[Support] unsafe pointer arithmetic in llvm_regcomp()

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.

Bug report: https://github.com/llvm/llvm-project/issues/47993

Reviewed By: MaskRay, vitalybuka

Differential Revision: https://reviews.llvm.org/D97129

diff --git a/llvm/lib/Support/regcomp.c b/llvm/lib/Support/regcomp.c
index ee2a1d8..24d0112 100644 (file)
--- a/llvm/lib/Support/regcomp.c
+++ b/llvm/lib/Support/regcomp.c
@@ -249,10 +249,10 @@ static char nuls[10];             /* place to point scanner in event of error */
  */
 #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++)
@@ -800,15 +800,17 @@ 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) {