Re: [PATCH 4/4] config: avoid calling `labs()` on too-large data type

-- >8 --
Subject: [PATCH] config: simplify unit suffix handling

parse_unit_factor() checks if a K, M or G is present after a number and
multiplies it by 2^10, 2^20 or 2^30, respectively.  One of its callers
checks if the result is smaller than the number alone to detect
overflows.  The other one passes 1 as the number and does multiplication
and overflow check itself in a similar manner.

This works, but is inconsistent, and it would break if we added support
for a bigger unit factor.  E.g. 16777217T expands to 2^64 + 2^40, which
is too big for a 64-bit number.  Modulo 2^64 we get 2^40 == 1TB, which
is bigger than the raw number 16777217 == 2^24 + 1, so the overflow
would go undetected by that method.

Move the multiplication out of parse_unit_factor() and rename it to
get_unit_factor() to signify its reduced functionality.  This partially
reverts c8deb5a146 ("Improve error messages when int/long cannot be
parsed from config", 2007-12-25), but keeps the improved error messages.
Use a return value of 0 to signal an invalid suffix.

And use unsigned_mult_overflows to check for an overflow *before* doing
the actual multiplication, which is simpler and can deal with larger
unit factors.

Signed-off-by: Rene Scharfe <redacted>
---
Patch generated with --function-context for easier reviewing.

 config.c | 39 ++++++++++++++++++---------------------
 1 file changed, 18 insertions(+), 21 deletions(-)

diff --git a/config.c b/config.c
index 01c6e9df23..61a8bbb5cd 100644
--- a/config.c
+++ b/config.c

@@ -834,51 +834,46 @@ static int git_parse_source(config_fn_t fn, void *data,
 	return error_return;
 }

-static int parse_unit_factor(const char *end, uintmax_t *val)
+static uintmax_t get_unit_factor(const char *end)
 {
 	if (!*end)
 		return 1;
-	else if (!strcasecmp(end, "k")) {
-		*val *= 1024;
-		return 1;
-	}
-	else if (!strcasecmp(end, "m")) {
-		*val *= 1024 * 1024;
-		return 1;
-	}
-	else if (!strcasecmp(end, "g")) {
-		*val *= 1024 * 1024 * 1024;
-		return 1;
-	}
+	if (!strcasecmp(end, "k"))
+		return 1024;
+	if (!strcasecmp(end, "m"))
+		return 1024 * 1024;
+	if (!strcasecmp(end, "g"))
+		return 1024 * 1024 * 1024;
 	return 0;
 }

 static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
 {
 	if (value && *value) {
 		char *end;
 		intmax_t val;
 		uintmax_t uval;
-		uintmax_t factor = 1;
+		uintmax_t factor;

 		errno = 0;
 		val = strtoimax(value, &end, 0);
 		if (errno == ERANGE)
 			return 0;
-		if (!parse_unit_factor(end, &factor)) {
+		factor = get_unit_factor(end);
+		if (!factor) {
 			errno = EINVAL;
 			return 0;
 		}
 		uval = val < 0 ? -val : val;
-		uval *= factor;
-		if (uval > max || (val < 0 ? -val : val) > uval) {
+		if (unsigned_mult_overflows(factor, uval) ||
+		    factor * uval > max) {
 			errno = ERANGE;
 			return 0;
 		}
 		val *= factor;
 		*ret = val;
 		return 1;
 	}
 	errno = EINVAL;
 	return 0;
 }

@@ -886,26 +881,28 @@ static int git_parse_signed(const char *value, intmax_t *ret, intmax_t max)
 static int git_parse_unsigned(const char *value, uintmax_t *ret, uintmax_t max)
 {
 	if (value && *value) {
 		char *end;
 		uintmax_t val;
-		uintmax_t oldval;
+		uintmax_t factor;

 		errno = 0;
 		val = strtoumax(value, &end, 0);
 		if (errno == ERANGE)
 			return 0;
-		oldval = val;
-		if (!parse_unit_factor(end, &val)) {
+		factor = get_unit_factor(end);
+		if (!factor) {
 			errno = EINVAL;
 			return 0;
 		}
-		if (val > max || oldval > val) {
+		if (unsigned_mult_overflows(factor, val) ||
+		    factor * val > max) {
 			errno = ERANGE;
 			return 0;
 		}
+		val *= factor;
 		*ret = val;
 		return 1;
 	}
 	errno = EINVAL;
 	return 0;
 }
--

2.22.0