x/sys/unix: fsconfig system call support

[IlyaHanov]

API proposal is in #59537 (comment)

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What is the feature?

Support for fsconfig system call which was added to the Linux 5.2 version.

Details

Looks like there already was an attempt to support fsconfig system call here, but I'm not sure why that hasn't been approved, I see 2 unresolved threads, but they're related to only comments, looks like patch is OK, is it needed only changes to comments? As I can see we can omit any links to documentation until it's merged, right? I can try to help if it needs some changes.

Cc: @ianlancetaylor @AlexeyPerevalov

[gopherbot]

Change https://go.dev/cl/484995 mentions this issue: unix: add Fsconfig syscall on linux

[IlyaHanov]

@ianlancetaylor , we talked to @AlexeyPerevalov and decided to create a new review for this (https://go-review.googlesource.com/c/sys/+/484995), this fixed issues you mentioned on the previous one, PTAL

[ianlancetaylor]

This is a very complicated system call. Please document the suggested Go API in this issue so that people can discuss it. In particular I wonder whether value should be string or []byte. Thanks.

[IlyaHanov]

I wonder whether value should be string or []byte

I wrote a version few weeks ago with []byte (IlyaHanov/sys@aeaec22), but I didn't test it so much, it worked for my own cases. As I can see from the previous review rounds there might be other cases that this version doesn't handle, of course, it will differ with string and going to be more complicated.

[IlyaHanov]

I can suggest to use interface{} for value:

• In case of FSCONFIG_BINARY do v, ok := value.([]byte)
• In case of FSCONFIG_SET_PATH AND FSCONFIG_SET_PATH_EMPTY AND FSCONFIG_SET_STRING do v, ok := value.(string)
• In other cases value is ignored and nil passed to the kernel

Fsconfig(fd int, cmd uint, key string, value interface{}, aux int) (err error)

[stagnation]

Hi, thanks for moving this forward, I too would like to use the syscall. What is the next step for the review, and can I help?

[IlyaHanov]

This is a very complicated system call. Please document the suggested Go API in this issue so that people can discuss it. In particular I wonder whether value should be string or []byte. Thanks.

@stagnation AFAIR, the next step is to define the interface, I've suggested few of them:

• Fsconfig(fd int, cmd uint, key string, value string, aux int) (err error)
• Fsconfig(fd int, cmd uint, key string, value []byte, aux int) (err error)
• Fsconfig(fd int, cmd uint, key string, value interface{}, aux int) (err error)

I think it's possible to move it forward if people and maintainers will be agreed on the interface. It's not a problem to implement any of them. WDYT?

[stagnation]

I do not have a strong opinion, I can see either option working fine. My main use case is mountat that just uses the SET_STRING command, which is convenient to call with a string argument, but interface{} works too, and []byte is not too much of a hassle.

In short:

 func mountat(dfd int, fstype, source, mountname string) (int, error) {
     // Mounts the `source` filesystem on `mountname` inside a directory
     // given as `dfd` file descriptor.
     // Using the `fstype` filesystem type.
     fd, err := unix.Fsopen(fstype, unix.FSOPEN_CLOEXEC)
     err = fsconfig(fd, FSCONFIG_SET_STRING, "source", source, 0)
     err = fsconfig(fd, FSCONFIG_CMD_CREATE, "", "", 0)
     mfd, err := unix.Fsmount(fd, unix.FSMOUNT_CLOEXEC, unix.MS_NOEXEC)
     err = unix.MoveMount(mfd, "", dfd, mountname, unix.MOVE_MOUNT_F_EMPTY_PATH)

     return mfd, nil

[IlyaHanov]

@ianlancetaylor would you mind to merge version that uses interface{} (I can prepare that on days) it looks the most attractive for users, but as @stagnation mentioned it seems that any of solutions can be adopted by the user if needed?