Hi On Sun, 9 Mar 2025, Zack Weinberg wrote: > I am developing a device-mapper client library in Rust. The language is > relevant here because Rust's standard library makes a strong distinction > between strings that are known to be encoded in UTF-8 and strings whose > encoding is not known (`str` vs `OsStr`), with the latter being awkward > to use compared to the former. > > The device-mapper ioctls use a lot of strings, and it's not clear > to me from either the documentation or the kernel's source code > what expectations the *kernel* has of the character encoding of these > strings. I don't expect the kernel to care about UTF-8 versus > ISO-8859-n or whatever, but it is plausible to me that at least some > of the strings used in device-mapper ioctls can be expected always to > be ASCII, in which case I can safely use `str` for them on the library > side and it'll be more ergonomic for end users. > > There are, I think, four different kinds of strings used in device- > mapper ioctls: target names, device names and UUIDs, table parameters, > and target messages. For each of these, I would like to ask: > > * Are there any circumstances where non-ASCII bytes *should* be > used in this type of string *when supplied to the kernel*? I think there's no reason to use non-ASCII characters in device mapper. > * Are there any circumstances where a client library should expect > non-ASCII bytes to *appear* in this type of string *when reported > back to the client by the kernel*? If the user creates a device name with non-ascii characters, it will be reported as such in the kernel ioctls. > * If there are any such circumstances (either way), does the kernel > try to interpret non-ASCII bytes in this type of string as characters > in a specific encoding, or does it just treat them as an opaque > token (as is typical e.g. for pathname components)? The kernel just passes non-ascii characters as opaque tokens. It doesn't attempt to interpret them. But udev mangles non-ascii characters - i.e. if you create dmsetup create ěščřžýáíé --table '0 3951256 zero' you get ls -la /dev/mapper/ '\xc4\x9b\xc5\xa1\xc4\x8d\xc5\x99\xc5\xbe\xc3\xbd\xc3\xa1\xc3\xad\xc3\xa9' -> ../dm-0 ls -al /dev/disk/by-id/ 'dm-name-\xc4\x9b\xc5\xa1\xc4\x8d\xc5\x99\xc5\xbe\xc3\xbd\xc3\xa1\xc3\xad\xc3\xa9' -> ../../dm-0 dmsetup table will show ěščřžýáíé: 0 3951256 zero dmsetup remove ěščřžýáíé will remove the device Mikulas > Thanks, > zw > > please cc: me directly on all responses, I'm not subscribed to the list >
