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

On Mon, Oct 21 2024 at 09:46, Björn Töpel wrote:

Celeste Liu coelacanthushex@gmail.com writes:

...

1. syscall_enter_from_user_mode() will do two things:
   1. the return value is only to inform whether the syscall should be skipped.
   2. regs will be modified by filters (seccomp or ptrace and so on).
2. for common entry user, there is two informations: syscall number and the return value of syscall_enter_from_user_mode() (called is_skipped below). so there is three situations:
   1. if syscall number is invalid, the syscall should not be performed, and we set a0 to -ENOSYS to inform userspace the syscall doesn't exist.
   2. if syscall number is valid, is_skipped will be used: a) if is_skipped is -1, which means there are some filters reject this syscall, so the syscall should not performed. (Of course, we can use bool instead to get better semantic) b) if is_skipped != -1, which means the filters approved this syscall, so we invoke syscall handler with modified regs.

In your design, the logical condition is not obvious. Why syscall_enter_from_user_mode() informed the syscall will be skipped but the syscall handler will be called when syscall number is invalid? The users need to think two things to get result: a) -1 means skip b) -1 < 0 in signed integer, so the skip condition is always a invalid syscall number.

In may way, the users only need to think one thing: The syscall_enter_from_user_mode() said -1 means the syscall should not be performed, so use it as a condition of reject directly. They just need to combine the informations that they get from API as the condition of control flow.

I'm all-in for simpler API usage! Maybe massage the syscall_enter_from_user_mode() (or a new one), so that additional syscall_get_nr() call is not needed?

It's completely unclear to me what the actual problem is. The flow how this works on all architectures is:

regs->orig_a0 = regs->a0 regs->a0 = -ENOSYS;

nr = syscall_enter_from_user_mode(....);

if (nr >= 0) regs->a0 = nr < MAX_SYSCALL ? syscall(nr) : -ENOSYS;

If syscall_trace_enter() returns -1 to skip the syscall, then regs->a0 is unmodified, unless one of the magic operations modified it.

If syscall_trace_enter() was not active (no tracer, no seccomp ...) then regs->a0 already contains -ENOSYS.

So what's the exact problem?

Thanks,

tglx