On Wed, 2014-08-27 at 13:08 +0800, Wang Nan wrote:
> When kprobe decoding instruction, original code calls instruction
> specific decoder if emulate is set to false. However, instructions with
> DECODE_TYPE_EMULATE are in fact don't have their decoder. What in the
> action table are in fact handlers. For example:
>
> /* LDRD (immediate) cccc 000x x1x0 xxxx xxxx xxxx 1101 xxxx */
> /* STRD (immediate) cccc 000x x1x0 xxxx xxxx xxxx 1111 xxxx */
> DECODE_EMULATEX (0x0e5000d0, 0x004000d0, PROBES_LDRSTRD,
> REGS(NOPCWB, NOPCX, 0, 0, 0)),
>
> and
>
> const union decode_action kprobes_arm_actions[NUM_PROBES_ARM_ACTIONS] = {
> ...
> [PROBES_LDRSTRD] = {.handler = emulate_ldrdstrd},
> ...
>
> In this situation, original code calls 'emulate_ldrdstrd' as a decoder,
> which is obviously incorrect.
Except that situation can't occur because when arm_probes_decode_insn()
is called with kprobes_arm_actions then emulate is set to true (see
arch_prepare_kprobe).
For the situation where emulate is false, i.e. when called from
arch_uprobe_analyze_insn(), then this provides these actions...
const union decode_action uprobes_probes_actions[] = {
...
[PROBES_LDRSTRD] = {.decoder = decode_pc_ro},
...
and we do want that decode function called. Basically, the code is
behaving as it was designed and when passed emulate=false it turns the
behaviour of emulated instruction forms into into the 'custom' action.
>
> This patch makes it returns INSN_GOOD directly when 'emulate' is not
> true.
>
> Signed-off-by: Wang Nan <[email protected]>
> Cc: "David A. Long" <[email protected]>
> Cc: Russell King <[email protected]>
> Cc: Jon Medhurst <[email protected]>
> Cc: Taras Kondratiuk <[email protected]>
> Cc: Ben Dooks <[email protected]>
> ---
> arch/arm/kernel/probes.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/arm/kernel/probes.c b/arch/arm/kernel/probes.c
> index a8ab540..1c77b8d 100644
> --- a/arch/arm/kernel/probes.c
> +++ b/arch/arm/kernel/probes.c
> @@ -436,8 +436,7 @@ probes_decode_insn(probes_opcode_t insn, struct
> arch_probes_insn *asi,
> struct decode_emulate *d = (struct decode_emulate *)h;
>
> if (!emulate)
> - return actions[d->handler.action].decoder(insn,
> - asi, h);
> + return INSN_GOOD;
>
> asi->insn_handler = actions[d->handler.action].handler;
> set_emulated_insn(insn, asi, thumb);
--
Tixy
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