Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat

[Catalin Marinas]

On Fri, Apr 16, 2021 at 03:55:31PM +0800, He Zhe wrote:
> The general version of is_syscall_success does not handle 32-bit
> compatible case, which would cause 32-bit negative return code to be
> recoganized as a positive number later and seen as a "success".
> 
> Since is_compat_thread is defined in compat.h, implementing
> is_syscall_success in ptrace.h would introduce build failure due to
> recursive inclusion of some basic headers like mutex.h. We put the
> implementation to ptrace.c
> 
> Signed-off-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/ptrace.h |  3 +++
>  arch/arm64/kernel/ptrace.c      | 10 ++++++++++
>  2 files changed, 13 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
> index e58bca832dff..3c415e9e5d85 100644
> --- a/arch/arm64/include/asm/ptrace.h
> +++ b/arch/arm64/include/asm/ptrace.h
> @@ -328,6 +328,9 @@ static inline void regs_set_return_value(struct pt_regs *regs, unsigned long rc)
>  	regs->regs[0] = rc;
>  }
>  
> +extern inline int is_syscall_success(struct pt_regs *regs);
> +#define is_syscall_success(regs) is_syscall_success(regs)
> +
>  /**
>   * regs_get_kernel_argument() - get Nth function argument in kernel
>   * @regs:	pt_regs of that context
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 170f42fd6101..3266201f8c60 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1909,3 +1909,13 @@ int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task)
>  	else
>  		return valid_native_regs(regs);
>  }
> +
> +inline int is_syscall_success(struct pt_regs *regs)
> +{
> +	unsigned long val = regs->regs[0];
> +
> +	if (is_compat_thread(task_thread_info(current)))
> +		val = sign_extend64(val, 31);
> +
> +	return !IS_ERR_VALUE(val);
> +}

It's better to use compat_user_mode(regs) here instead of
is_compat_thread(). It saves us from worrying whether regs are for the
current context.

I think we should change regs_return_value() instead. This function
seems to be called from several other places and it has the same
potential problems if called on compat pt_regs.

-- 
Catalin