Subject: [PATCH v4 02/16] xmon: Move out-of-line instructions to text section

From: Jordan Niethe <[email-masked]>
Date: Fri, 20 Mar 2020 16:17:55 +1100

To execute an instruction out of line after a breakpoint, the NIP is set
to the address of struct bpt::instr. Here a copy of the instruction that
was replaced with a breakpoint is kept, along with a trap so normal flow
can be resumed after XOLing. The struct bpt's are located within the
data section. This is problematic as the data section may be marked as
no execute.

Instead of each struct bpt holding the instructions to be XOL'd, make a
new array, bpt_table[], with enough space to hold instructions for the
number of supported breakpoints. Place this array in the text section.
Make struct bpt::instr a pointer to the instructions in bpt_table[]
associated with that breakpoint. This association is a simple mapping:
bpts[n] -> bpt_table[n * words per breakpoint]. Currently we only need
the copied instruction followed by a trap, so 2 words per breakpoint.

Signed-off-by: Jordan Niethe <[email-masked]>
---
v4: New to series
---
 arch/powerpc/kernel/vmlinux.lds.S |  2 +-
 arch/powerpc/xmon/xmon.c          | 22 +++++++++++++---------
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index b4c89a1acebb..e90845b8c300 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -86,7 +86,7 @@ SECTIONS
 		ALIGN_FUNCTION();
 #endif
 		/* careful! __ftr_alt_* sections need to be close to .text */
-		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text);
+		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely .fixup __ftr_alt_* .ref.text .text.xmon_bpts);
 #ifdef CONFIG_PPC64
 		*(.tramp.ftrace.text);
 #endif
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 02e3bd62cab4..7875d1a37770 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -97,7 +97,7 @@ static long *xmon_fault_jmp[NR_CPUS];
 /* Breakpoint stuff */
 struct bpt {
 	unsigned long	address;
-	unsigned int	instr[2];
+	unsigned int	*instr;
 	atomic_t	ref_count;
 	int		enabled;
 	unsigned long	pad;
@@ -109,6 +109,7 @@ struct bpt {
 #define BP_DABR		4
 
 #define NBPTS	256
+#define BPT_WORDS	2
 static struct bpt bpts[NBPTS];
 static struct bpt dabr;
 static struct bpt *iabr;
@@ -116,6 +117,8 @@ static unsigned bpinstr = 0x7fe00008;	/* trap */
 
 #define BP_NUM(bp)	((bp) - bpts + 1)
 
+static unsigned int __section(.text.xmon_bpts) bpt_table[NBPTS * BPT_WORDS];
+
 /* Prototypes */
 static int cmds(struct pt_regs *);
 static int mread(unsigned long, void *, int);
@@ -852,16 +855,16 @@ static struct bpt *at_breakpoint(unsigned long pc)
 static struct bpt *in_breakpoint_table(unsigned long nip, unsigned long *offp)
 {
 	unsigned long off;
+	unsigned long bp_off;
 
-	off = nip - (unsigned long) bpts;
-	if (off >= sizeof(bpts))
+	off = nip - (unsigned long) bpt_table;
+	if (off >= sizeof(bpt_table))
 		return NULL;
-	off %= sizeof(struct bpt);
-	if (off != offsetof(struct bpt, instr[0])
-	    && off != offsetof(struct bpt, instr[1]))
+	bp_off = off % (sizeof(unsigned int) * BPT_WORDS);
+	if (bp_off != 0 && bp_off != 4)
 		return NULL;
-	*offp = off - offsetof(struct bpt, instr[0]);
-	return (struct bpt *) (nip - off);
+	*offp = bp_off;
+	return bpts + ((off - bp_off) / (sizeof(unsigned int) * BPT_WORDS));
 }
 
 static struct bpt *new_breakpoint(unsigned long a)
@@ -876,7 +879,8 @@ static struct bpt *new_breakpoint(unsigned long a)
 	for (bp = bpts; bp < &bpts[NBPTS]; ++bp) {
 		if (!bp->enabled && atomic_read(&bp->ref_count) == 0) {
 			bp->address = a;
-			patch_instruction(&bp->instr[1], bpinstr);
+			bp->instr = bpt_table + ((bp - bpts) * BPT_WORDS);
+			patch_instruction(bp->instr + 1, bpinstr);
 			return bp;
 		}
 	}
-- 
2.17.1