From 30641e36aa5b547eee48565caa3078b0a2e7c185 Mon Sep 17 00:00:00 2001 From: Ian Lance Taylor Date: Sun, 14 Feb 2021 17:14:41 -0800 Subject: [PATCH] internal/poll: if copy_file_range returns 0, assume it failed 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 Run-TryBot: Ian Lance Taylor TryBot-Result: Go Bot Reviewed-by: Russ Cox --- src/internal/poll/copy_file_range_linux.go | 10 ++++++- src/os/readfrom_linux_test.go | 32 ++++++++++++++++++++++ 2 files changed, 41 insertions(+), 1 deletion(-) diff --git a/src/internal/poll/copy_file_range_linux.go b/src/internal/poll/copy_file_range_linux.go index fc34aef4cb..01b242a4ea 100644 --- a/src/internal/poll/copy_file_range_linux.go +++ b/src/internal/poll/copy_file_range_linux.go @@ -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 diff --git a/src/os/readfrom_linux_test.go b/src/os/readfrom_linux_test.go index 37047175e6..1d145dadb0 100644 --- a/src/os/readfrom_linux_test.go +++ b/src/os/readfrom_linux_test.go @@ -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) + } +}