regcomp.c uses the "start + count < end" idiom to check that there are
"count" bytes available in an array of char "start" and "end" both point
to.
This is fine, unless "start + count" goes beyond the last element of the
array. In this case, pedantic interpretation of the C standard makes
the comparison of such a pointer against "end" undefined, and optimizers
from hell will happily remove as much code as possible because of this.
An example of this occurs in regcomp.c's bothcases(), which defines
bracket[3], sets "next" to "bracket" and "end" to "bracket + 2". Then it
invokes p_bracket(), which starts with "if (p->next + 5 < p->end)"...
Because bothcases() and p_bracket() are static functions in regcomp.c,
there is a real risk of miscompilation if aggressive inlining happens.
The following diff rewrites the "start + count < end" constructs into
"end - start > count". Assuming "end" and "start" are always pointing in
the array (such as "bracket[3]" above), "end - start" is well-defined
and can be compared without trouble.
As a bonus, MORE2() implies MORE() therefore SEETWO() can be simplified
a bit.
Index: regcomp.c
===================================================================
RCS file: /OpenBSD/src/lib/libc/regex/regcomp.c,v
retrieving revision 1.35
diff -u -p -r1.35 regcomp.c
--- regcomp.c 13 Oct 2020 04:42:28 -0000 1.35
+++ regcomp.c 29 Dec 2020 10:24:18 -0000
@@ -113,10 +110,10 @@ static char nuls[10]; /* place to point
*/
#define PEEK() (*p->next)
#define PEEK2() (*(p->next+1))
-#define MORE() (p->next < p->end)
-#define MORE2() (p->next+1 < p->end)
+#define MORE() (p->end - p->next > 0)
+#define MORE2() (p->end - p->next > 1)
#define SEE(c) (MORE() && PEEK() == (c))
-#define SEETWO(a, b) (MORE() && MORE2() && PEEK() == (a) && PEEK2()
== (b))
+#define SEETWO(a, b) (MORE2() && PEEK() == (a) && PEEK2() == (b))
#define EAT(c) ((SEE(c)) ? (NEXT(), 1) : 0)
#define EATTWO(a, b) ((SEETWO(a, b)) ? (NEXT2(), 1) : 0)
#define NEXT() (p->next++)
@@ -623,15 +620,17 @@ p_bracket(struct parse *p)
int invert = 0;
/* Dept of Truly Sickening Special-Case Kludges */
- if (p->next + 5 < p->end && strncmp(p->next, "[:<:]]", 6) == 0) {
- EMIT(OBOW, 0);
- NEXTn(6);
- return;
- }
- if (p->next + 5 < p->end && strncmp(p->next, "[:>:]]", 6) == 0) {
- EMIT(OEOW, 0);
- NEXTn(6);
- return;
+ if (p->end - p->next > 5) {
+ if (strncmp(p->next, "[:<:]]", 6) == 0) {
+ EMIT(OBOW, 0);
+ NEXTn(6);
+ return;
+ }
+ if (strncmp(p->next, "[:>:]]", 6) == 0) {
+ EMIT(OEOW, 0);
+ NEXTn(6);
+ return;
+ }
}
if ((cs = allocset(p)) == NULL) {
