Subject: Re: [PATCH v7 02/28] powerpc/xmon: Move breakpoint instructions to own array

From: Alistair Popple <[email-masked]>
Date: Mon, 04 May 2020 15:41:03 +1000

On Friday, 1 May 2020 1:41:54 PM AEST Jordan Niethe wrote:
> 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 <[email-masked]>
> ---
> v4: New to series
> v5: - Do not use __section(), use a .space directive in .S file
>     - Simplify in_breakpoint_table() calculation
>     - Define BPT_SIZE
> v6: - Seperate moving to text section
> ---
>  arch/powerpc/xmon/xmon.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index f91ae2c9adbe..6ba7f66c1dd0 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/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))

Minor nit-pick but IMHO this would be more logical if you defined BPT_WORDS 
first like so:

#define BPT_WORDS      (2)
#define BPT_SIZE       (sizeof(unsigned int) * BPT_WORDS)

Otherwise this looks good and I think the offset calculations below are correct 
so:

Reviewed-by: Alistair Popple <[email-masked]>

> +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;
>  		}
>  	}