powerpc/xmon: Move breakpoint instructions to own array

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. A later patch will move this to 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 <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Alistair Popple <alistair@popple.id.au>
Link: https://lore.kernel.org/r/20200506034050.24806-3-jniethe5@gmail.com

arch/powerpc/xmon/xmon.c

@@ -98,7 +98,7 @@ 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;
@@ -117,6 +117,10 @@ static unsigned bpinstr = 0x7fe00008;	/* trap */
 
 #define BP_NUM(bp)	((bp) - bpts + 1)
 
+#define BPT_SIZE       (sizeof(unsigned int) * 2)
+#define BPT_WORDS      (BPT_SIZE / sizeof(unsigned int))
+static unsigned int bpt_table[NBPTS * BPT_WORDS];
+
 /* Prototypes */
 static int cmds(struct pt_regs *);
 static int mread(unsigned long, void *, int);
@@ -854,15 +858,13 @@ static struct bpt *in_breakpoint_table(unsigned long nip, unsigned long *offp)
 {
 	unsigned long 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]))
+	*offp = off % BPT_SIZE;
+	if (*offp != 0 && *offp != 4)
 		return NULL;
-	*offp = off - offsetof(struct bpt, instr[0]);
-	return (struct bpt *) (nip - off);
+	return bpts + (off / BPT_SIZE);
 }
 
 static struct bpt *new_breakpoint(unsigned long a)
@@ -877,7 +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;
 		}
 	}