tmpfiles: use write(2) for the 'w' action

This resolves problems with filesystems which do not implement the
aio_write file operation. In this case, the kernel will fall back using
a loop writing technique for each pointer in a received iovec. The
result is strange errors in dmesg such as:

[   31.855871] elevator: type  not found
[   31.856262] elevator: switch to
[   31.856262]  failed

It does not make sense to implement a synchronous aio_write method for
sysfs as this isn't a real filesystem where a reasonable use case for
using writev exists, nor is there an expectation that tmpfiles will be
used to write more data than can be reasonably written in a single write
syscall.

In addition, some sysfs attrs are currently buggy and will NOT reject
the second write with the newline, causing the sysfs value to be zeroed
out. This of course should be fixed in the kernel regardless of any
wrongdoing in userspace, but this simple change makes us immune to such
a bug.

This change means that we do not write a trailing newline by default, as
the expected use case of 'w' is for sysfs and procfs. In exchange, honor
C-style backslash escapes so that if the newline is really needed, the
user can add it.

2 files changed, 11 insertions, 16 deletions

diff --git a/man/tmpfiles.d.xml b/man/tmpfiles.d.xml
index c6325a457b..a86ef33695 100644
--- a/man/tmpfiles.d.xml
+++ b/man/tmpfiles.d.xml
@@ -114,7 +114,10 @@ L    /tmp/foobar -    -    -    -   /dev/null</programlisting>
 
                                 <varlistentry>
                                         <term><varname>w</varname></term>
-                                        <listitem><para>Write the argument parameter to a file, if it exists. Lines of this type accept shell-style globs in place of normal path names.</para></listitem>
+                                        <listitem><para>Write the argument parameter to a file, if the file exists.
+                                            Lines of this type accept shell-style globs in place of normal path
+                                            names. The argument parameter will be written without a trailing
+                                            newline. C-style backslash escapes are interpreted.</para></listitem>
                                 </varlistentry>
 
                                 <varlistentry>
diff --git a/src/tmpfiles/tmpfiles.c b/src/tmpfiles/tmpfiles.c
index ed51ec8a7f..5d0f571bea 100644
--- a/src/tmpfiles/tmpfiles.c
+++ b/src/tmpfiles/tmpfiles.c
@@ -42,6 +42,7 @@
 
 #include "log.h"
 #include "util.h"
+#include "macro.h"
 #include "mkdir.h"
 #include "path-util.h"
 #include "strv.h"
@@ -503,24 +504,15 @@ static int write_one_file(Item *i, const char *path) {
         if (i->argument) {
                 ssize_t n;
                 size_t l;
-                struct iovec iovec[2];
-                static const char new_line = '\n';
+                _cleanup_free_ char *unescaped;
 
-                l = strlen(i->argument);
+                unescaped = cunescape(i->argument);
+                if (unescaped == NULL)
+                        return log_oom();
 
-                zero(iovec);
-                iovec[0].iov_base = i->argument;
-                iovec[0].iov_len = l;
+                l = strlen(unescaped);
+                n = write(fd, unescaped, l);
 
-                iovec[1].iov_base = (void*) &new_line;
-                iovec[1].iov_len = 1;
-
-                n = writev(fd, iovec, 2);
-
-                /* It's OK if we don't write the trailing
-                 * newline, hence we check for l, instead of
-                 * l+1 here. Files in /sys often refuse
-                 * writing of the trailing newline. */
                 if (n < 0 || (size_t) n < l) {
                         log_error("Failed to write file %s: %s", path, n < 0 ? strerror(-n) : "Short write");
                         close_nointr_nofail(fd);