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 <jniet...@gmail.com>
Reviewed-by: Nicholas Piggin <npig...@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
>
>
