[Libguestfs] [PATCH nbdkit] server: Try hard to maintain invariant that fds 0, 1 and 2 are always open.

[Richard W.M. Jones]

https://www.redhat.com/archives/libguestfs/2019-August/thread.html#00347

Thanks: Eric Blake and Daniel P. Berrangé
---
 common/utils/utils.h |  1 +
 server/connections.c |  4 ++--
 server/crypto.c      |  5 +++--
 server/main.c        | 23 +++++++++++++++++++++++
 common/utils/utils.c | 29 +++++++++++++++++++++++++++++
 5 files changed, 58 insertions(+), 4 deletions(-)

diff --git a/common/utils/utils.h b/common/utils/utils.h
index ebd5f66..a77d2cd 100644
--- a/common/utils/utils.h
+++ b/common/utils/utils.h
@@ -38,5 +38,6 @@ extern void uri_quote (const char *str, FILE *fp);
 extern int exit_status_to_nbd_error (int status, const char *cmd);
 extern int set_cloexec (int fd);
 extern int set_nonblock (int fd);
+extern void close_or_nullify_fd (int fd);
 
 #endif /* NBDKIT_UTILS_H */
diff --git a/server/connections.c b/server/connections.c
index c173df8..f57ab3e 100644
--- a/server/connections.c
+++ b/server/connections.c
@@ -489,7 +489,7 @@ static void
 raw_close (struct connection *conn)
 {
   if (conn->sockin >= 0)
-    close (conn->sockin);
+    close_or_nullify_fd (conn->sockin);
   if (conn->sockout >= 0 && conn->sockin != conn->sockout)
-    close (conn->sockout);
+    close_or_nullify_fd (conn->sockout);
 }
diff --git a/server/crypto.c b/server/crypto.c
index 9cd1bb0..6f97f2c 100644
--- a/server/crypto.c
+++ b/server/crypto.c
@@ -47,6 +47,7 @@
 #include <assert.h>
 
 #include "internal.h"
+#include "utils.h"
 
 #ifdef HAVE_GNUTLS
 
@@ -404,9 +405,9 @@ crypto_close (struct connection *conn)
   gnutls_bye (session, GNUTLS_SHUT_RDWR);
 
   if (sockin >= 0)
-    close (sockin);
+    close_or_nullify_fd (sockin);
   if (sockout >= 0 && sockin != sockout)
-    close (sockout);
+    close_or_nullify_fd (sockout);
 
   gnutls_deinit (session);
   conn->crypto_session = NULL;
diff --git a/server/main.c b/server/main.c
index 65025a6..65bef30 100644
--- a/server/main.c
+++ b/server/main.c
@@ -60,6 +60,7 @@ static struct backend *open_filter_so (struct backend *next, size_t i, const cha
 static void start_serving (void);
 static void write_pidfile (void);
 static bool is_config_key (const char *key, size_t len);
+static void open_std_file_descriptors (void);
 
 struct debug_flag *debug_flags; /* -D */
 bool exit_with_parent;          /* --exit-with-parent */
@@ -149,6 +150,13 @@ main (int argc, char *argv[])
   size_t i;
   const char *magic_config_key;
 
+  /* Ensures that fds 0, 1 and 2 are open (on /dev/null if nothing
+   * else).  This is so that nbdkit and plugins can assume these file
+   * descriptors are always open, which makes certain code easier to
+   * write.
+   */
+  open_std_file_descriptors ();
+
   threadlocal_init ();
 
   /* The default setting for TLS depends on whether we were
@@ -930,3 +938,18 @@ is_config_key (const char *key, size_t len)
 
   return true;
 }
+
+static void
+open_std_file_descriptors (void)
+{
+  int fds[] = { STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO };
+  int i, fd, fl;
+
+  for (i = 0; i < sizeof fds / sizeof fds[0]; ++i) {
+    fd = fds[i];
+    fl = fcntl (fd, F_GETFL, NULL);
+    if (fl == -1 && errno == EBADF)
+      /* This is best effort - don't fail. */
+      open ("/dev/null", fd == STDIN_FILENO ? O_RDONLY : O_WRONLY);
+  }
+}
diff --git a/common/utils/utils.c b/common/utils/utils.c
index 32bc96a..c818d4c 100644
--- a/common/utils/utils.c
+++ b/common/utils/utils.c
@@ -190,3 +190,32 @@ set_nonblock (int fd) {
   }
   return fd;
 }
+
+/* Calls close (fd).  As a special case if fd is stdin/stdout/stderr
+ * then it reopens the file descriptor on /dev/null.  This allows us
+ * to maintain certain invariants for nbdkit and plugins.  In all
+ * cases this ignores errors.
+ */
+void
+close_or_nullify_fd (int fd)
+{
+  int flags;
+  int nfd;
+
+  close (fd);
+
+  if (fd == STDIN_FILENO)
+    flags = O_RDONLY;
+  else if (fd == STDOUT_FILENO || fd == STDERR_FILENO)
+    flags = O_WRONLY;
+  else
+    return;
+
+  nfd = open ("/dev/null", flags);
+  if (nfd == -1)
+    return;
+  if (nfd != fd) {
+    dup2 (nfd, fd);
+    close (nfd);
+  }
+}
-- 
2.22.0