[PATCH] linux-user/mmap.c: fix integer underflow in target_mremap

[Jonathan Marler]

Fixes: https://bugs.launchpad.net/bugs/1876373

This code path in mmap occurs when a page size is decreased with mremap.  When 
a section of pages is shrunk, qemu calls mmap_reserve on the pages that were 
released.  However, it has the diff operation reversed, subtracting the larger 
old_size from the smaller new_size.  Instead, it should be subtracting the 
smaller new_size from the larger old_size.  You can also see in the previous 
line of the change that this mmap_reserve call only occurs when old_size > 
new_size.

Signed-off-by: Jonathan Marler <address@hidden>
---
 linux-user/mmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index e378033797..caab62909e 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -708,7 +708,7 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong 
old_size,
         if (prot == 0) {
             host_addr = mremap(g2h(old_addr), old_size, new_size, flags);
             if (host_addr != MAP_FAILED && reserved_va && old_size > new_size) 
{
-                mmap_reserve(old_addr + old_size, new_size - old_size);
+                mmap_reserve(old_addr + old_size, old_size - new_size);
             }
         } else {
             errno = ENOMEM;
-- 
2.23.1