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internal/poll: if copy_file_range returns 0, assume it failed

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On current Linux kernels copy_file_range does not correctly handle
files in certain special file systems, such as /proc. For those file
systems it fails to copy any data and returns zero. This breaks Go's
io.Copy for those files.

Fix the problem by assuming that if copy_file_range returns 0 the
first time it is called on a file, that that file is not supported.
In that case fall back to just using read. This will force an extra
system call when using io.Copy to copy a zero-sized normal file,
but at least it will work correctly.

For #36817
Fixes #44272

Change-Id: I02e81872cb70fda0ce5485e2ea712f219132e614
Reviewed-on: https://go-review.googlesource.com/c/go/+/291989
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
This commit is contained in:
Ian Lance Taylor 2021-02-14 17:14:41 -08:00
parent 33d72fd412
commit 30641e36aa
2 changed files with 41 additions and 1 deletions
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10
src/internal/poll/copy_file_range_linux.go
View File

@ -112,7 +112,15 @@ func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err
return 0, false, nil
case nil:
if n == 0 {
// src is at EOF, which means we are done.
// If we did not read any bytes at all,
// then this file may be in a file system
// where copy_file_range silently fails.
// https://lore.kernel.org/linux-fsdevel/20210126233840.GG4626@dread.disaster.area/T/#m05753578c7f7882f6e9ffe01f981bc223edef2b0
if written == 0 {
return 0, false, nil
}
// Otherwise src is at EOF, which means
// we are done.
return written, true, nil
}
remain -= n

32
src/os/readfrom_linux_test.go
View File

@ -361,3 +361,35 @@ func (h *copyFileRangeHook) install() {
func (h *copyFileRangeHook) uninstall() {
*PollCopyFileRangeP = h.original
}
// On some kernels copy_file_range fails on files in /proc.
func TestProcCopy(t *testing.T) {
const cmdlineFile = "/proc/self/cmdline"
cmdline, err := os.ReadFile(cmdlineFile)
if err != nil {
t.Skipf("can't read /proc file: %v", err)
}
in, err := os.Open(cmdlineFile)
if err != nil {
t.Fatal(err)
}
defer in.Close()
outFile := filepath.Join(t.TempDir(), "cmdline")
out, err := os.Create(outFile)
if err != nil {
t.Fatal(err)
}
if _, err := io.Copy(out, in); err != nil {
t.Fatal(err)
}
if err := out.Close(); err != nil {
t.Fatal(err)
}
copy, err := os.ReadFile(outFile)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(cmdline, copy) {
t.Errorf("copy of %q got %q want %q\n", cmdlineFile, copy, cmdline)
}
}