Subject: [PATCH 1/2] Fix child spawning closing standard fds prematurely

From: Josselin Poiret <[email-masked]>
Date: Mon, 27 Dec 2021 22:35:37 +0100

* libguile/posix.c (start_child): Close standard file descriptors only
after all of them have been dup2'd.
---
 libguile/posix.c | 40 +++++++++++++++++++---------------------
 1 file changed, 19 insertions(+), 21 deletions(-)

diff --git a/libguile/posix.c b/libguile/posix.c
index 3ab12b99e..148ebeb3d 100644
--- a/libguile/posix.c
+++ b/libguile/posix.c
@@ -1357,27 +1357,25 @@ start_child (const char *exec_file, char **exec_argv,
   if (err == -1)
     err = open ("/dev/null", O_WRONLY);
 
-  if (in > 0)
-    {
-      if (out == 0)
-        out = renumber_file_descriptor (out, err);
-      if (err == 0)
-        err = renumber_file_descriptor (err, err);
-      do dup2 (in, 0); while (errno == EINTR);
-      close (in);
-    }
-  if (out > 1)
-    {
-      if (err == 1)
-        err = renumber_file_descriptor (err, err);
-      do dup2 (out, 1); while (errno == EINTR);
-      close (out);
-    }
-  if (err > 2)
-    {
-      do dup2 (err, 2); while (errno == EINTR);
-      close (err);
-    }
+  /* Dup each non-yet-dup2'd fd that's in the way to the next available fd,
+     so that we can safely dup2 to 0/1/2 without potentially overwriting
+     in/out/err.  Note that dup2 doesn't do anything if its arguments are
+     equal. */
+  if (out == 0)
+    do out = dup (out); while (errno == EINTR);
+  if (err == 0)
+    do err = dup (err); while (errno == EINTR);
+  do dup2 (in, 0); while (errno == EINTR);
+
+  if (err == 1)
+    do err = dup (err); while (errno == EINTR);
+  do dup2 (out, 1); while (errno == EINTR);
+
+  do dup2 (err, 2); while (errno == EINTR);
+
+  if (in > 2) close (in);
+  if (out > 2) close (out);
+  if (err > 2) close (err);
 
   execvp (exec_file, exec_argv);
 
-- 
2.34.0