[PATCH] be paranoid about closed stdin/stdout/stderr

It is in general unsafe to start git with one or more of file descriptors
0/1/2 closed.  Karl Chen for example noticed that stat_command does this
in order to rename a pipe file descriptor to 0:

    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.  Another extremely rare but nasty problem
would occur if an "important" file ends up in file descriptor 2, and is
corrupted by a call to die().

This patch fixes these problems by opening all of the "low" descriptors
to /dev/null in main.

Signed-off-by: Paolo Bonzini <redacted>
---
 git.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/git.c b/git.c
index 89e4645..be227b2 100644
--- a/git.c
+++ b/git.c

@@ -420,6 +420,19 @@ int main(int argc, const char **argv)
 	const char *cmd = argv[0] && *argv[0] ? argv[0] : "git-help";
 	char *slash = (char *)cmd + strlen(cmd);
 	int done_alias = 0;
+	int devnull_fd;
+
+	/*
+	 * Always open file descriptors 0/1/2 to avoid clobbering files
+	 * in die().  It also avoids not messing up when the pipes are
+	 * dup'ed onto stdin/stdout/stderr in the child processes we spawn.
+	 */
+	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));
+	close (devnull_fd);
 
 	/*
 	 * Take the basename of argv[0] as the command

-- 
1.5.5