exact > semantics was that we used to do things like the mount option copying > with a "copy_from_user()" iirc. > > And that could take a fault at the end of the stack etc, because > "copy_mount_options()" is nasty and doesn't get a size, and just > copies "up to 4kB" of data. > > It's a mistake in the interface, but it is what it is. But we've > always handled the inexact count there anyway by originally doing byte > accesses, and at some point you optimized it to just look at where > page boundaries might be.. And we may have to change this again since, with arm64 MTE, the page boundary check is insufficient: https://lore.kernel.org/linux-fsdevel/20200715170844.30064-25-catalin.marinas@xxxxxxx/ While currently the fault path is unlikely to trigger, with MTE in user space it's a lot more likely since the buffer (e.g. a string) is normally less than 4K and the adjacent addresses would have a different colour. I looked (though briefly) into passing the copy_from_user() problem to filesystems that would presumably know better how much to copy. In most cases the options are string, so something like strncpy_from_user() would work. For mount options as binary blobs (IIUC btrfs) maybe the fs has a better way to figure out how much to copy. > I think that was the only truly _valid_ case of "we actually copy data > from user space, and we might need to handle a partial case", and > exactly because of that, it had already long avoided the whole "assume > copy_from_user gives us byte-accurate data before the fault". With MTE, we didn't find any other instance of copy_from_user() where the byte accuracy matters. The close relative, strncpy_from_user(), already handles exact copying via a fall back to byte at a time. -- Catalin
