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libc: regex: rework unsafe pointer arithmetic

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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.

PR:		252403
This commit is contained in:
Miod Vallat 2021-01-08 12:59:00 -06:00 committed by Kyle Evans
parent c6951fac78
commit d36b5dbe28
1 changed files with 14 additions and 12 deletions
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26
lib/libc/regex/regcomp.c
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@ -177,10 +177,10 @@ 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 SEESPEC(a) (p->bre ? SEETWO('\\', a) : SEE(a))
#define EAT(c) ((SEE(c)) ? (NEXT(), 1) : 0)
#define EATTWO(a, b) ((SEETWO(a, b)) ? (NEXT2(), 1) : 0)
@ -1013,15 +1013,17 @@ p_bracket(struct parse *p)
wint_t ch;
/* 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)