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net: IPMask.MarshalText silently returns empty output for masks that are not 4 or 16 bytes long #81749
Description
net.IPMask.MarshalText gained a new text format in the Go 1.28 cycle (issue #29678), gated behind GODEBUG=netmarshal=1. For a mask whose length is not 0, 4, or 16 bytes, the new path returns empty bytes with a nil error instead of an error, and the data is silently destroyed:
m := net.IPMask{255, 255, 255, 0, 1}
b, err := m.MarshalText() // GODEBUG=netmarshal=1: b="", err=nil
j, _ := json.Marshal(m) // j = ""
var m2 net.IPMask
json.Unmarshal(j, &m2) // m2 = [] -- the original 5 bytes are gone(The old base64 format round-trips the same mask fine.)
Root cause in src/net/ip.go:
addr, _ := netip.AddrFromSlice(m)
return addr.AppendTo(nil), nilThe ok result is discarded; on failure addr is the zero Addr and AppendTo(nil) yields nothing. The sibling IP.MarshalText returns an honest error (address 0102030405: invalid IP address) for invalid lengths, so this is also internally inconsistent. The existing ipMaskJSONTests table only covers nil/4-byte/16-byte masks, which is why it slipped through.
Expected: MarshalText returns a non-nil error for a mask that cannot be represented as an IP address.
Seen on master at 8190b02c (2026-09-25), linux/amd64, go1.28-devel.
Activity
gabyhelp commentedon Sep 25, 2026
Related Issues
(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)
dajiaohuang commentedon Sep 25, 2026
prattmic commentedon Sep 29, 2026
cc @neild @nicholashusin @seankhliao
prattmic commentedon Sep 29, 2026
Marked as a release blocker to ensure this is investigated before the 1.28 release.
ianlancetaylor commentedon Sep 29, 2026
I will take a look at this.
@dajiaohuang Please never send code patches in the issue tracker. For copyright reasons we can only use patches that go through the contribution process (https://go.dev/doc/contribute). Patches in the issue tracker make our work harder, not easier. Thanks.
dajiaohuang commentedon Oct 7, 2026
非常抱歉在issue中直接贴了代码patch，给各位维护者添麻烦了。我理解Go项目只能通过Gerrit贡献流程接收patch（https://go.dev/doc/contribute），这是出于版权原因的必要流程。后续如果我认为有合适的修复方案，会严格按照贡献流程通过Gerrit提交。再次致歉！
cagedmantis commentedon Oct 13, 2026
Just a note: IPMask.MarshalText has been discussed in the proposal committee recently. There may be a need to revisit this.
gopherbot commentedon Oct 13, 2026
Change https://go.dev/cl/848108 mentions this issue:
net: marshal and unmarshal invalid IPMask valuesianlancetaylor commentedon Oct 14, 2026
I sent https://go.dev/cl/848108, which uses the old marshaling for invalid
IPMaskvalues rather than returning an error. That seems best for backward compatibility.net: marshal and unmarshal invalid IPMask values