syscall: wrong Dirent struct on mips64le

[rfjakob]

The maintainer of the Debian package just reported that gocryptfs is broken on mips64le: rfjakob/gocryptfs#200 . The reason seems to be that syscall.Dirent does not match the used getdents syscall. Analysis below:

On mips64le, getdents is used instead of getdents64:

go/src/syscall/syscall_linux_mips64x.go

Line 13 in bcc86d5

// Linux introduced getdents64 syscall for N64 ABI only in 3.10

	_SYS_getdents  = SYS_GETDENTS

And getdents returns linux_dirent, which defined in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/readdir.c?h=v4.15#n147 :

struct linux_dirent {
	unsigned long	d_ino;
	unsigned long	d_off;
	unsigned short	d_reclen;
	char		d_name[1];
};

In Go mips64le, syscall.Dirent is defined here

go/src/syscall/ztypes_linux_mips64le.go

Line 133 in bcc86d5

// Note: on mips64, we're using the getdents syscall,

:

type Dirent struct {
	Ino       uint64
	Off       int64
	Reclen    uint16
	Name      [256]int8
	Type      uint8
	Pad_cgo_0 [5]byte
}

And this looks wrong, because it has a Type field, which the the linux_dirent struct has not.

[ianlancetaylor]

CC @minux @cherrymui

[rfjakob]

PS: I believe this only affects people who parse the Getdents output themselves. gocryptfs does for performance, so does https://github.com/golang/tools/blob/master/imports/fastwalk_unix.go , but probably not much else. The fix for gocryptfs will be to switch over to unix.Getdents from x/sys/unix, which does not have this problem.

[cherrymui]

We should probably move to use getdents64 on MIPS64, just like all other architectures. At the time the code was written, getdents64 was rather new in MIPS64 Linux kernel. But it is not new anymore, and we probably do require newer kernels.

[cherrymui]

x/sys/unix also uses getdents64 on MIPS64.

[ianlancetaylor]

I would certainly encourage anybody who wants to call Getdents themselves to use golang.org/x/sys/unix rather than syscall. But of course syscall should at least be internally consistent.

[minux]

Yeah, I think we can fix this (with a api-breaking change) now as 3.10 is almost 6 years old. I do think the syscall package should at least work for the most basic things (e.g. not supporting bpf is fine, but Getdirent should work.)

[tklauser]

Yeah, I think we can fix this (with a api-breaking change) now as 3.10 is
almost 6 years old.

According to https://github.com/golang/go/wiki/MinimumRequirements the minimum kernel version required for linux/mips64le is 4.8, so that shouldn't be a problem.

[gopherbot]

Change https://golang.org/cl/91055 mentions this issue: syscall: use SYS_GETDENTS64 on linux/mips64{,le}

[lechner]

Hi, we use syscall in other places as well. Older documentation encourages external packages to use x/sys/unix instead. Is the recommendation still valid? Thank you!

[bradfitz]

@lechner, x/sys/unix is gets new stuff and is updated more regularly. You can use syscall if it meets your needs, but it doesn't change often (ideally never), and when it does it's tied to releases. So using x/sys/unix is almost always a better option.

[lechner]

@bradfitz, which module would you recommend for Go-Fuse, a package that provides the filesystem-in-userspace (FUSE) bindings used by gocryptfs, please? Thank you!

[bradfitz]

@lechner, whatever the Go-Fuse authors are using is fine, if their code is working. If they hit problems and they're using syscall, the answer might be to not use syscall.

[lechner]

@bradfitz Thank you!