There is a problem in the use of dup2+close in start_command()
when one or more of file descriptors 0/1/2 are closed. In order
to rename a pipe file descriptor to 0, it does
dup2(from, 0);
close(from);
... but if stdin was closed (for example) from == 0, so that
dup2(0, 0);
close(0);
just ends up closing the pipe. This patch fixes it by opening all of
the "low" descriptors to /dev/null.
In most cases this patch will not cause any additional system calls;
actually by reusing the /dev/null descriptor when possible (instead
of opening a fresh one in dup_devnull) it may even save a handful in
some cases. :-)
Signed-off-by: Paolo Bonzini <bonzini@xxxxxxx>
---
run-command.c | 35 ++++++++++++++++++++++-------------
1 files changed, 22 insertions(+), 13 deletions(-)
diff --git a/run-command.c b/run-command.c
index caab374..4619494 100644
--- a/run-command.c
+++ b/run-command.c
@@ -2,25 +2,34 @@
#include "run-command.h"
#include "exec_cmd.h"
+static int devnull_fd = -1;
+
static inline void close_pair(int fd[2])
{
close(fd[0]);
close(fd[1]);
}
-static inline void dup_devnull(int to)
-{
- int fd = open("/dev/null", O_RDWR);
- dup2(fd, to);
- close(fd);
-}
-
int start_command(struct child_process *cmd)
{
int need_in, need_out, need_err;
int fdin[2], fdout[2], fderr[2];
/*
+ * Make sure that all file descriptors <= 2 are open, otherwise we
+ * mess them up when dup'ing pipes onto stdin/stdout/stderr. Since
+ * we are at it, open a file descriptor on /dev/null to use it later.
+ */
+ if (devnull_fd == -1)
+ {
+ devnull_fd = open("/dev/null", O_RDWR);
+ while (devnull_fd >= 0 && devnull_fd <= 2)
+ devnull_fd = dup(devnull_fd);
+ if (devnull_fd == -1)
+ die("opening /dev/null failed (%s)", strerror(errno));
+ }
+
+ /*
* In case of errors we must keep the promise to close FDs
* that have been passed in via ->in and ->out.
*/
@@ -72,7 +81,7 @@ int start_command(struct child_process *cmd)
cmd->pid = fork();
if (!cmd->pid) {
if (cmd->no_stdin)
- dup_devnull(0);
+ dup2(devnull_fd, 0);
else if (need_in) {
dup2(fdin[0], 0);
close_pair(fdin);
@@ -82,14 +91,14 @@ int start_command(struct child_process *cmd)
}
if (cmd->no_stderr)
- dup_devnull(2);
+ dup2(devnull_fd, 2);
else if (need_err) {
dup2(fderr[1], 2);
close_pair(fderr);
}
if (cmd->no_stdout)
- dup_devnull(1);
+ dup2(devnull_fd, 1);
else if (cmd->stdout_to_stderr)
dup2(2, 1);
else if (need_out) {
@@ -127,7 +136,7 @@ int start_command(struct child_process *cmd)
if (cmd->no_stdin) {
s0 = dup(0);
- dup_devnull(0);
+ dup2(devnull_fd, 0);
} else if (need_in) {
s0 = dup(0);
dup2(fdin[0], 0);
@@ -138,7 +147,7 @@ int start_command(struct child_process *cmd)
if (cmd->no_stderr) {
s2 = dup(2);
- dup_devnull(2);
+ dup2(devnull_fd, 2);
} else if (need_err) {
s2 = dup(2);
dup2(fderr[1], 2);
@@ -146,7 +155,7 @@ int start_command(struct child_process *cmd)
if (cmd->no_stdout) {
s1 = dup(1);
- dup_devnull(1);
+ dup2(devnull_fd, 1);
} else if (cmd->stdout_to_stderr) {
s1 = dup(1);
dup2(2, 1);
--
1.5.5
--
To unsubscribe from this list: send the line "unsubscribe git" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
