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

[Björn Töpel]

Thanks for helping out to dissect this! Much appreciated!

Thomas Gleixner <tglx at linutronix.de> writes:

> Let me look at your failure analysis from your first reply:
>
>>  1. strace "tracing": Requires that regs->a0 is not tampered with prior
>>     ptrace notification
>> 
>>     E.g.:
>>     | # ./strace /
>>     | execve("/", ["/"], 0x7ffffaac3890 /* 21 vars */) = -1 EACCES (Permission denied)
>>     | ./strace: exec: Permission denied
>>     | +++ exited with 1 +++
>>     | # ./disable_ptrace_get_syscall_info ./strace /
>>     | execve(0xffffffffffffffda, ["/"], 0x7fffd893ce10 /* 21 vars */) = -1 EACCES (Permission denied)
>>     | ./strace: exec: Permission denied
>>     | +++ exited with 1 +++
>> 
>>     In the second case, arg0 is prematurely set to -ENOSYS
>>     (0xffffffffffffffda).
>
> That's expected if ptrace_get_syscall_info() is not used. Plain dumping
> registers will give you the current value on all architectures.
> ptrace_get_syscall_info() exist exactly for that reason.

Noted! So this shouldn't be considered as a regression. IOW, the
existing upstream code is OK for this scenario.

>>  2. strace "syscall tampering": Requires that ENOSYS is returned for
>>     syscall(-1), and not skipped w/o a proper return value.
>> 
>>     E.g.:
>>     | ./strace -a0 -ewrite -einject=write:error=enospc echo helloject=write:error=enospc echo hello   
>> 
>>     Here, strace expects that injecting -1, would result in a ENOSYS.
>
> No. It expects ENOSPC with the above command line. man strace:
>
>        If :error=errno option is specified, a fault is injected into a
>        syscall invocation: the syscall number is replaced by -1 which
>        corresponds to an invalid syscall (unless a syscall is specified
>        with :syscall= option), and the error code is specified using a
>        symbolic errno value like ENOSYS or a numeric value within
>        1..4095 range.
>
> Similar for -einject:retval=$N
>
> So you cannot overwrite a0 with ENOSYS if the syscall needs to be
> skipped.

ACK!