From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date: Fri, 25 Apr 2025 17:28:22 +0000 (-0300)
Subject: string: Remove UB on ffs/ffsll
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=da2b1f328eadc9e242212cff6c2b627d97392344;p=thirdparty%2Fglibc.git

string: Remove UB on ffs/ffsll

Building with ubsan on 32 bit architecture, tst-ffs shows:

ffsll(0x4000000000000000) as expected 63
UBSAN: Undefined behaviour in ffsll.c:37:34 negation of 9223372036854775808 cannot be represented in type 'long long int'

Since the idea is to isolate the least significant bit, use unsigned
types.
---

diff --git a/string/ffs.c b/string/ffs.c
index a2aa5b1242..eda9f49197 100644
--- a/string/ffs.c
+++ b/string/ffs.c
@@ -40,7 +40,9 @@ __ffs (int i)
       8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8
     };
   unsigned int a;
-  unsigned int x = i & -i;
+  /* Isolate the least significant one.  */
+  unsigned int u_i = i;
+  unsigned int x = u_i & -u_i;
 
   a = x <= 0xffff ? (x <= 0xff ? 0 : 8) : (x <= 0xffffff ?  16 : 24);
 
diff --git a/string/ffsll.c b/string/ffsll.c
index 1b01d8fb12..ed2c877ad0 100644
--- a/string/ffsll.c
+++ b/string/ffsll.c
@@ -29,7 +29,9 @@ __ffsll (long long int i)
 #if USE_FFSLL_BUILTIN
   return __builtin_ffsll (i);
 #else
-  unsigned long long int x = i & -i;
+  /* Isolate the least significant one.  */
+  unsigned long long int i_u = i;
+  unsigned long long int x = i_u & -i_u;
 
   if (x <= 0xffffffff)
     return ffs (i);