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

[Thomas Gleixner <tglx@linutronix.de>]

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