[PATCH v3 1/2] kho: history: track previous kernel version

[Breno Leitao <leitao@debian.org>]

Store and display the kernel version from the previous kexec boot.

The current kernel's release string is saved to the "previous-release"
property in the KHO FDT before kexec. On the next boot, if this property
exists, the previous kernel version is retrieved and printed during
early boot.

This helps diagnose bugs that only manifest when kexecing from specific
kernel versions, making it easier to correlate crashes with the kernel
that initiated the kexec.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 include/linux/kho/abi/kexec_handover.h |  3 +++
 kernel/liveupdate/kexec_handover.c     | 25 ++++++++++++++++++++++++-
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h
index 285eda8a36e4..f4f31e8f575b 100644
--- a/include/linux/kho/abi/kexec_handover.h
+++ b/include/linux/kho/abi/kexec_handover.h
@@ -84,6 +84,9 @@
 /* The FDT property for sub-FDTs. */
 #define KHO_FDT_SUB_TREE_PROP_NAME "fdt"
 
+/* The FDT property to track previous kernel (kexec caller) */
+#define KHO_PROP_PREVIOUS_RELEASE "previous-release"
+
 /**
  * DOC: Kexec Handover ABI for vmalloc Preservation
  *
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 3cf2dc6840c9..b2d57868d22f 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -22,6 +22,7 @@
 #include <linux/page-isolation.h>
 #include <linux/unaligned.h>
 #include <linux/vmalloc.h>
+#include <linux/utsname.h>
 
 #include <asm/early_ioremap.h>
 
@@ -1246,6 +1247,7 @@ struct kho_in {
 	phys_addr_t fdt_phys;
 	phys_addr_t scratch_phys;
 	phys_addr_t mem_map_phys;
+	char previous_release[__NEW_UTS_LEN + 1];
 	struct kho_debugfs dbg;
 };
 
@@ -1325,6 +1327,8 @@ static __init int kho_out_fdt_setup(void)
 	err |= fdt_property_string(root, "compatible", KHO_FDT_COMPATIBLE);
 	err |= fdt_property(root, KHO_FDT_MEMORY_MAP_PROP_NAME, &empty_mem_map,
 			    sizeof(empty_mem_map));
+	err |= fdt_property_string(root, KHO_PROP_PREVIOUS_RELEASE,
+				   init_uts_ns.name.release);
 	err |= fdt_end_node(root);
 	err |= fdt_finish(root);
 
@@ -1436,6 +1440,22 @@ void __init kho_memory_init(void)
 	}
 }
 
+static int __init kho_print_previous_kernel(const void *fdt)
+{
+	const char *prev_release;
+	int len;
+
+	prev_release = fdt_getprop(fdt, 0, KHO_PROP_PREVIOUS_RELEASE, &len);
+	if (!prev_release || len <= 0)
+		return -ENOENT;
+
+	strscpy(kho_in.previous_release, prev_release,
+		sizeof(kho_in.previous_release));
+	pr_info("exec from: %s\n", kho_in.previous_release);
+
+	return 0;
+}
+
 void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
 			 phys_addr_t scratch_phys, u64 scratch_len)
 {
@@ -1513,7 +1533,10 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
 	kho_in.scratch_phys = scratch_phys;
 	kho_in.mem_map_phys = mem_map_phys;
 	kho_scratch_cnt = scratch_cnt;
-	pr_info("found kexec handover data.\n");
+
+	if (kho_print_previous_kernel(fdt))
+		/* Fallback message when previous kernel info unavailable */
+		pr_info("found kexec handover data.\n");
 
 out:
 	if (fdt)

-- 
2.47.3