internal/poll: use copy_file_range only on Linux kernel >= 5.3

https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS states:

  A major rework of the kernel implementation occurred in 5.3.  Areas
  of the API that weren't clearly defined were clarified and the API
  bounds are much more strictly checked than on earlier kernels.
  Applications should target the behaviour and requirements of 5.3
  kernels.

Rather than attempting to detect the file system for source and
destination files (which means two additional statfs syscalls) and skip
copy_file_range in case of known defects (e.g. CIFS -> CIFS), just
assume copy_file_range to be broken on kernels < 5.3.

Fixes #42400

Change-Id: I3a531296182c1d6e341772cc9d2be5bf83e52575
Reviewed-on: https://go-review.googlesource.com/c/go/+/268338
Trust: Tobias Klauser <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/internal/poll/copy_file_range_linux.go

@@ -10,15 +10,60 @@ import (
 	"syscall"
 )
 
-var copyFileRangeSupported int32 = 1 // accessed atomically
+var copyFileRangeSupported int32 = -1 // accessed atomically
 
 const maxCopyFileRangeRound = 1 << 30
 
+func kernelVersion() (major int, minor int) {
+	var uname syscall.Utsname
+	if err := syscall.Uname(&uname); err != nil {
+		return
+	}
+
+	rl := uname.Release
+	var values [2]int
+	vi := 0
+	value := 0
+	for _, c := range rl {
+		if '0' <= c && c <= '9' {
+			value = (value * 10) + int(c-'0')
+		} else {
+			// Note that we're assuming N.N.N here.  If we see anything else we are likely to
+			// mis-parse it.
+			values[vi] = value
+			vi++
+			if vi >= len(values) {
+				break
+			}
+		}
+	}
+	switch vi {
+	case 0:
+		return 0, 0
+	case 1:
+		return values[0], 0
+	case 2:
+		return values[0], values[1]
+	}
+	return
+}
+
 // CopyFileRange copies at most remain bytes of data from src to dst, using
 // the copy_file_range system call. dst and src must refer to regular files.
 func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) {
-	if atomic.LoadInt32(&copyFileRangeSupported) == 0 {
+	if supported := atomic.LoadInt32(&copyFileRangeSupported); supported == 0 {
 		return 0, false, nil
+	} else if supported == -1 {
+		major, minor := kernelVersion()
+		if major > 5 || (major == 5 && minor >= 3) {
+			atomic.StoreInt32(&copyFileRangeSupported, 1)
+		} else {
+			// copy_file_range(2) is broken in various ways on kernels older than 5.3,
+			// see issue #42400 and
+			// https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS
+			atomic.StoreInt32(&copyFileRangeSupported, 0)
+			return 0, false, nil
+		}
 	}
 	for remain > 0 {
 		max := remain