From 308659acbff34c51e57693e4dbfa43cbbc910fa1 Mon Sep 17 00:00:00 2001
From: Ahmad Khalifa <vexeduxr@FreeBSD.org>
Date: Wed, 21 May 2025 23:41:15 +0300
Subject: [PATCH] libkvm/kvm_amd64: account for relocatable kernels

amd64 kernels don't have to be loaded at 2M physical anymore, they can
be anywhere in the lower 4G of the physical address space. The kernel
now provides its physical address in an ELF section in the dump, so
account for it.

Reviewed by: markj
Approved by: imp (mentor)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1706
---
 lib/libkvm/kvm_amd64.c | 46 +++++++++++++++++++++++++++++++++---------
 1 file changed, 37 insertions(+), 9 deletions(-)

diff --git a/lib/libkvm/kvm_amd64.c b/lib/libkvm/kvm_amd64.c
index e3640cf1224..acc7ac017a3 100644
--- a/lib/libkvm/kvm_amd64.c
+++ b/lib/libkvm/kvm_amd64.c
@@ -109,8 +109,9 @@ _amd64_initvtop(kvm_t *kd)
 {
 	struct kvm_nlist nl[2];
 	amd64_physaddr_t pa;
-	kvaddr_t kernbase;
+	kvaddr_t kernbase, kernphys;
 	amd64_pml4e_t *PML4;
+	int found = 0;
 
 	kd->vmst = (struct vmstate *)_kvm_malloc(kd, sizeof(*kd->vmst));
 	if (kd->vmst == NULL) {
@@ -123,16 +124,43 @@ _amd64_initvtop(kvm_t *kd)
 		if (_kvm_read_core_phdrs(kd, &kd->vmst->phnum,
 		    &kd->vmst->phdr) == -1)
 			return (-1);
+
+		for (size_t i = 0; i < kd->vmst->phnum; i++) {
+			if (kd->vmst->phdr[i].p_type == PT_DUMP_DELTA) {
+				/* Account for the 2M hole at KERNBASE. */
+				kernphys = kd->vmst->phdr[i].p_paddr -
+				    kd->vmst->phdr[i].p_align;
+				kernbase = kd->vmst->phdr[i].p_vaddr;
+
+				found = 1;
+				break;
+			}
+		}
 	}
 
-	nl[0].n_name = "kernbase";
-	nl[1].n_name = 0;
+	if (found == 0) {
+		nl[0].n_name = "kernbase";
+		nl[1].n_name = 0;
 
-	if (kvm_nlist2(kd, nl) != 0) {
-		_kvm_err(kd, kd->program, "bad namelist - no kernbase");
-		return (-1);
+		if (kvm_nlist2(kd, nl) != 0) {
+			_kvm_err(kd, kd->program, "bad namelist - no kernbase");
+			return (-1);
+		}
+
+		nl[0].n_name = "kernphys";
+		nl[1].n_name = 0;
+
+		/* XXX
+		 * Relocatable kernels can still be loaded at 2M.
+		 */
+		if (kvm_nlist2(kd, nl) != 1) {
+			_kvm_err(kd, kd->program, "cannot determine kernphys");
+			return (-1);
+		}
+
+		kernphys = 0;
+		kernbase = nl[0].n_value;
 	}
-	kernbase = nl[0].n_value;
 
 	nl[0].n_name = "KPML4phys";
 	nl[1].n_name = 0;
@@ -141,8 +169,8 @@ _amd64_initvtop(kvm_t *kd)
 		_kvm_err(kd, kd->program, "bad namelist - no KPML4phys");
 		return (-1);
 	}
-	if (kvm_read2(kd, (nl[0].n_value - kernbase), &pa, sizeof(pa)) !=
-	    sizeof(pa)) {
+	if (kvm_read2(kd, (nl[0].n_value - kernbase + kernphys), &pa,
+	    sizeof(pa)) != sizeof(pa)) {
 		_kvm_err(kd, kd->program, "cannot read KPML4phys");
 		return (-1);
 	}