[musl] [PATCH] add close_range() syscall wrapper

[Natanael Copa <ncopa@alpinelinux.org>]

close_range() is a syscall present in FreeBSD 8.0 and Linux 5.9. glibc
2.34 added a wrapper.

Expose it under _GNU_SOURCE similar to what GNU libc does. Also expose
it under _BSD_SOURCE since it is also a FreeBSD function.
---

This is a re-take of close_range() syscall wrapper. It was previously
discussed in mailing list: https://www.openwall.com/lists/musl/2022/08/18/5

Difference from previous submission:

- Use correct values for CLOSE_RANGE_UNSHARE and CLOSE_RANGE_CLOEXEC.
- Set errno on errors.
- Drop the (unsigned int) cast for flags, as it raised questions last
  time.

I think it is a good idea to add this to musl because it is difficult to
the close before exec properly without it. 

Most workaounrds currently out there are either parsing /proc or try to
close everything to maxfd reported by getrlimit(), sysconf() or
getdtablesize().

opendir("/proc/self/fd") is problematic becase 1) /proc may not be
mounted and 2) some versions of musl hangs on malloc between fork and 
exec.

Trying to close everything between a maxfd is also problematic. On some
systems (under docker for example) the maxfd can be 1G, which
effectively results in a hang. (See https://github.com/k0sproject/k0s/pull/3436)

I think its better to encourage the use of close_range().

See also: https://github.com/OpenRC/openrc/pull/645

 include/unistd.h        | 3 +++
 src/linux/close_range.c | 8 ++++++++
 2 files changed, 11 insertions(+)
 create mode 100644 src/linux/close_range.c

diff --git a/include/unistd.h b/include/unistd.h
index 5bc7f798..d89e3d4c 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -161,6 +161,9 @@ unsigned ualarm(unsigned, unsigned);
 #define L_INCR 1
 #define L_XTND 2
 int brk(void *);
+#define CLOSE_RANGE_UNSHARE	(1U << 1)
+#define CLOSE_RANGE_CLOEXEC	(1U << 2)
+int close_range(unsigned int, unsigned int, int);
 void *sbrk(intptr_t);
 pid_t vfork(void);
 int vhangup(void);
diff --git a/src/linux/close_range.c b/src/linux/close_range.c
new file mode 100644
index 00000000..258ba8bd
--- /dev/null
+++ b/src/linux/close_range.c
@@ -0,0 +1,8 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include "syscall.h"
+
+int close_range(unsigned int first, unsigned int last, int flags)
+{
+	return __syscall_ret(syscall(SYS_close_range, first, last, flags));
+}
-- 
2.42.0