[PATCH] xfs_io: Make HAVE_MAP_SYNC more robust

[Florian Fainelli <f.fainelli@gmail.com>]

MIPS platforms building with recent kernel headers and the musl-libc toolchain
will expose the following build failure:

mmap.c: In function 'mmap_f':
mmap.c:196:12: error: 'MAP_SYNC' undeclared (first use in this function); did you mean 'MS_SYNC'?
  196 |    flags = MAP_SYNC | MAP_SHARED_VALIDATE;
      |            ^~~~~~~~
      |            MS_SYNC
mmap.c:196:12: note: each undeclared identifier is reported only once for each function it appears in
make[4]: *** [../include/buildrules:81: mmap.o] Error 1

The reason for that is that the linux.h header file which intends to provide a fallback definition for MAP_SYNC and MAP_SHARED_VALIDATE is included too early through:

input.h -> libfrog/projects.h -> xfs.h -> linux.h and this happens
*before* sys/mman.h is included.

sys/mman.h -> bits/mman.h which has a:
  #undef MAP_SYNC

see: https://git.musl-libc.org/cgit/musl/tree/arch/mips/bits/mman.h#n21

The end result is that sys/mman.h being included for the first time
ends-up overriding the HAVE_MAP_SYNC fallbacks.

To remedy that, make sure that linux.h is updated to include sys/mman.h
such that its fallbacks are independent of the inclusion order. As a
consequence this forces us to ensure that we do not re-define
accidentally MAP_SYNC or MAP_SHARED_VALIDATE so we protect against that.

Fixes: dad796834cb9 ("xfs_io: add MAP_SYNC support to mmap()")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 include/linux.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux.h b/include/linux.h
index 3d9f4e3dca80..c3cc8e30c677 100644
--- a/include/linux.h
+++ b/include/linux.h
@@ -252,8 +252,13 @@ struct fsxattr {
 #endif
 
 #ifndef HAVE_MAP_SYNC
+#include <sys/mman.h>
+#ifndef MAP_SYNC
 #define MAP_SYNC 0
+#endif
+#ifndef MAP_SHARED_VALIDATE
 #define MAP_SHARED_VALIDATE 0
+#endif
 #else
 #include <asm-generic/mman.h>
 #include <asm-generic/mman-common.h>
-- 
2.25.1