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[PATCH v3 2/2] util/cutils: Fix incorrect integer->float conversion caug
From: |
Markus Armbruster |
Subject: |
[PATCH v3 2/2] util/cutils: Fix incorrect integer->float conversion caught by clang |
Date: |
Fri, 22 Nov 2019 09:00:39 +0100 |
From: Fangrui Song <address@hidden>
Clang does not like do_strtosz()'s code to guard against overflow:
qemu/util/cutils.c:245:23: error: implicit conversion from 'unsigned long'
to 'double' changes value from 18446744073709550592 to 18446744073709551616
[-Werror,-Wimplicit-int-float-conversion]
The warning will be enabled by default in clang 10. It is not
available for clang <= 9.
val * mul >= 0xfffffffffffffc00 is indeed wrong. 0xfffffffffffffc00
is not representable exactly as double. It's half-way between the
representable values 0xfffffffffffff800 and 0x10000000000000000.
Which one we get is implementation-defined. Bad.
We want val * mul > (the largest uint64_t exactly representable as
double). That's 0xfffffffffffff800. Write it as nextafter(0x1p64, 0)
with a suitable comment.
Signed-off-by: Fangrui Song <address@hidden>
Reviewed-by: Markus Armbruster <address@hidden>
[Patch split, commit message improved]
Signed-off-by: Markus Armbruster <address@hidden>
---
util/cutils.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/util/cutils.c b/util/cutils.c
index fd591cadf0..77acadc70a 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -239,10 +239,12 @@ static int do_strtosz(const char *nptr, const char **end,
goto out;
}
/*
- * Values >= 0xfffffffffffffc00 overflow uint64_t after their trip
- * through double (53 bits of precision).
+ * Values near UINT64_MAX overflow to 2**64 when converting to double
+ * precision. Compare against the maximum representable double precision
+ * value below 2**64, computed as "the next value after 2**64 (0x1p64) in
+ * the direction of 0".
*/
- if ((val * mul >= 0xfffffffffffffc00) || val < 0) {
+ if ((val * mul > nextafter(0x1p64, 0)) || val < 0) {
retval = -ERANGE;
goto out;
}
--
2.21.0