From 088a7eef95b1f1919fe6eee722a57c4d4e1e0656 Mon Sep 17 00:00:00 2001
From: mhorne <mhorne@FreeBSD.org>
Date: Fri, 18 Dec 2020 16:06:46 -0400
Subject: [PATCH] ddb: add ability to print user registers

The debugger is always entered after some kind of kernel trap, often a
breakpoint in kdb_enter(). This means that the most recent trapframe
will include kernel state at the time of the trap, when often it is
desirable to the developer to view the contents of the previous
trapframe. This trapframe often corresponds to the entry from userspace.

The ddb(4) man page claims the ability to display user register state
via the 'u' modifier to `show registers`, but this appears untrue. It is
not obvious from a quick search of the history when this feature was
added, or when it was removed. (Re)implement this feature in
db_show_regs, noting that it is not necessarily populated with userspace
state.

Reviewed by:	jhb (earlier version), markj, bcr (manpages)
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27705
---
 share/man/man4/ddb.4 | 10 +++-------
 sys/ddb/db_print.c   | 24 +++++++++++++++++++++++-
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/share/man/man4/ddb.4 b/share/man/man4/ddb.4
index 5dab5c42a2c..91b179e4e6f 100644
--- a/share/man/man4/ddb.4
+++ b/share/man/man4/ddb.4
@@ -955,14 +955,10 @@ at address
 Display the register set.
 If the
 .Cm u
-modifier is specified, it displays user registers instead of
-kernel registers or the currently saved one.
+modifier is specified, the register contents of the thread's previous
+trapframe are displayed instead.
+Usually, this corresponds to the saved state from userspace.
 .Pp
-.Sy Warning :
-The support of the
-.Cm u
-modifier depends on the machine.
-If not supported, incorrect information will be displayed.
 .\"
 .Pp
 .It Ic show Cm rman Ar addr
diff --git a/sys/ddb/db_print.c b/sys/ddb/db_print.c
index 3ff0b781532..da929457984 100644
--- a/sys/ddb/db_print.c
+++ b/sys/ddb/db_print.c
@@ -49,12 +49,32 @@ __FBSDID("$FreeBSD$");
 #include <ddb/db_sym.h>
 
 void
-db_show_regs(db_expr_t _1, bool _2, db_expr_t _3, char *_4)
+db_show_regs(db_expr_t _1, bool _2, db_expr_t _3, char *modif)
 {
+	struct trapframe *oldtf;
 	struct db_variable *regp;
 	db_expr_t value, offset;
 	const char *name;
 
+	/*
+	 * The 'u' modifier instructs us to print the previous trapframe, most
+	 * often containing state from userspace. This is done by temporarily
+	 * switching out kdb_frame.
+	 *
+	 * NB: curthread is used instead of kdb_thread, so that behaviour is
+	 * consistent with regular `show registers`, which always prints
+	 * curthread's trapframe.
+	 */
+	oldtf = kdb_frame;
+	if (modif[0] == 'u') {
+		if (curthread->td_frame == NULL ||
+		    curthread->td_frame == oldtf) {
+			db_printf("previous trapframe unavailable");
+			return;
+		}
+		kdb_frame = curthread->td_frame;
+	}
+
 	for (regp = db_regs; regp < db_eregs; regp++) {
 		if (!db_read_variable(regp, &value))
 			continue;
@@ -70,4 +90,6 @@ db_show_regs(db_expr_t _1, bool _2, db_expr_t _3, char *_4)
 		db_printf("\n");
 	}
 	db_print_loc_and_inst(PC_REGS());
+
+	kdb_frame = oldtf;
 }