[PATCH for-4.19 1/3] tools/libxs: Fix CLOEXEC handling in get_dev()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Move the O_CLOEXEC compatibility outside of an #ifdef USE_PTHREAD block.

Introduce set_cloexec() to wrap fcntl() setting FD_CLOEXEC.  It will be reused
for other CLOEXEC fixes too.

Use set_cloexec() when O_CLOEXEC isn't available as a best-effort fallback.

Fixes: f4f2f3402b2f ("tools/libxs: Open /dev/xen/xenbus fds as O_CLOEXEC")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Anthony PERARD <anthony@xxxxxxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
CC: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
 tools/libs/store/xs.c | 38 ++++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/tools/libs/store/xs.c b/tools/libs/store/xs.c
index 14985150737e..037e79d98b58 100644
--- a/tools/libs/store/xs.c
+++ b/tools/libs/store/xs.c
@@ -40,6 +40,10 @@
 #include <xentoolcore_internal.h>
 #include <xen_list.h>
 
+#ifndef O_CLOEXEC
+#define O_CLOEXEC 0
+#endif
+
 struct xs_stored_msg {
        XEN_TAILQ_ENTRY(struct xs_stored_msg) list;
        struct xsd_sockmsg hdr;
@@ -54,10 +58,6 @@ struct xs_stored_msg {
 #include <dlfcn.h>
 #endif
 
-#ifndef O_CLOEXEC
-#define O_CLOEXEC 0
-#endif
-
 struct xs_handle {
        /* Communications channel to xenstore daemon. */
        int fd;
@@ -176,6 +176,16 @@ static bool setnonblock(int fd, int nonblock) {
        return true;
 }
 
+static bool set_cloexec(int fd)
+{
+       int flags = fcntl(fd, F_GETFL);
+
+       if (flags < 0)
+               return false;
+
+       return fcntl(fd, flags | FD_CLOEXEC) >= 0;
+}
+
 int xs_fileno(struct xs_handle *h)
 {
        char c = 0;
@@ -230,8 +240,24 @@ static int get_socket(const char *connect_to)
 
 static int get_dev(const char *connect_to)
 {
-       /* We cannot open read-only because requests are writes */
-       return open(connect_to, O_RDWR | O_CLOEXEC);
+       int fd, saved_errno;
+
+       fd = open(connect_to, O_RDWR | O_CLOEXEC);
+       if (fd < 0)
+               return -1;
+
+       /* Compat for non-O_CLOEXEC environments.  Racy. */
+       if (!O_CLOEXEC && !set_cloexec(fd))
+               goto error;
+
+       return fd;
+
+error:
+       saved_errno = errno;
+       close(fd);
+       errno = saved_errno;
+
+       return -1;
 }
 
 static int all_restrict_cb(Xentoolcore__Active_Handle *ah, domid_t domid) {
-- 
2.39.2