Re: Syscall table AKA hijacking syscalls

Helge Hafting
2004-01-03T15:30:22Z

On Sat, Jan 03, 2004 at 12:46:17AM +0100, Libor Vanek wrote:
> >>I'm writing some project which needs to hijack some syscalls in VFS
> >>layer. AFAIK in 2.6 is this "not-wanted" solution (even that there are
> >>some very nasty ways of doing it - see
> >>http://mail.nl.linux.org/kernelnewbies/2002-12/msg00266.html )
> >
> >
> >And it will fail miserably on many non x86 architectures for
> >various reasons:
> >
> >1. ppc64 and ia64 use function descriptors
> >2. sparc64 uses a 32bit call out table
> >
> >In short its not only an awful hack, its horribly non portable :)
>
> But in short you always get some syscall from userspace and have some table
> with function vectors assigned to each syscall, don't you?
>
> So you can have something like
> "append_this_function_before_syscall_sys_open" and
> "append_this_function_after_syscall_sys_open" which would be platform
> independent but will have platform dependent implementation.

Why bother overriding syscalls?
If you want a different sys_open, just modify/rewrite it. Then you get a kernel
that works your way without touching the syscall table (or other implementation of it)
at all.

Of course this sort of rewrite cannot be acitvated/deactivated by loading/unloading
a module. But that isn't necessary, use a writeable flag in /proc instead.

i.e.:
sys_open(...)
{
if (activated_in_proc) my_sys_open(...); else standard_sys_open(...);
}

Helge Hafting