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linux: let the kernel resolve the tun.dev %d template atomically

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Resolving the template in userspace (LinkList then TUNSETIFF) races with
concurrent instances: both can pick nebula0, and with multiqueue the loser
silently attaches as a second queue to the winner's device instead of
failing. TUNSETIFF natively substitutes a single %d via dev_alloc_name,
atomically and always allocating a fresh device, so keep the fail-fast
validation but pass the template through and read the resolved name back.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fg7oD3uZ4jZjjQ4gRsPAwB
This commit is contained in:
JackDoan
2026-07-09 18:37:30 -05:00
parent 57e1a9b6af
commit bc04261d0b
3 changed files with 17 additions and 106 deletions
Download Patch File Download Diff File Expand all files Collapse all files
overlay/tun_linux_test.go
+4 -56
View File
@@ -38,14 +38,6 @@ func TestTunAdvMSS(t *testing.T) {
}
}
func nameSet(names ...string) map[string]struct{} {
used := make(map[string]struct{}, len(names))
for _, n := range names {
used[n] = struct{}{}
}
return used
}
func TestValidateTunName(t *testing.T) {
// A device name must be shorter than IFNAMSIZ (i.e. IFNAMSIZ-1 chars max).
maxLenName := strings.Repeat("a", unix.IFNAMSIZ-1)
@@ -61,11 +53,12 @@ func TestValidateTunName(t *testing.T) {
{"trailing template is fine", "nebula%d", false},
{"mid-string template is fine", "neb%dprod", false},
{"leading template is fine", "%dnebula", false},
{"template at the max static length is fine", strings.Repeat("a", unix.IFNAMSIZ-2) + "%d", false},
{"template at the max length is fine", strings.Repeat("a", unix.IFNAMSIZ-3) + "%d", false},
{"template at IFNAMSIZ is rejected", strings.Repeat("a", unix.IFNAMSIZ-2) + "%d", true},
{"bare %d is rejected", "%d", true},
{"multiple %d is rejected", "neb%d%dprod", true},
{"template with no room for a digit is rejected", strings.Repeat("a", unix.IFNAMSIZ-1) + "%d", true},
{"mid-string template with no room for a digit is rejected", "neb%d" + strings.Repeat("a", unix.IFNAMSIZ-3), true},
{"over-long template is rejected", strings.Repeat("a", unix.IFNAMSIZ-1) + "%d", true},
{"over-long mid-string template is rejected", "neb%d" + strings.Repeat("a", unix.IFNAMSIZ-3), true},
}
for _, tt := range tests {
@@ -80,48 +73,3 @@ func TestValidateTunName(t *testing.T) {
})
}
}
func TestNextTunName(t *testing.T) {
// A prefix long enough that only single-digit suffixes (0-9) fit within
// IFNAMSIZ, so marking all ten used exercises running out of names.
longPrefix := strings.Repeat("a", unix.IFNAMSIZ-2)
longUsed := make([]string, 0, 10)
for i := 0; i < 10; i++ {
longUsed = append(longUsed, longPrefix+string(rune('0'+i)))
}
tests := []struct {
name string
tmpl string
used map[string]struct{}
want string
wantErr bool
}{
{"nothing used picks zero", "nebula%d", nil, "nebula0", false},
{"skips taken names", "nebula%d", nameSet("nebula0", "nebula1"), "nebula2", false},
{"picks the lowest free index", "nebula%d", nameSet("nebula0", "nebula2"), "nebula1", false},
{"ignores unrelated names", "nebula%d", nameSet("eth0", "tun5"), "nebula0", false},
{"mid-string placeholder picks zero", "neb%dprod", nil, "neb0prod", false},
{"mid-string placeholder skips taken", "neb%dprod", nameSet("neb0prod", "neb1prod"), "neb2prod", false},
{"leading placeholder picks zero", "%dnebula", nameSet("tun0"), "0nebula", false},
{"runs out of names within IFNAMSIZ", longPrefix + "%d", nameSet(longUsed...), "", true},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
got, err := nextTunName(tt.tmpl, tt.used)
if tt.wantErr {
if err == nil {
t.Fatalf("expected an error, got name %q", got)
}
return
}
if err != nil {
t.Fatalf("unexpected error: %v", err)
}
if got != tt.want {
t.Errorf("got %q, want %q", got, tt.want)
}
})
}
}