libnbd/0006-copy-Pass-in-dummy-variable-rather-than-errno-to-cal.patch

From 22572f8ac13e2e8daf91d227eac2f384303fb5b4 Mon Sep 17 00:00:00 2001
From: Eric Blake <eblake@redhat.com>
Date: Thu, 3 Feb 2022 14:25:57 -0600
Subject: [PATCH] copy: Pass in dummy variable rather than &errno to callback

In several places where asynch handlers manually call the provided
nbd_completion_callback, the value of errno is indeterminate (for
example, in file-ops.c:file_asynch_read(), the previous call to
file_synch_read() already triggered exit() on error, but does not
guarantee what is left in errno on success).  As the callback should
be paying attention to the value of *error (to be fixed in the next
patch), we are better off ensuring that we pass in a pointer to a
known-zero value.  Besides, passing in &errno carries a risk that if
the callback uses any other library function that alters errno prior
to dereferncing *error, it will no longer see the value we passed in.
Thus, it is easier to use a dummy variable on the stack than to mess
around with errno and it's magic macro expansion into a thread-local
storage location.

Note that several callsites then check if the callback returned -1,
and if so assume that the callback has caused errno to now have a sane
value to pass on to perror.  In theory, the fact that we are no longer
passing in &errno means that if the callback assigns into *error but
did not otherwise affect errno (a tenuous assumption, given our
argument above that we could not even guarantee that the callback does
not accidentally alter errno prior to reading *error), our perror call
would no longer reflect the intended error value from the callback.
But in practice, since the callback never actually returned -1, nor
even assigned into *error, the call to perror is dead code; although I
have chosen to defer that additional cleanup to the next patch.

Message-Id: <20220203202558.203013-5-eblake@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>
Acked-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
(cherry picked from commit 794c8ce06e995ebd282e8f2b9465a06140572112)
Conflicts:
	copy/file-ops.c - no backport of d5f65e56 ("copy: Do not use trim
 for zeroing"), so asynch_trim needed same treatment
	copy/multi-thread-copying.c - context due to missing refactoring
	copy/null-ops.c - no backport of 0b16205e "copy: Implement "null:"
  destination."
(cherry picked from commit 26e3dcf80815fe2db320d3046aabc2580c2f7a0d)
---
 copy/file-ops.c             | 22 +++++++++++++---------
 copy/multi-thread-copying.c |  8 +++++---
 2 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/copy/file-ops.c b/copy/file-ops.c
index 086348a..cc312b4 100644
--- a/copy/file-ops.c
+++ b/copy/file-ops.c
@@ -1,5 +1,5 @@
 /* NBD client library in userspace.
- * Copyright (C) 2020 Red Hat Inc.
+ * Copyright (C) 2020-2022 Red Hat Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -158,10 +158,11 @@ file_asynch_read (struct rw *rw,
                   struct command *command,
                   nbd_completion_callback cb)
 {
+  int dummy = 0;
+
   file_synch_read (rw, slice_ptr (command->slice),
                    command->slice.len, command->offset);
-  errno = 0;
-  if (cb.callback (cb.user_data, &errno) == -1) {
+  if (cb.callback (cb.user_data, &dummy) == -1) {
     perror (rw->name);
     exit (EXIT_FAILURE);
   }
@@ -172,10 +173,11 @@ file_asynch_write (struct rw *rw,
                    struct command *command,
                    nbd_completion_callback cb)
 {
+  int dummy = 0;
+
   file_synch_write (rw, slice_ptr (command->slice),
                     command->slice.len, command->offset);
-  errno = 0;
-  if (cb.callback (cb.user_data, &errno) == -1) {
+  if (cb.callback (cb.user_data, &dummy) == -1) {
     perror (rw->name);
     exit (EXIT_FAILURE);
   }
@@ -185,10 +187,11 @@ static bool
 file_asynch_trim (struct rw *rw, struct command *command,
                   nbd_completion_callback cb)
 {
+  int dummy = 0;
+
   if (!file_synch_trim (rw, command->offset, command->slice.len))
     return false;
-  errno = 0;
-  if (cb.callback (cb.user_data, &errno) == -1) {
+  if (cb.callback (cb.user_data, &dummy) == -1) {
     perror (rw->name);
     exit (EXIT_FAILURE);
   }
@@ -199,10 +202,11 @@ static bool
 file_asynch_zero (struct rw *rw, struct command *command,
                   nbd_completion_callback cb)
 {
+  int dummy = 0;
+
   if (!file_synch_zero (rw, command->offset, command->slice.len))
     return false;
-  errno = 0;
-  if (cb.callback (cb.user_data, &errno) == -1) {
+  if (cb.callback (cb.user_data, &dummy) == -1) {
     perror (rw->name);
     exit (EXIT_FAILURE);
   }
diff --git a/copy/multi-thread-copying.c b/copy/multi-thread-copying.c
index a7aaa7d..2593ff7 100644
--- a/copy/multi-thread-copying.c
+++ b/copy/multi-thread-copying.c
@@ -1,5 +1,5 @@
 /* NBD client library in userspace.
- * Copyright (C) 2020 Red Hat Inc.
+ * Copyright (C) 2020-2022 Red Hat Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -391,6 +391,7 @@ finished_read (void *vp, int *error)
     bool last_is_hole = false;
     uint64_t i;
     struct command *newcommand;
+    int dummy = 0;
 
     /* Iterate over whole blocks in the command, starting on a block
      * boundary.
@@ -473,7 +474,7 @@ finished_read (void *vp, int *error)
     /* Free the original command since it has been split into
      * subcommands and the original is no longer needed.
      */
-    free_command (command, &errno);
+    free_command (command, &dummy);
   }
 
   return 1; /* auto-retires the command */
@@ -498,6 +499,7 @@ static void
 fill_dst_range_with_zeroes (struct command *command)
 {
   char *data;
+  int dummy = 0;
 
   if (destination_is_zero)
     goto free_and_return;
@@ -541,7 +543,7 @@ fill_dst_range_with_zeroes (struct command *command)
   free (data);
 
  free_and_return:
-  free_command (command, &errno);
+  free_command (command, &dummy);
 }
 
 static int
-- 
2.31.1