[PATCH 4/8] linux_dep: Fix CLOSE_RANGE_UNSHARE availability handling

[David Gibson <david@gibson.dropbear.id.au>]

If CLOSE_RANGE_UNSHARE isn't defined, we define a fallback version of
close_range() which is a (successful) no-op.  This is broken in several
ways:
 * It doesn't actually fix compile if using old kernel headers, because
   the caller of close_range() still directly uses CLOSE_RANGE_UNSHARE
   unprotected by ifdefs
 * Even if it did fix the compile, it means inconsistent behaviour between
   a compile time failure to find the value (we silently don't close files)
   and a runtime failure (we die with an error from close_range())
 * Silently not closing the files we intend to close for security reasons
   is probably not a good idea in any case

As bonus this fixes a cppcheck error I see with some different options I'm
looking to apply in future.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 linux_dep.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/linux_dep.h b/linux_dep.h
index 3a41e42..240f50a 100644
--- a/linux_dep.h
+++ b/linux_dep.h
@@ -127,22 +127,18 @@ struct tcp_info_linux {
 
 #include <linux/close_range.h>
 
-#ifdef CLOSE_RANGE_UNSHARE	/* Linux kernel >= 5.9 */
 /* glibc < 2.34 and musl as of 1.2.5 need these */
 #ifndef SYS_close_range
 #define SYS_close_range		436
 #endif
+#ifndef CLOSE_RANGE_UNSHARE	/* Linux kernel < 5.9 */
+#define CLOSE_RANGE_UNSHARE	(1U << 1)
+#endif
+
 __attribute__ ((weak))
 /* cppcheck-suppress funcArgNamesDifferent */
 int close_range(unsigned int first, unsigned int last, int flags) {
 	return syscall(SYS_close_range, first, last, flags);
 }
-#else
-/* No reasonable fallback option */
-/* cppcheck-suppress funcArgNamesDifferent */
-int close_range(unsigned int first, unsigned int last, int flags) {
-	return 0;
-}
-#endif
 
 #endif /* LINUX_DEP_H */
-- 
@@ -127,22 +127,18 @@ struct tcp_info_linux {
 
 #include <linux/close_range.h>
 
-#ifdef CLOSE_RANGE_UNSHARE	/* Linux kernel >= 5.9 */
 /* glibc < 2.34 and musl as of 1.2.5 need these */
 #ifndef SYS_close_range
 #define SYS_close_range		436
 #endif
+#ifndef CLOSE_RANGE_UNSHARE	/* Linux kernel < 5.9 */
+#define CLOSE_RANGE_UNSHARE	(1U << 1)
+#endif
+
 __attribute__ ((weak))
 /* cppcheck-suppress funcArgNamesDifferent */
 int close_range(unsigned int first, unsigned int last, int flags) {
 	return syscall(SYS_close_range, first, last, flags);
 }
-#else
-/* No reasonable fallback option */
-/* cppcheck-suppress funcArgNamesDifferent */
-int close_range(unsigned int first, unsigned int last, int flags) {
-	return 0;
-}
-#endif
 
 #endif /* LINUX_DEP_H */
-- 
2.47.0