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hclsyntax: Impose an upper limit on a refined prefix in TemplateExpr #617

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Jul 7, 2023
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hclsyntax: Impose an upper limit on a refined prefix in TemplateExpr #617

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11 changes: 10 additions & 1 deletion hclsyntax/expression_template.go
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Original file line number Diff line number Diff line change
Expand Up @@ -94,7 +94,16 @@ func (e *TemplateExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics)
ret = cty.UnknownVal(cty.String)
if !diags.HasErrors() { // Invalid input means our partial result buffer is suspect
if knownPrefix := buf.String(); knownPrefix != "" {
ret = ret.Refine().StringPrefix(knownPrefix).NewValue()
byteLen := len(knownPrefix)
// Impose a reasonable upper limit to avoid producing too long a prefix.
// The 128 B is about 10% of the safety limits in cty's msgpack decoder.
// @see https://github.com/zclconf/go-cty/blob/v1.13.2/cty/msgpack/unknown.go#L170-L175
//
// This operation is safe because StringPrefix removes incomplete trailing grapheme clusters.
if byteLen > 128 { // arbitrarily-decided threshold
byteLen = 128
}
ret = ret.Refine().StringPrefix(knownPrefix[:byteLen]).NewValue()
}
}
} else {
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11 changes: 11 additions & 0 deletions hclsyntax/expression_template_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
package hclsyntax

import (
"strings"
"testing"

"github.com/hashicorp/hcl/v2"
Expand Down Expand Up @@ -307,6 +308,16 @@ trim`,
cty.UnknownVal(cty.String).Refine().NotNull().StringPrefixFull("test_known_").NewValue(),
0,
},
{ // can preserve a static prefix as a refinement, but the length is limited to 128 B
strings.Repeat("_", 130) + `${unknown}`,
&hcl.EvalContext{
Variables: map[string]cty.Value{
"unknown": cty.UnknownVal(cty.String),
},
},
cty.UnknownVal(cty.String).Refine().NotNull().StringPrefixFull(strings.Repeat("_", 128)).NewValue(),
0,
},
{ // marks from uninterpolated values are ignored
`hello%{ if false } ${target}%{ endif }`,
&hcl.EvalContext{
Expand Down