[PATCH] linux-user: mprotect() should returns 0 when len is 0.

[Soichiro Isshiki]

From: sisshiki1969 <sisshiki@mac.com>

For now, qemu-x86_64 returns ENOMEM when mprotect() was called with an argument
len is 0 from a guest process.
This behavior is incompatible with the current Linux implementation,
which mprotect() with len = 0 does nothing and returns 0,
although it does not appear to be explicitly described in man.

This is due to the following function which always returns false if len = 0.

```C
static inline bool guest_range_valid_untagged(abi_ulong start, abi_ulong len)
{
    return len - 1 <= GUEST_ADDR_MAX && start <= GUEST_ADDR_MAX - len + 1;
}

```

This patch fix this incompatibility problem.

Signed-off-by: sisshiki1969 <sisshiki@mac.com>
---
 linux-user/mmap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 28f3bc85ed..1ed79459ea 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -130,12 +130,12 @@ int target_mprotect(abi_ulong start, abi_ulong len, int 
target_prot)
     }
     len = TARGET_PAGE_ALIGN(len);
     end = start + len;
-    if (!guest_range_valid_untagged(start, len)) {
-        return -TARGET_ENOMEM;
-    }
     if (len == 0) {
         return 0;
     }
+    if (!guest_range_valid_untagged(start, len)) {
+        return -TARGET_ENOMEM;
+    }
 
     mmap_lock();
     host_start = start & qemu_host_page_mask;
-- 
2.25.1