On Tue, Jul 07, 2015 at 03:23:13AM +0200, Oleg Nesterov wrote:
> The previous change documents that cleanup_return_instances() can't
> always detect the dead frames, the stack can grow. But there is one
> special case which imho worth fixing: arch_uretprobe_is_alive() can
> return true when the stack didn't actually grow, but the next "call"
> insn uses the already invalidated frame.
>
> Test-case:
>
> #include <stdio.h>
> #include <setjmp.h>
>
> jmp_buf jmp;
> int nr = 1024;
>
> void func_2(void)
> {
> if (--nr == 0)
> return;
> longjmp(jmp, 1);
> }
>
> void func_1(void)
> {
> setjmp(jmp);
> func_2();
> }
>
> int main(void)
> {
> func_1();
> return 0;
> }
>
> If you ret-probe func_1() and func_2() prepare_uretprobe() hits the
> MAX_URETPROBE_DEPTH limit and "return" from func_2() is not reported.
>
> When we know that the new call is not chained, we can do the more
> strict check. In this case "sp" points to the new ret-addr, so every
> frame which uses the same "sp" must be dead. The only complication is
> that arch_uretprobe_is_alive() needs to know was it chained or not, so
> we add the new RP_CHECK_CHAIN_CALL enum and change prepare_uretprobe()
> to pass RP_CHECK_CALL only if !chained.
>
> Note: arch_uretprobe_is_alive() could also re-read *sp and check if
> this word is still trampoline_vaddr. This could obviously improve the
> logic, but I would like to avoid another copy_from_user() especially
> in the case when we can't avoid the false "alive == T" positives.
>
> Signed-off-by: Oleg Nesterov <o...@redhat.com>
Acked-by: Anton Arapov <ara...@gmail.com>
> ---
> arch/x86/kernel/uprobes.c | 5 ++++-
> include/linux/uprobes.h | 1 +
> kernel/events/uprobes.c | 14 +++++++-------
> 3 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 67eb168..a5c59f2 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -997,5 +997,8 @@ arch_uretprobe_hijack_return_addr(unsigned long
> trampoline_vaddr, struct pt_regs
> bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx,
> struct pt_regs *regs)
> {
> - return regs->sp <= ret->stack;
> + if (ctx == RP_CHECK_CALL) /* sp was just decremented by "call" insn */
> + return regs->sp < ret->stack;
> + else
> + return regs->sp <= ret->stack;
> }
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index c0a5402..0bdc72f 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -104,6 +104,7 @@ struct return_instance {
>
> enum rp_check {
> RP_CHECK_CALL,
> + RP_CHECK_CHAIN_CALL,
> RP_CHECK_RET,
> };
>
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index df5661a..0f370ef 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1511,10 +1511,11 @@ static unsigned long get_trampoline_vaddr(void)
> return trampoline_vaddr;
> }
>
> -static void cleanup_return_instances(struct uprobe_task *utask, struct
> pt_regs *regs)
> +static void cleanup_return_instances(struct uprobe_task *utask, bool chained,
> + struct pt_regs *regs)
> {
> struct return_instance *ri = utask->return_instances;
> - enum rp_check ctx = RP_CHECK_CALL;
> + enum rp_check ctx = chained ? RP_CHECK_CHAIN_CALL : RP_CHECK_CALL;
>
> while (ri && !arch_uretprobe_is_alive(ri, ctx, regs)) {
> ri = free_ret_instance(ri);
> @@ -1528,7 +1529,7 @@ static void prepare_uretprobe(struct uprobe *uprobe,
> struct pt_regs *regs)
> struct return_instance *ri;
> struct uprobe_task *utask;
> unsigned long orig_ret_vaddr, trampoline_vaddr;
> - bool chained = false;
> + bool chained;
>
> if (!get_xol_area())
> return;
> @@ -1554,14 +1555,15 @@ static void prepare_uretprobe(struct uprobe *uprobe,
> struct pt_regs *regs)
> goto fail;
>
> /* drop the entries invalidated by longjmp() */
> - cleanup_return_instances(utask, regs);
> + chained = (orig_ret_vaddr == trampoline_vaddr);
> + cleanup_return_instances(utask, chained, regs);
>
> /*
> * We don't want to keep trampoline address in stack, rather keep the
> * original return address of first caller thru all the consequent
> * instances. This also makes breakpoint unwrapping easier.
> */
> - if (orig_ret_vaddr == trampoline_vaddr) {
> + if (chained) {
> if (!utask->return_instances) {
> /*
> * This situation is not possible. Likely we have an
> @@ -1570,8 +1572,6 @@ static void prepare_uretprobe(struct uprobe *uprobe,
> struct pt_regs *regs)
> uprobe_warn(current, "handle tail call");
> goto fail;
> }
> -
> - chained = true;
> orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
> }
>
> --
> 1.5.5.1
>
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