From ca0927e1683d3bc816d264987c0218df247671b5 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Fri, 22 May 2026 03:50:05 +0000
Subject: [PATCH] runtime: handle underscore in Linux kernel release strings

Some embedded Linux vendors append a platform identifier after an
underscore in the uname release field. For example, Synology NVRs
report a kernel release like "3.4.35_hi3535". parseRelease only
stripped suffixes after '-' or '+', so the patch component
"35_hi3535" failed to parse as an integer and the runtime threw
"failed to parse kernel version from uname" before malloc heap
initialization, making the program unbootable.

Also strip everything after '_', mirroring the existing '-' and '+'
handling.

Regressed in f4de14a515 / CL 758902 on 2026-03-24, which started
calling parseRelease unconditionally during osinit on 32-bit Linux
to decide between 32- and 64-bit time syscalls.

Updates golang/go#79612
Updates tailscale/go#162
Updates tailscale/tailscale#6860
---
 src/runtime/export_linux_test.go  |  1 +
 src/runtime/os_linux.go           |  6 ++++--
 src/runtime/runtime_linux_test.go | 25 +++++++++++++++++++++++++
 3 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/src/runtime/export_linux_test.go b/src/runtime/export_linux_test.go
index 52afd28666e9af..f8c4b4fdc37ea2 100644
--- a/src/runtime/export_linux_test.go
+++ b/src/runtime/export_linux_test.go
@@ -11,6 +11,7 @@ const SigeventMaxSize = _sigev_max_size
 
 var NewOSProc0 = newosproc0
 var Mincore = mincore
+var ParseRelease = parseRelease
 
 type Siginfo siginfo
 type Sigevent sigevent
diff --git a/src/runtime/os_linux.go b/src/runtime/os_linux.go
index 493567b5303673..400dd18e7d98ad 100644
--- a/src/runtime/os_linux.go
+++ b/src/runtime/os_linux.go
@@ -962,9 +962,11 @@ func getKernelVersion() kernelVersion {
 // semver syntax, but allows the minor and patch versions to be
 // elided.
 func parseRelease(rel string) (major, minor, patch int, ok bool) {
-	// Strip anything after a dash or plus.
+	// Strip anything after a dash, plus, or underscore. Some
+	// embedded Linux vendors (e.g., Synology) append a platform
+	// identifier with an underscore, like "3.4.35_hi3535".
 	for i := 0; i < len(rel); i++ {
-		if rel[i] == '-' || rel[i] == '+' {
+		if rel[i] == '-' || rel[i] == '+' || rel[i] == '_' {
 			rel = rel[:i]
 			break
 		}
diff --git a/src/runtime/runtime_linux_test.go b/src/runtime/runtime_linux_test.go
index ab2452c9e72910..8bd3a40c99a69e 100644
--- a/src/runtime/runtime_linux_test.go
+++ b/src/runtime/runtime_linux_test.go
@@ -53,6 +53,31 @@ func TestMincoreErrorSign(t *testing.T) {
 	}
 }
 
+func TestParseRelease(t *testing.T) {
+	tests := []struct {
+		in                  string
+		major, minor, patch int
+		ok                  bool
+	}{
+		{"6.1.0", 6, 1, 0, true},
+		{"5.15.0-91-generic", 5, 15, 0, true},
+		{"4.19.0+", 4, 19, 0, true},
+		// Synology embedded Linux appends a platform identifier
+		// after an underscore.
+		{"3.4.35_hi3535", 3, 4, 35, true},
+		{"2.6.32_synology", 2, 6, 32, true},
+		{"3.10", 3, 10, 0, true},
+		{"bogus", 0, 0, 0, false},
+	}
+	for _, tt := range tests {
+		major, minor, patch, ok := ParseRelease(tt.in)
+		if major != tt.major || minor != tt.minor || patch != tt.patch || ok != tt.ok {
+			t.Errorf("ParseRelease(%q) = (%d, %d, %d, %v); want (%d, %d, %d, %v)",
+				tt.in, major, minor, patch, ok, tt.major, tt.minor, tt.patch, tt.ok)
+		}
+	}
+}
+
 func TestKernelStructSize(t *testing.T) {
 	// Check that the Go definitions of structures exchanged with the kernel are
 	// the same size as what the kernel defines.