Re: [PATCH] ptrace: add PTRACE_GET_RSEQ_CONFIGURATION request

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Feb 22, 2021, at 5:04 AM, Piotr Figiel figiel@google.com wrote:

> For userspace checkpoint and restore (C/R) a way of getting process state
> containing RSEQ configuration is needed.
> 
> There are two ways this information is going to be used:
> - to re-enable RSEQ for threads which had it enabled before C/R
> - to detect if a thread was in a critical section during C/R
> 
> Since C/R preserves TLS memory and addresses RSEQ ABI will be restored
> using the address registered before C/R.
> 
> Detection whether the thread is in a critical section during C/R is needed
> to enforce behavior of RSEQ abort during C/R. Attaching with ptrace()
> before registers are dumped itself doesn't cause RSEQ abort.
> Restoring the instruction pointer within the critical section is
> problematic because rseq_cs may get cleared before the control is passed
> to the migrated application code leading to RSEQ invariants not being
> preserved. C/R code will use RSEQ ABI address to find the abort handler
> to which the instruction pointer needs to be set.
> 
> To achieve above goals expose the RSEQ ABI address and the signature value
> with the new ptrace request PTRACE_GET_RSEQ_CONFIGURATION.
> 
> This new ptrace request can also be used by debuggers so they are aware
> of stops within restartable sequences in progress.
> 
> Signed-off-by: Piotr Figiel <figiel@google.com>
> Reviewed-by: Michal Miroslaw <emmir@google.com>
> 
> ---
> include/uapi/linux/ptrace.h |  8 ++++++++
> kernel/ptrace.c             | 23 +++++++++++++++++++++++
> 2 files changed, 31 insertions(+)
> 
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index 83ee45fa634b..d54cf6b6ce7c 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -102,6 +102,14 @@ struct ptrace_syscall_info {
> 	};
> };
> 
> +#define PTRACE_GET_RSEQ_CONFIGURATION	0x420f
> +
> +struct ptrace_rseq_configuration {
> +	__u64 rseq_abi_pointer;
> +	__u32 signature;
> +	__u32 pad;
> +};

I notice that other structures defined in this UAPI header are not packed as well.
Should we add an attribute packed on new structures ? It seems like it is
generally a safer course of action, even though each field is naturally aligned
here (there is no padding/hole in the structure).

> +
> /*
>  * These values are stored in task->ptrace_message
>  * by tracehook_report_syscall_* to describe the current syscall-stop.
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 61db50f7ca86..a936af66cf6f 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -31,6 +31,7 @@
> #include <linux/cn_proc.h>
> #include <linux/compat.h>
> #include <linux/sched/signal.h>
> +#include <linux/minmax.h>
> 
> #include <asm/syscall.h>	/* for syscall_get_* */
> 
> @@ -779,6 +780,22 @@ static int ptrace_peek_siginfo(struct task_struct *child,
> 	return ret;
> }
> 
> +#ifdef CONFIG_RSEQ
> +static long ptrace_get_rseq_configuration(struct task_struct *task,
> +					  unsigned long size, void __user *data)
> +{
> +	struct ptrace_rseq_configuration conf = {
> +		.rseq_abi_pointer = (u64)(uintptr_t)task->rseq,
> +		.signature = task->rseq_sig,
> +	};
> +
> +	size = min_t(unsigned long, size, sizeof(conf));
> +	if (copy_to_user(data, &conf, size))
> +		return -EFAULT;
> +	return size;

See other email about returning 0 here.

Thanks,

Mathieu

> +
> 	default:
> 		break;
> 	}
> --
> 2.30.0.617.g56c4b15f3c-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com