Re: [PATCH] riscv/entry: get correct syscall number from syscall_get_nr()

From: Thomas Gleixner <tglx@linutronix.de>
To: "Celeste Liu" <coelacanthushex@gmail.com>,
	"Björn Töpel" <bjorn@kernel.org>,
	"Celeste Liu via B4 Relay"
	<devnull+CoelacanthusHex.gmail.com@kernel.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Björn Töpel" <bjorn@rivosinc.com>
Cc: Palmer Dabbelt <palmer@rivosinc.com>,
	Alexandre Ghiti <alex@ghiti.fr>,
	"Dmitry V. Levin" <ldv@strace.io>,
	Andrea Bolognani <abologna@redhat.com>,
	Felix Yan <felixonmars@archlinux.org>,
	Ruizhe Pan <c141028@gmail.com>,
	Shiqi Zhang <shiqi@isrc.iscas.ac.cn>, Guo Ren <guoren@kernel.org>,
	Yao Zi <ziyao@disroot.org>, Han Gao <gaohan@iscas.ac.cn>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH] riscv/entry: get correct syscall number from syscall_get_nr()
Date: Sun, 27 Oct 2024 16:56:24 +0100	[thread overview]
Message-ID: <87a5ep4k0n.ffs@tglx> (raw)
In-Reply-To: <3dc10d89-6c0c-4654-95ed-dd6f19efbad4@gmail.com>

On Sun, Oct 27 2024 at 23:29, Celeste Liu wrote:
> On 2024-10-27 04:21, Thomas Gleixner wrote:
>> The real problem is that orig_a0 is not exposed in the user view of the
>> registers. Changing that struct breaks the existing applications
>> obviously.
>> 
>> But you can expose it without changing the struct by exposing a regset
>> for orig_a0 which allows you to read and write it similar to what ARM64
>> does for the syscall number.
>
> If we add something like NT_SYSCALL_NR to UAPI, it cannot solve anything: We 
> already have PTRACE_GET_SYSCALL_INFO to get syscall number, which was introduced 
> in 5.3 kernel. The problem is only in the kernel before 5.3. So we can't fix 
> this issue unless we also backport NT_SYSCALL_NR to 4.19 LTS. But if we can 
> backport it, we can backport PTRACE_GET_SYSCALL_INFO directly instead.

PTRACE_GET_SYSCALL_INFO only solves half of the problem. It correctly
returns orig_a0, but there is no way to modify orig_a0, which is
required to change arg0.

On x86 AX contains the syscall number and is used for the return
value. So the tracer has do modify orig_AX when it wants to change the
syscall number.

Equivalently you need to be able to modify orig_a0 for changing arg0,
no?

Thanks,

        tglx

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