[PATCH] libs/port/mbtowc: Convert 1 character at a time if dst overlaps into src

[Gabriel Ivăncescu]

Some applications partially overlap the two buffers. For such rare corner
cases, do a single check so that there will be no performance regressions,
as unsigned operations are defined to wrap around.

Bug discovered by Jason Edmeades, revised from bug report.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=38558
Signed-off-by: Gabriel Ivăncescu 
---
 libs/port/mbtowc.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/libs/port/mbtowc.c b/libs/port/mbtowc.c
index 4977c82..6ed2c2e 100644
--- a/libs/port/mbtowc.c
+++ b/libs/port/mbtowc.c
@@ -65,6 +65,17 @@ static inline int mbstowcs_sbcs( const struct sbcs_table *table, int flags,
         ret = -1;
     }

+    /* dst can overlap into src, in which case we need to handle 1 char at a time,
+       but when src < dst, there is no issue no matter how many we handle at once */
+    if ((UINT_PTR) ((UINT_PTR)src - (UINT_PTR)dst) < srclen * sizeof(*dst))
+    {
+        do
+            *dst++ = cp2uni[*src++];
+        while (--srclen);
+        return ret;
+    }
+
+    /* dst doesn't overlap into src */
     for (;;)
     {
         switch(srclen)