Implement kvm_kerndisp

This change adds a new libkvm function, kvm_kerndisp(), that can be used to
retrieve the kernel displacement, that is the difference between the kernel's
base virtual address at run time and the kernel base virtual address specified
in the kernel image file.

This will be used by kgdb, to properly relocate kernel symbols, when needed.

Reviewed by:	jhb
Differential Revision:	https://reviews.freebsd.org/D23285

lib/libkvm/Makefile

@@ -25,8 +25,8 @@ INCS=	kvm.h
 LIBADD=	elf
 
 MAN=	kvm.3 kvm_getcptime.3 kvm_geterr.3 kvm_getloadavg.3 \
-	kvm_getpcpu.3 kvm_getprocs.3 kvm_getswapinfo.3 kvm_native.3 \
-	kvm_nlist.3 kvm_open.3 kvm_read.3
+	kvm_getpcpu.3 kvm_getprocs.3 kvm_getswapinfo.3 kvm_kerndisp.3 \
+	kvm_native.3 kvm_nlist.3 kvm_open.3 kvm_read.3
 
 MLINKS+=kvm_getpcpu.3 kvm_getmaxcpu.3 \
 	kvm_getpcpu.3 kvm_dpcpu_setcpu.3 \

lib/libkvm/kvm.3

@@ -32,7 +32,7 @@
 .\"     @(#)kvm.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd April 30, 2016
+.Dd February 5, 2020
 .Dt KVM 3
 .Os
 .Sh NAME
@@ -133,7 +133,8 @@ respectively.
 Finally, only a limited subset of operations are supported for non-native
 crash dumps:
 .Fn kvm_close ,
-.Fn kvm_geterr
+.Fn kvm_geterr ,
+.Fn kvm_kerndisp ,
 .Fn kvm_open2 ,
 .Fn kvm_native ,
 .Fn kvm_nlist2 ,
@@ -147,6 +148,7 @@ and
 .Xr kvm_getloadavg 3 ,
 .Xr kvm_getprocs 3 ,
 .Xr kvm_getswapinfo 3 ,
+.Xr kvm_kerndisp 3 ,
 .Xr kvm_native 3 ,
 .Xr kvm_nlist 3 ,
 .Xr kvm_nlist2 3 ,

lib/libkvm/kvm.c

@@ -46,6 +46,7 @@ __SCCSID("@(#)kvm.c	8.2 (Berkeley) 2/13/94");
 #include <sys/linker.h>
 #include <sys/pcpu.h>
 #include <sys/stat.h>
+#include <sys/sysctl.h>
 #include <sys/mman.h>
 
 #include <net/vnet.h>
@@ -499,3 +500,32 @@ kvm_walk_pages(kvm_t *kd, kvm_walk_pages_cb_t *cb, void *closure)
 
 	return (kd->arch->ka_walk_pages(kd, cb, closure));
 }
+
+kssize_t
+kvm_kerndisp(kvm_t *kd)
+{
+	unsigned long kernbase, rel_kernbase;
+	size_t kernbase_len = sizeof(kernbase);
+	size_t rel_kernbase_len = sizeof(rel_kernbase);
+
+	if (ISALIVE(kd)) {
+		if (sysctlbyname("kern.base_address", &kernbase,
+		    &kernbase_len, NULL, 0) == -1) {
+			_kvm_syserr(kd, kd->program,
+				"failed to get kernel base address");
+			return (0);
+		}
+		if (sysctlbyname("kern.relbase_address", &rel_kernbase,
+		    &rel_kernbase_len, NULL, 0) == -1) {
+			_kvm_syserr(kd, kd->program,
+				"failed to get relocated kernel base address");
+			return (0);
+		}
+		return (rel_kernbase - kernbase);
+	}
+
+	if (kd->arch->ka_kerndisp == NULL)
+		return (0);
+
+	return (kd->arch->ka_kerndisp(kd));
+}

lib/libkvm/kvm.h

@@ -124,6 +124,7 @@ ssize_t	  kvm_read(kvm_t *, unsigned long, void *, size_t);
 ssize_t	  kvm_read_zpcpu(kvm_t *, unsigned long, void *, size_t, int);
 ssize_t	  kvm_read2(kvm_t *, kvaddr_t, void *, size_t);
 ssize_t	  kvm_write(kvm_t *, unsigned long, const void *, size_t);
+kssize_t  kvm_kerndisp(kvm_t *);
 
 typedef int kvm_walk_pages_cb_t(struct kvm_page *, void *);
 int kvm_walk_pages(kvm_t *, kvm_walk_pages_cb_t *, void *);

lib/libkvm/kvm_kerndisp.3

@@ -0,0 +1,57 @@
+.\"
+.\" Copyright (c) 2020 Leandro Lupori <luporl@FreeBSD.org>
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd February 5, 2020
+.Dt KVM_KERNDISP 3
+.Os
+.Sh NAME
+.Nm kvm_kerndisp
+.Nd get kernel displacement
+.Sh LIBRARY
+.Lb libkvm
+.Sh SYNOPSIS
+.In kvm.h
+.Ft kssize_t
+.Fn kvm_kerndisp "kvm_t *kd"
+.Sh DESCRIPTION
+.Fn kvm_kerndisp
+returns the number of bytes by which the kernel referenced by
+.Fa kd
+is displaced.
+This is the difference between the kernel's base virtual address at run time
+and the kernel base virtual address specified in the kernel image file.
+.Pp
+Note that if the kernel is moved to a lower memory address,
+the displacement will be negative.
+.Sh RETURN VALUES
+.Fn kvm_kerndisp
+returns the number of bytes by which the kernel is displaced.
+If the kernel is not displaced or if it is not possible to find the
+displacement then 0 is returned.
+.Sh SEE ALSO
+.Xr kvm 3 ,
+.Xr kvm_close 3 ,
+.Xr kvm_open 3

lib/libkvm/kvm_minidump_powerpc64.c

@@ -184,6 +184,12 @@ _powerpc64_native(kvm_t *kd __unused)
 #endif
 }
 
+static kssize_t
+_powerpc64_kerndisp(kvm_t *kd)
+{
+	return (kd->vmst->hdr.startkernel - PPC64_KERNBASE);
+}
+
 static int
 _powerpc64_minidump_walk_pages(kvm_t *kd, kvm_walk_pages_cb_t *cb, void *arg)
 {
@@ -197,6 +203,7 @@ static struct kvm_arch kvm_powerpc64_minidump = {
 	.ka_kvatop	= _powerpc64_minidump_kvatop,
 	.ka_walk_pages	= _powerpc64_minidump_walk_pages,
 	.ka_native	= _powerpc64_native,
+	.ka_kerndisp	= _powerpc64_kerndisp,
 };
 
 KVM_ARCH(kvm_powerpc64_minidump);

lib/libkvm/kvm_private.h

@@ -47,6 +47,7 @@ struct kvm_arch {
 	int	(*ka_kvatop)(kvm_t *, kvaddr_t, off_t *);
 	int	(*ka_native)(kvm_t *);
 	int	(*ka_walk_pages)(kvm_t *, kvm_walk_pages_cb_t *, void *);
+	kssize_t (*ka_kerndisp)(kvm_t *);
 };
 
 #define	KVM_ARCH(ka)	DATA_SET(kvm_arch, ka)

sys/sys/types.h

@@ -260,6 +260,7 @@ typedef	struct cap_rights	cap_rights_t;
 typedef	__uint64_t	kpaddr_t;
 typedef	__uint64_t	kvaddr_t;
 typedef	__uint64_t	ksize_t;
+typedef	__int64_t	kssize_t;
 
 typedef	__vm_offset_t	vm_offset_t;
 typedef	__uint64_t	vm_ooffset_t;