Export various helper variables describing the layout and size of

certain kernel structures for use by debuggers. This mostly aids
in examining cores from a kernel without debug symbols as a debugger
can infer these values if debug symbols are available.

One set of variables describes the layout of 'struct linker_file' to
walk the list of loaded kernel modules.

A second set of variables describes the layout of 'struct proc' and
'struct thread' to walk the list of processes in the kernel and the
threads in each process.

The 'pcb_size' variable is used to index into the stoppcbs[] array.

The 'vm_maxuser_address' is used to distinguish kernel virtual addresses
from user addresses. This doesn't have to be perfect, and
'vm_maxuser_address' is a cheap and simple way to differentiate kernel
pointers from simple values like TIDs and PIDs.

While here, annotate the fields in struct pcb used by kgdb on amd64
and i386 to note that their ABI should be preserved.  Annotations for
other platforms will be added in the future.

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D3773

sys/amd64/include/pcb.h

@@ -44,15 +44,19 @@
 #include <machine/segments.h>
 
 #ifdef __amd64__
+/*
+ * NB: The fields marked with (*) are used by kernel debuggers.  Their
+ * ABI should be preserved.
+ */
 struct pcb {
-	register_t	pcb_r15;
-	register_t	pcb_r14;
-	register_t	pcb_r13;
-	register_t	pcb_r12;
-	register_t	pcb_rbp;
-	register_t	pcb_rsp;
-	register_t	pcb_rbx;
-	register_t	pcb_rip;
+	register_t	pcb_r15;	/* (*) */
+	register_t	pcb_r14;	/* (*) */
+	register_t	pcb_r13;	/* (*) */
+	register_t	pcb_r12;	/* (*) */
+	register_t	pcb_rbp;	/* (*) */
+	register_t	pcb_rsp;	/* (*) */
+	register_t	pcb_rbx;	/* (*) */
+	register_t	pcb_rip;	/* (*) */
 	register_t	pcb_fsbase;
 	register_t	pcb_gsbase;
 	register_t	pcb_kgsbase;

sys/i386/include/pcb.h

@@ -44,13 +44,17 @@
 #endif
 #include <machine/npx.h>
 
+/*
+ * NB: The fields marked with (*) are used by kernel debuggers.  Their
+ * ABI should be preserved.
+ */
 struct pcb {
-	int	pcb_edi;
-	int	pcb_esi;
-	int	pcb_ebp;
-	int	pcb_esp;
-	int	pcb_ebx;
-	int	pcb_eip;
+	int	pcb_edi;	/* (*) */
+	int	pcb_esi;	/* (*) */
+	int	pcb_ebp;	/* (*) */
+	int	pcb_esp;	/* (*) */
+	int	pcb_ebx;	/* (*) */
+	int	pcb_eip;	/* (*) */
 	struct segment_descriptor pcb_fsd;
 	struct segment_descriptor pcb_gsd;
 	int	pcb_ds;

sys/kern/kern_linker.c

@@ -70,6 +70,12 @@ SYSCTL_INT(_debug, OID_AUTO, kld_debug, CTLFLAG_RWTUN,
     &kld_debug, 0, "Set various levels of KLD debug");
 #endif
 
+/* These variables are used by kernel debuggers to enumerate loaded files. */
+const int kld_off_address = offsetof(struct linker_file, address);
+const int kld_off_filename = offsetof(struct linker_file, filename);
+const int kld_off_pathname = offsetof(struct linker_file, pathname);
+const int kld_off_next = offsetof(struct linker_file, link.tqe_next);
+
 /*
  * static char *linker_search_path(const char *name, struct mod_depend
  * *verinfo);

sys/kern/kern_mib.c

@@ -580,6 +580,11 @@ SYSCTL_INT(_debug_sizeof, OID_AUTO, buf, CTLFLAG_RD,
 SYSCTL_INT(_debug_sizeof, OID_AUTO, kinfo_proc, CTLFLAG_RD,
     SYSCTL_NULL_INT_PTR, sizeof(struct kinfo_proc), "sizeof(struct kinfo_proc)");
 
+/* Used by kernel debuggers. */
+const int pcb_size = sizeof(struct pcb);
+SYSCTL_INT(_debug_sizeof, OID_AUTO, pcb, CTLFLAG_RD,
+    SYSCTL_NULL_INT_PTR, sizeof(struct pcb), "sizeof(struct pcb)");
+
 /* XXX compatibility, remove for 6.0 */
 #include <sys/imgact.h>
 #include <sys/imgact_elf.h>

sys/kern/kern_proc.c

@@ -139,6 +139,21 @@ struct sx proctree_lock;
 struct mtx ppeers_lock;
 uma_zone_t proc_zone;
 
+/*
+ * The offset of various fields in struct proc and struct thread.
+ * These are used by kernel debuggers to enumerate kernel threads and
+ * processes.
+ */
+const int proc_off_p_pid = offsetof(struct proc, p_pid);
+const int proc_off_p_comm = offsetof(struct proc, p_comm);
+const int proc_off_p_list = offsetof(struct proc, p_list);
+const int proc_off_p_threads = offsetof(struct proc, p_threads);
+const int thread_off_td_tid = offsetof(struct thread, td_tid);
+const int thread_off_td_name = offsetof(struct thread, td_name);
+const int thread_off_td_oncpu = offsetof(struct thread, td_oncpu);
+const int thread_off_td_pcb = offsetof(struct thread, td_pcb);
+const int thread_off_td_plist = offsetof(struct thread, td_plist);
+
 int kstack_pages = KSTACK_PAGES;
 SYSCTL_INT(_kern, OID_AUTO, kstack_pages, CTLFLAG_RD, &kstack_pages, 0,
     "Kernel stack size in pages");

sys/vm/vm_kern.c

@@ -94,6 +94,9 @@ vm_map_t pipe_map;
 const void *zero_region;
 CTASSERT((ZERO_REGION_SIZE & PAGE_MASK) == 0);
 
+/* NB: Used by kernel debuggers. */
+const u_long vm_maxuser_address = VM_MAXUSER_ADDRESS;
+
 SYSCTL_ULONG(_vm, OID_AUTO, min_kernel_address, CTLFLAG_RD,
     SYSCTL_NULL_ULONG_PTR, VM_MIN_KERNEL_ADDRESS, "Min kernel address");