cmd/compile: fix length overflow when appending elements to a slice

Instead of testing len(slice)+numNewElements > cap(slice) use
uint(len(slice)+numNewElements) > uint(cap(slice)) to test
if a slice needs to be grown in an append operation.

This prevents a possible overflow when len(slice) is near the maximum
int value and the addition of a constant number of new elements
makes it overflow and wrap around to a negative number which is
smaller than the capacity of the slice.

Appending a slice to a slice with append(s1, s2...) already used
a uint comparison to test slice capacity and therefore was not
vulnerable to the same overflow issue.

Fixes: #29190

Change-Id: I41733895838b4f80a44f827bf900ce931d8be5ca
Reviewed-on: https://go-review.googlesource.com/c/154037
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/test/prove.go b/test/prove.go
index 0de6bd63b4..a881b2d6e2 100644
--- a/test/prove.go
+++ b/test/prove.go
@@ -530,7 +530,7 @@ func fence1(b []int, x, y int) {
 	}
 	if len(b) < cap(b) {
 		// This eliminates the growslice path.
-		b = append(b, 1) // ERROR "Disproved Greater64$"
+		b = append(b, 1) // ERROR "Disproved Greater64U$"
 	}
 }