x/tools/gopls: SEGV in string comparison in mapaccess

[adonovan]

#!stacks
"sigpanic" && "memequal" && "getWithoutKeySmallFastStr"

Issue created by stacks.

The memequal(a, b) call panics reading from b. More memory corruption.

This stack fJSOUQ was reported by telemetry:

• crash/crash
• runtime.throw:+9,+0x33
• runtime.sigpanic:+33,+0x213
• runtime.memequal:+65,+0xd8
• internal/runtime/maps.(*Map).getWithoutKeySmallFastStr:+48,+0xdf
• runtime.mapaccess1_faststr:+17,+0xbb
• encoding/json.(*decodeState).object:+95,+0x6c3
• encoding/json.(*decodeState).value:+17,+0x3f
• encoding/json.(*decodeState).unmarshal:+10,+0xff
• encoding/json.Unmarshal:+11,+0xdb
• golang.org/x/tools/internal/jsonrpc2.DecodeMessage:+2,+0x4f
• golang.org/x/tools/internal/jsonrpc2.(*headerStream).Read:+44,+0x2f7
• golang.org/x/tools/internal/jsonrpc2.(*conn).run:+4,+0x73
• golang.org/x/tools/internal/jsonrpc2.(*conn).Go.gowrap1:+0,+0x2b
• runtime.goexit:+0,+0x3

golang.org/x/tools/gopls@v0.20.0 go1.25.1 darwin/arm64 gopls.client=other,vscode (1)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=darwin GOARCH=arm64 GOTOOLCHAIN=go1.25.1 go install golang.org/x/tools/gopls@v0.20.0 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less

[gabyhelp]

Related Issues

• x/tools/gopls: SEGV accessing g in function prolog (R28 is nil or invalid) #76158
• x/tools/gopls: SEGV in json.Marshal #75740
• x/tools/gopls: SEGV in mapassign #75673
• x/tools/gopls: SEGV accessing map[string]... #73387
• x/tools/gopls: sigpanic in methods of persistent.Map (corrupt g) #74870
• x/tools/gopls: impossible nil panic in reflect.ValueOf #73853
• x/tools/gopls: panic in aeshashbody on behalf of mapaccess2_faststr: string memory corrupted #71367
• x/tools/gopls: SEGV in HasSuffix (corrupt string) #74348
• x/tools/gopls: MCP: nil panic in summarizeDiagnostics (CodeAction without WorkspaceEdit) #75489 (closed)
• x/tools/gopls: SEGV updating non-nil map in analysis.Validate #73437

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