syscall, internal/syscall/unix: fix fstatat on linux/mips64

On linux/mips64, the syscall.Stat_t struct does not match the
kernel version of the struct. Functions that operate on a Stat_t
translate between it and the kernel struct.

The fstatat function was not doing this translation.
Make it do so.

Export a syscall.Fstatat on mips64 for usage by
internal/syscall/unix. Perhaps we should just do this on all
architectures, but this is the smaller change for now.

Fixes #70659

Change-Id: I38e36473689be25861953b418c9abc5b270a7bcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/633280
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: neild

src/internal/syscall/unix/at_fstatat.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build dragonfly || (linux && !loong64) || netbsd || (openbsd && mips64)
+//go:build dragonfly || (linux && !(loong64 || mips64)) || netbsd || (openbsd && mips64)
 
 package unix
 

src/internal/syscall/unix/at_fstatat2.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build freebsd || (linux && loong64)
+//go:build freebsd || (linux && (loong64 || mips64))
 
 package unix
 

src/syscall/syscall_linux_mips64x.go

@@ -16,7 +16,6 @@ const (
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)
-//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sys	Ftruncate(fd int, length int64) (err error)
 //sysnb	Getegid() (egid int)
 //sysnb	Geteuid() (euid int)
@@ -126,10 +125,22 @@ type stat_t struct {
 	Blocks     int64
 }
 
+//sys	fstatatInternal(dirfd int, path string, stat *stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sys	fstat(fd int, st *stat_t) (err error)
 //sys	lstat(path string, st *stat_t) (err error)
 //sys	stat(path string, st *stat_t) (err error)
 
+func fstatat(fd int, path string, s *Stat_t, flags int) (err error) {
+	st := &stat_t{}
+	err = fstatatInternal(fd, path, st, flags)
+	fillStat_t(s, st)
+	return
+}
+
+func Fstatat(fd int, path string, s *Stat_t, flags int) (err error) {
+	return fstatat(fd, path, s, flags)
+}
+
 func Fstat(fd int, s *Stat_t) (err error) {
 	st := &stat_t{}
 	err = fstat(fd, st)