[PATCH] setup: avoid unconditional open with write flags

["Christian Göttsche" <cgzones@googlemail.com>]

Commit 57f5d52a942 ("common-main: call sanitize_stdfds()") added the
sanitization for standard file descriptors (stdin, stdout, stderr) to
all binaries.  The lead to all binaries unconditionally opening
/dev/null with the flag O_RDWR (read and write).  Most of the time the
standard file descriptors should be set up properly and the sanitization
ends up doing nothing.

There are many non modifying git operations, like `git status` or `git
stash list`, which might be called by a parent to gather information
about the repository.  That parent might run under a seccomp filter to
avoid accidental modification or unwanted command execution on memory
corruptions.  As part of that seccomp filter open(2) and openat(2) might
be only allowed in read-only mode (O_RDONLY), thus preventing git's
sanitation and stopping the application.

Check before opening /dev/null to populate a possible non-present
standard file descriptor if actually any is missing.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
Alternatively one could add a command line argument
(`--no-stdfd-sanitization`).
---
 setup.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/setup.c b/setup.c
index cefd5f6..2af7170 100644
--- a/setup.c
+++ b/setup.c
@@ -1669,7 +1669,14 @@ const char *resolve_gitdir_gently(const char *suspect, int *return_error_code)
 /* if any standard file descriptor is missing open it to /dev/null */
 void sanitize_stdfds(void)
 {
-	int fd = xopen("/dev/null", O_RDWR);
+	int fd;
+
+	if (fcntl(0, F_GETFD) != -1 &&
+	    fcntl(1, F_GETFD) != -1 &&
+	    fcntl(2, F_GETFD) != -1)
+		return;
+
+	fd = xopen("/dev/null", O_RDWR);
 	while (fd < 2)
 		fd = xdup(fd);
 	if (fd > 2)
-- 
2.38.1