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From: Mark Rutland <mark.rutland@arm.com>
To: He Zhe <zhe.he@windriver.com>
Cc: catalin.marinas@arm.com, oleg@redhat.com,
	linux-kernel@vger.kernel.org, linux-audit@redhat.com,
	will@kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return_value for compat
Date: Wed, 21 Apr 2021 18:41:05 +0100
Message-ID: <20210421174105.GB52940@C02TD0UTHF1T.local> (raw)
In-Reply-To: <20210416075533.7720-2-zhe.he@windriver.com>

On Fri, Apr 16, 2021 at 03:55:32PM +0800, He Zhe wrote:
> Add sign extension handling in syscall_get_return_value so that it can
> handle 32-bit compatible case and can be used by for example audit, just
> like what syscall_get_error does.

If a compat syscall can ever legitimately return a non-error value with
bit 31 set, and this sign-extends it, is that ever going to reach
userspace as a 64-bit value?

IIUC things like mmap() can return pointers above 2GiB for a compat
task, so I'm a bit uneasy that we'd handle those wrong. I can't see a
way of preventing that unless we keep the upper 32 bits for errors.

Mark.

> 
> Signed-off-by: He Zhe <zhe.he@windriver.com>
> ---
>  arch/arm64/include/asm/syscall.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
> index cfc0672013f6..cd7a22787aeb 100644
> --- a/arch/arm64/include/asm/syscall.h
> +++ b/arch/arm64/include/asm/syscall.h
> @@ -44,7 +44,12 @@ static inline long syscall_get_error(struct task_struct *task,
>  static inline long syscall_get_return_value(struct task_struct *task,
>  					    struct pt_regs *regs)
>  {
> -	return regs->regs[0];
> +	long val = regs->regs[0];
> +
> +	if (is_compat_thread(task_thread_info(task)))
> +		val = sign_extend64(val, 31);
> +
> +	return val;
>  }
>  
>  static inline void syscall_set_return_value(struct task_struct *task,
> -- 
> 2.17.1
> 

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  parent reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-16  7:55 [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat He Zhe
2021-04-16  7:55 ` [PATCH 2/3] arm64: syscall.h: Add sign extension handling in syscall_get_return_value for compat He Zhe
2021-04-16  9:43   ` Oleg Nesterov
2021-04-20  8:38     ` He Zhe
2021-04-21 17:41   ` Mark Rutland [this message]
2021-04-22 16:55     ` Mark Rutland
2021-04-16  7:55 ` [PATCH 3/3] audit: Use syscall_get_return_value to get syscall return code in audit_syscall_exit He Zhe
2021-04-16 12:33 ` [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat Catalin Marinas
2021-04-16 13:34   ` Mark Rutland
2021-04-17 13:19     ` David Laight
2021-04-19 12:19     ` Will Deacon
2021-04-20  8:54       ` He Zhe
2021-04-21 17:10       ` Mark Rutland
2021-04-22 16:07         ` Will Deacon
2021-04-22 16:42           ` Mark Rutland
2021-04-22 18:57             ` Dmitry V. Levin
2021-04-20  8:42   ` He Zhe
2021-04-21 17:17     ` Mark Rutland

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