On 2019-11-21, Eric Blake wrote:
On 11/19/19 2:49 PM, Fangrui Song wrote:
Can we simply drop the offending line statement instead?
Fixed in the new patch.
The first val * mul above this range is 0x1p64. Rejecting it is
correct, because it overflows yint64_t.
I am not subscribed, so apologize that this email may be off the thread.
(The binutils mailing list allows a user to download the raw email so I
can still reply to a specific email, but this list does not provide such
feature.)
Actually, it's better to post a v2 patch as a new top-level thread,
rather than buried as an attachment to a reply to v1, because our CI
tooling doesn't see through the attachment (nor was it easy for me to
reply to the v2 patch - I had to open the attachment to paste its text
inline below...).
More patch submission hints at https://wiki.qemu.org/Contribute/SubmitAPatch
Retitled to [PATCH v2]
From 5f1c5a42794ddcbabb63d9af920d9f437ea90a9f Mon Sep 17 00:00:00 2001
From: Fangrui Song <i...@maskray.me>
Date: Fri, 15 Nov 2019 16:27:47 -0800
Subject: [PATCH] Fix incorrect integer->float conversions caught by clang
-Wimplicit-int-float-conversion
To: qemu-devel@nongnu.org
The warning will be enabled by default in clang 10. It is not available for clang
<= 9.
+++ b/migration/migration.c
@@ -2035,11 +2035,10 @@ void qmp_migrate_set_downtime(double value, Error
**errp)
}
value *= 1000; /* Convert to milliseconds */
- value = MAX(0, MIN(INT64_MAX, value));
MigrateSetParameters p = {
.has_downtime_limit = true,
- .downtime_limit = value,
+ .downtime_limit = (int64_t)value,
};
The explicit cast looks odd without a comment (generally, we try to
avoid casts, so a comment such as /* explicit cast to silence compiler
*/ can be useful)
downtime_limit is an int64_t while value is a double.
There is a diagnostic (-Wfloat-conversion, included by -Wconversion)
warning: conversion from ‘double’ to ‘int64_t’ {aka ‘long int’} may change
value [-Wfloat-conversion]
but it is not enabled by -Wall or -Wextra.
I am not familiar with qemu coding style, but I strongly feel it is a good
thing to add an explicit cast. If it does not fit the style, I hope a
maintainer can delete that for me.
qmp_migrate_set_parameters(&p, errp);
diff --git a/util/cutils.c b/util/cutils.c
index fd591cadf0..2b4484c015 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -239,10 +239,10 @@ static int do_strtosz(const char *nptr, const char **end,
goto out;
}
/*
- * Values >= 0xfffffffffffffc00 overflow uint64_t after their trip
+ * Values > nextafter(0x1p64, 0) overflow uint64_t after their trip
* through double (53 bits of precision).
I thought we agreed on more text than just this (in particular, that
the nextafter() call represents 2^64 rounded towards zero).
*/
- if ((val * mul >= 0xfffffffffffffc00) || val < 0) {
+ if ((val * mul > nextafter(0x1p64, 0)) || val < 0) {
retval = -ERANGE;
goto out;
}
Sorry, I uploaded the wrong patch file. Attaching the correct one now.
>From d533585df39083e88adc50b881a4be74125c837e Mon Sep 17 00:00:00 2001
From: Fangrui Song <i...@maskray.me>
Date: Fri, 15 Nov 2019 16:27:47 -0800
Subject: [PATCH] Fix incorrect integer->float conversion caught by clang
-Wimplicit-int-float-conversion
To: qemu-devel@nongnu.org
The warning will be enabled by default in clang 10. It is not available for clang <= 9.
qemu/migration/migration.c:2038:24: error: implicit conversion from 'long' to 'double' changes value from 9223372036854775807 to 9223372036854775808 [-Werror,-Wimplicit-int-float-conversion]
...
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]
Signed-off-by: Fangrui Song <i...@maskray.me>
---
migration/migration.c | 3 +--
util/cutils.c | 8 +++++---
2 files changed, 6 insertions(+), 5 deletions(-)
diff --git a/migration/migration.c b/migration/migration.c
index 354ad072fa..09b150663f 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2035,11 +2035,10 @@ void qmp_migrate_set_downtime(double value, Error **errp)
}
value *= 1000; /* Convert to milliseconds */
- value = MAX(0, MIN(INT64_MAX, value));
MigrateSetParameters p = {
.has_downtime_limit = true,
- .downtime_limit = value,
+ .downtime_limit = (int64_t)value,
};
qmp_migrate_set_parameters(&p, errp);
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;
}
--
2.24.0
