linux: handle UNAME26 personality (libuv#4109)

src/unix/linux.c

@@ -328,6 +328,31 @@ unsigned uv__kernel_version(void) {
   if (3 != sscanf(u.release, "%u.%u.%u", &major, &minor, &patch))
     return 0;
 
+  /* Handle it when the process runs under the UNAME26 personality:
+   *
+   * - kernels >= 3.x identify as 2.6.40+x
+   * - kernels >= 4.x identify as 2.6.60+x
+   *
+   * UNAME26 is a poorly conceived hack that doesn't let us distinguish
+   * between 4.x kernels and 5.x/6.x kernels so we conservatively assume
+   * that 2.6.60+x means 4.x.
+   *
+   * Fun fact of the day: it's technically possible to observe the actual
+   * kernel version for a brief moment because uname() first copies out the
+   * real release string before overwriting it with the backcompat string.
+   */
+  if (major == 2 && minor == 6) {
+    if (patch >= 60) {
+      major = 4;
+      minor = patch - 60;
+      patch = 0;
+    } else if (patch >= 40) {
+      major = 3;
+      minor = patch - 40;
+      patch = 0;
+    }
+  }
+
   version = major * 65536 + minor * 256 + patch;
   atomic_store_explicit(&cached_version, version, memory_order_relaxed);