util: in make_stdio() use dup2() rather than dup3()

dup3() allows setting O_CLOEXEC which we are not interested in. However,
it also fails if called with the same fd as input and output, which is
something we don't want. Hence use dup2().

Also, we need to explicitly turn off O_CLOEXEC for the fds, in case the
input fd was O_CLOEXEC and < 3.

1 files changed, 8 insertions, 4 deletions

diff --git a/src/shared/util.c b/src/shared/util.c
index ee95a4b6f7..364f61885b 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -2468,9 +2468,9 @@ int make_stdio(int fd) {
 
         assert(fd >= 0);
 
-        r = dup3(fd, STDIN_FILENO, 0);
-        s = dup3(fd, STDOUT_FILENO, 0);
-        t = dup3(fd, STDERR_FILENO, 0);
+        r = dup2(fd, STDIN_FILENO);
+        s = dup2(fd, STDOUT_FILENO);
+        t = dup2(fd, STDERR_FILENO);
 
         if (fd >= 3)
                 safe_close(fd);
@@ -2478,7 +2478,11 @@ int make_stdio(int fd) {
         if (r < 0 || s < 0 || t < 0)
                 return -errno;
 
-        /* We rely here that the new fd has O_CLOEXEC not set */
+        /* Explicitly unset O_CLOEXEC, since if fd was < 3, then
+         * dup2() was a NOP and the bit hence possibly set. */
+        fd_cloexec(STDIN_FILENO, false);
+        fd_cloexec(STDOUT_FILENO, false);
+        fd_cloexec(STDERR_FILENO, false);
 
         return 0;
 }