[PATCH] unlzma: avoid UB shift

[Jan Beulich <jbeulich@xxxxxxxx>]

Shifting signed quantities has restrictions. Since the wrapping macro of
read_int() type-casts the result anyway, switch function return type as
well as the local variable to the corresponding unsigned type.

Reported-by: Kamil Frankowicz <kamil.frankowicz@xxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
We've inherited that code from Linux, and the same code still exists
there. As I'm entirely uncertain whether they would even care, I'd prefer
to not take the route of posting a patch against Linux first.

--- a/xen/common/unlzma.c
+++ b/xen/common/unlzma.c
@@ -30,10 +30,10 @@
 
 #include "decompress.h"
 
-static long long __init read_int(unsigned char *ptr, int size)
+static unsigned long long __init read_int(unsigned char *ptr, int size)
 {
        int i;
-       long long ret = 0;
+       unsigned long long ret = 0;
 
        for (i = 0; i < size; i++)
                ret = (ret << 8) | ptr[size-i-1];