[PATCH] copy: handle system security config issues with copy_file_range()

[Pádraig Brady]

* src/copy.c (sparse_copy): Upon EPERM from copy_file_range(),
fall back to a standard copy, which will give a more accurate
error as to whether the issue is with the source or destination.
Also this will avoid the issue where seccomp or apparmor are
not configured to handle copy_file_range(), in which case
the fall back standard copy would succeed without issue.
This specific issue with seccomp was noticed for example in:
https://github.com/golang/go/issues/40900
---
 src/copy.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/copy.c b/src/copy.c
index 69ba94b11..d945a5d94 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -294,6 +294,15 @@ sparse_copy (int src_fd, int dest_fd, char *buf, size_t 
buf_size,
                 || errno == EINVAL || errno == EBADF
                 || errno == EXDEV || errno == ETXTBSY)
               break;
+
+            /* copy_file_range might not be enabled in seccomp filters,
+               so retry with a standard copy.  EPERM can also occur
+               for immutable files, but that would only be in the edge case
+               where the file is made immutable after creating/truncating,
+               in which case the (more accurate) error is still shown.  */
+            if (errno == EPERM && *total_n_read == 0)
+              break;
+
             if (errno == EINTR)
               n_copied = 0;
             else
-- 
2.26.2