[PATCH] x86/atomic: Fix UB on various atomic helpers

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

The atomic interface is signed. There's UB if a caller adds over INT_MAX
or subtracts below INT_MIN where the addition/subtraction is done in
C, just like in a regular int. Make the outward facing interface respect
two's complement on wraparound by round-tripping through unsigned for
arithmetic.

While at it, fix the while loop to have Xen style.

Fixes: e0efc0738163 ("atomic: replace atomic_compareandswap() with 
atomic_cmpxchg()")
Fixes: c98c80084d77 ("asm/atomic.h: implement missing and add common 
prototypes")
Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
---
I'd prefer to turn this into an unsigned interface, but then some
callers would need changing in non-trivial ways.

OTOH, one could argue the atomics merely highlight the UB already
present in signed arithmetic, and thus this doesn't need fixing. I'd
prefer to remove UB where possible.
---
 xen/arch/x86/include/asm/atomic.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/include/asm/atomic.h 
b/xen/arch/x86/include/asm/atomic.h
index ed4e09a503..8f227e0e6f 100644
--- a/xen/arch/x86/include/asm/atomic.h
+++ b/xen/arch/x86/include/asm/atomic.h
@@ -122,7 +122,7 @@ static inline void atomic_add(int i, atomic_t *v)
 
 static inline int atomic_add_return(int i, atomic_t *v)
 {
-    return i + arch_fetch_and_add(&v->counter, i);
+    return (unsigned int)i + arch_fetch_and_add(&v->counter, i);
 }
 
 static inline void atomic_sub(int i, atomic_t *v)
@@ -135,7 +135,9 @@ static inline void atomic_sub(int i, atomic_t *v)
 
 static inline int atomic_sub_return(int i, atomic_t *v)
 {
-    return arch_fetch_and_add(&v->counter, -i) - i;
+    unsigned int ui = i;
+
+    return arch_fetch_and_add(&v->counter, -ui) - ui;
 }
 
 static inline int atomic_sub_and_test(int i, atomic_t *v)
@@ -221,8 +223,9 @@ static inline int atomic_add_unless(atomic_t *v, int a, int 
u)
     int c, old;
 
     c = atomic_read(v);
-    while (c != u && (old = atomic_cmpxchg(v, c, c + a)) != c)
+    while ( c != u && (old = atomic_cmpxchg(v, c, (unsigned int)c + a)) != c )
         c = old;
+
     return c;
 }
 

base-commit: 7168cc02f86fb069da6e7405afbf5c7a1c215b0d
-- 
2.43.0