Re: [PATCH 1/2] tracing/syscalls: allow multiple syscall numbers per syscall

On Tue, Aug 30, 2016 at 12:29 PM, Steven Rostedt [off-list ref] wrote:

On Tue, 30 Aug 2016 11:52:39 -0700
Andy Lutomirski [off-list ref] wrote:

quoted

Okay, I think I see what's going on.  init_ftrace_syscalls() does:

        meta = find_syscall_meta(addr);

Unless I'm missing some reason why this is a sensible thing to do,
this seems overcomplicated and incorrect.  There is exactly one caller
of find_syscall_meta() and that caller knows the syscall number.  Why
doesn't it just look up the metadata by *number* instead of by syscall
implementation address?  There are plenty of architectures for which
multiple logically different syscalls can share an implementation
(e.g. pretty much everything that calls in_compat_syscall()).

The problem is that the meta data is created at the syscalls
themselves. Look at all the macro magic in include/linux/syscalls.h,
and search for __syscall_metadata. The meta data is created via linker
magic, and the find_syscall_meta() is what finds a specific system call
and the meta data associated with it.

Egads!  OK, I see why this is a mess.

I guess we should be creating the metadata from the syscall tables
instead of from the syscall definitions, but I guess that's currently
a nasty per-arch mess.

Could we at least have an array of (arch, nr) instead of just an array
of nrs in the metadata?

--Andy