SMBIOS reader: check the serial-number string index before reading it

ISO copy of the reader, kept identical to the disk copy in vm-config-dist, plus
the matching gitlink bump (00dd919..52d2087, reachable on org-ai-assisted).

Type 1 offset 7 holds a string INDEX, and 0 means "no string" (DSP0134 6.1.3), so
an unguarded --get-string 7 makes GRUB print 'error: failed to retrieve the
structure field.' on every ordinary boot of an image built with the reader -- the
QEMU gate passes on any QEMU guest, and the serial is set only when a tester
injects. Probe the index byte first; read the string only when it is non-zero.

Found by coderabbit on org-ai-assisted/vm-config-dist#8.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: claude

live-build-data/grub-config/smbios-reader.cfg

@@ -36,9 +36,11 @@
 # passed, so it holds across SeaBIOS and OVMF, i440fx and q35, amd64 and arm64.
 # Product Name carries the machine type and Type 0 vendor differs per firmware, so
 # either would break a boot-test leg on a machine type nobody thought about.
-# Reading Manufacturer FIRST also keeps an ordinary boot quiet: DSP0134 requires
+# Reading Manufacturer FIRST also keeps a BARE-METAL boot quiet: DSP0134 requires
 # Manufacturer to be non-null but leaves the serial number optional, and GRUB prints
-# 'error: failed to retrieve the structure field.' for an absent string.
+# 'error: failed to retrieve the structure field.' for an absent string. Under QEMU
+# the gate passes, so that alone is not enough -- the serial-index check inside the
+# gate is what keeps an ordinary QEMU boot quiet.
 # dm-smbios-reader: begin
 insmod regexp
 insmod smbios
@@ -48,14 +50,24 @@ insmod serial
 # so a value surviving from an earlier load_env would select the tester path on an
 # ordinary boot.
 set dm_smbios_vendor=
+set dm_smbios_serial_index=0
 set dm_smbios_oem=
 set dm_smbios_extra=
 # Nothing below runs off QEMU: the serial number is only consulted once the
 # manufacturer identifies a QEMU guest.
 smbios --type 1 --get-string 4 --set dm_smbios_vendor
 if [ "${dm_smbios_vendor}" = "QEMU" ]; then
-    smbios --type 1 --get-string 7 --set dm_smbios_oem
-    regexp --set 1:dm_smbios_extra "^dm-cmdline=(.*)" "${dm_smbios_oem}"
+    # Offset 7 holds a string INDEX, and 0 means "no string" (DSP0134 6.1.3).
+    # Asking for the string when the index is 0 makes GRUB print 'error: failed to
+    # retrieve the structure field.' on the console -- which is EVERY ordinary boot
+    # of an image built with the reader, because the serial is set only when a
+    # tester injects. Reading the index byte first never errors, so the noisy call
+    # happens only when there is something to read.
+    smbios --type 1 --get-byte 7 --set dm_smbios_serial_index
+    if [ "${dm_smbios_serial_index}" != "0" ]; then
+        smbios --type 1 --get-string 7 --set dm_smbios_oem
+        regexp --set 1:dm_smbios_extra "^dm-cmdline=(.*)" "${dm_smbios_oem}"
+    fi
 fi
 # Route GRUB to the serial console ONLY when a boot-tester injected a cmdline via
 # SMBIOS (dm_smbios_extra non-empty), so the headless firmware -> GRUB -> kernel