greatly improve freopen behavior

1. don't open /dev/null just as a basis to copy flags; use shared
__fmodeflags function to get the right file flags for the mode.

2. handle the case (probably invalid, but whatever) case where the
original stream's file descriptor was closed; previously, the logic
re-closed it.

3. accept the "e" mode flag for close-on-exec; update dup3 to fallback
to using dup2 so we can simply call __dup3 instead of putting fallback
logic in freopen itself.

3 files changed, 27 insertions, 15 deletions

diff --git a/src/stdio/__fmodeflags.c b/src/stdio/__fmodeflags.c
new file mode 100644
index 00000000..da9f23b6
--- /dev/null
+++ b/src/stdio/__fmodeflags.c
@@ -0,0 +1,16 @@
+#include <fcntl.h>
+#include <string.h>
+
+int __fmodeflags(const char *mode)
+{
+	int flags;
+	if (strchr(mode, '+')) flags = O_RDWR;
+	else if (*mode == 'r') flags = O_RDONLY;
+	else flags = O_WRONLY;
+	if (strchr(mode, 'x')) flags |= O_EXCL;
+	if (strchr(mode, 'e')) flags |= O_CLOEXEC;
+	if (*mode != 'r') flags |= O_CREAT;
+	if (*mode == 'w') flags |= O_TRUNC;
+	if (*mode == 'a') flags |= O_APPEND;
+	return flags;
+}
diff --git a/src/stdio/fopen.c b/src/stdio/fopen.c
index 03c10cd1..c741aede 100644
--- a/src/stdio/fopen.c
+++ b/src/stdio/fopen.c
@@ -13,14 +13,7 @@ FILE *fopen(const char *restrict filename, const char *restrict mode)
 	}
 
 	/* Compute the flags to pass to open() */
-	if (strchr(mode, '+')) flags = O_RDWR;
-	else if (*mode == 'r') flags = O_RDONLY;
-	else flags = O_WRONLY;
-	if (strchr(mode, 'x')) flags |= O_EXCL;
-	if (strchr(mode, 'e')) flags |= O_CLOEXEC;
-	if (*mode != 'r') flags |= O_CREAT;
-	if (*mode == 'w') flags |= O_TRUNC;
-	if (*mode == 'a') flags |= O_APPEND;
+	flags = __fmodeflags(mode);
 
 	fd = syscall_cp(SYS_open, filename, flags|O_LARGEFILE, 0666);
 	if (fd < 0) return 0;
diff --git a/src/stdio/freopen.c b/src/stdio/freopen.c
index 5b4f126d..c80ce3b4 100644
--- a/src/stdio/freopen.c
+++ b/src/stdio/freopen.c
@@ -7,24 +7,27 @@
 /* Locking is not necessary because, in the event of failure, the stream
  * passed to freopen is invalid as soon as freopen is called. */
 
+int __dup3(int, int, int);
+
 FILE *freopen(const char *restrict filename, const char *restrict mode, FILE *restrict f)
 {
-	int fl;
+	int fl = __fmodeflags(mode);
 	FILE *f2;
 
 	fflush(f);
 
 	if (!filename) {
-		f2 = fopen("/dev/null", mode);
-		if (!f2) goto fail;
-		fl = __syscall(SYS_fcntl, f2->fd, F_GETFL, 0);
+		if (fl&O_CLOEXEC)
+			__syscall(SYS_fcntl, f->fd, F_SETFD, FD_CLOEXEC);
+		fl &= ~(O_CREAT|O_EXCL|O_CLOEXEC);
 		if (syscall(SYS_fcntl, f->fd, F_SETFL, fl) < 0)
-			goto fail2;
+			goto fail;
+		return f;
 	} else {
 		f2 = fopen(filename, mode);
 		if (!f2) goto fail;
-		if (syscall(SYS_dup2, f2->fd, f->fd) < 0)
-			goto fail2;
+		if (f2->fd == f->fd) f2->fd = -1; /* avoid closing in fclose */
+		else if (__dup3(f2->fd, f->fd, fl&O_CLOEXEC)<0) goto fail2;
 	}
 
 	f->flags = (f->flags & F_PERM) | f2->flags;