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[dev.typeparams] cmd/compile: refuse excessively long constants

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The compiler uses 512 bit of precision for untyped constant
arithmetic but didn't restrict the length of incoming constant
literals in any way, possibly opening the door for excessively
long constants that could bring compilation to a crawl.

Add a simple check that refuses excessively long constants.
Add test.

Change-Id: I797cb2a8e677b8da2864eb92d686d271ab8a004d
Reviewed-on: https://go-review.googlesource.com/c/go/+/289049
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
This commit is contained in:
Robert Griesemer 2021-02-02 13:20:03 -08:00
parent 3db6e18468
commit c910fd7b77
3 changed files with 108 additions and 0 deletions
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14
src/cmd/compile/internal/noder/noder.go
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@ -1455,6 +1455,20 @@ func (p *noder) basicLit(lit *syntax.BasicLit) constant.Value {
switch lit.Kind {
case syntax.IntLit, syntax.FloatLit, syntax.ImagLit:
checkLangCompat(lit)
// The max. mantissa precision for untyped numeric values
// is 512 bits, or 4048 bits for each of the two integer
// parts of a fraction for floating-point numbers that are
// represented accurately in the go/constant package.
// Constant literals that are longer than this many bits
// are not meaningful; and excessively long constants may
// consume a lot of space and time for a useless conversion.
// Cap constant length with a generous upper limit that also
// allows for separators between all digits.
const limit = 10000
if len(lit.Value) > limit {
p.errorAt(lit.Pos(), "excessively long constant: %s... (%d chars)", lit.Value[:10], len(lit.Value))
return constant.MakeUnknown()
}
}
v := constant.MakeFromLiteral(lit.Value, tokenForLitKind[lit.Kind], 0)