[hackers] [sic] util: trim() fix for UB on pointer arithmetic || Hiltjo Posthuma

[git]

commit 058547e707e961e0cb7f8af4877f1b92f4c6d888
Author:     Hiltjo Posthuma <hil...@codemadness.org>
AuthorDate: Thu May 6 12:18:21 2021 +0200
Commit:     Hiltjo Posthuma <hil...@codemadness.org>
CommitDate: Thu May 6 12:20:30 2021 +0200

    util: trim() fix for UB on pointer arithmetic
    
    Follow-up fix on commit df4c0611366bf361fa263fbc57009cbe68456855
    
    "
    While it is true reversing the condition solves a single-byte read at
    one before s, there is a second instance of UB.
    
    Having a pointer to one before an object is in of itself UB in C, it's
    on the side of language lawyering, but it's UB.
    I add here a quote from a C standard draft:
    > When an expression that has integer type is added to or subtracted
    > from a pointer, the result has the type of the pointer operand.
    > If both the pointer operand and the result point to elements of the
    > same array object, or one past the last element of the array object,
    > the evaluation shall not produce an overflow; otherwise, the
    > behavior is undefined.
    Taken from: http://www.iso-9899.info/n1570.html#6.5.6p8
    "
    
    Thanks Guilherme Janczak <guilherme.janc...@yandex.com>

diff --git a/util.c b/util.c
index cb966d4..8cea883 100644
--- a/util.c
+++ b/util.c
@@ -60,8 +60,7 @@ static void
 trim(char *s) {
        char *e;
 
-       e = s + strlen(s) - 1;
-       while(e > s && isspace((unsigned char)*e))
-               e--;
-       *(e + 1) = '\0';
+       for (e = s + strlen(s); e > s && isspace((unsigned char)*(e - 1)); e--)
+               ;
+       *e = '\0';
 }