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

[Nicholas Piggin]

Jordan Niethe's on March 20, 2020 3:17 pm:
> 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 <jniethe5 at gmail.com>

Reviewed-by: Nicholas Piggin <npiggin at gmail.com>

> ---
> 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
> 
>