Re: Is adding an argument to an existing syscall okay?

[Peter Oskolkov <posk@google.com>]

My assumption here was that applications that are aware of the new API
will always provide three parameters, while older applications will
continue calling the syscall with two.

I can't think of a situation/architecture where this will break anything.

Thanks,
Peter


On Tue, Nov 17, 2020 at 10:44 AM Florian Weimer <fw@deneb.enyo.de> wrote:
>
> * Segher Boessenkool:
>
> > But this isn't variadic in the sense of "..." -- on Power that always
> > passes the unspecified arguments in memory, while in this case it just
> > passes in either two or three registers.  I don't know any arg where
> > that would not work, given the Linux system call restrictions.
> >
> > This is similar to the "open" system call.
>
> Exactly.  You cannot call the open function through a non-variadic
> function pointer.  I've seen it cause stack corruption in practice:
>
> commit c7774174beffe9a8d29dd4fb38bbed43ece1cecd
> Author: Andreas Schneider <asn@samba.org>
> Date:   Wed Aug 2 13:21:59 2017 +0200
>
>     swrap: Fix prototype of open[64] to prevent segfault on ppc64le
>
>     The calling conventions for vaarg are different on ppc64le. The patch
>     fixes segfaults on that platform.
>
>     Thanks to Florian Weimer who helped debugging it!
>
>     Signed-off-by: Andreas Schneider <asn@samba.org>
>     Reviewed-by: Stefan Metzmacher <metze@samba.org>
>
> <https://git.samba.org/?p=socket_wrapper.git;a=commitdiff;h=c7774174beffe>
>
> It is possible to implement the open function in such a way that it
> does not have this problem (simply do not use the parameter save area,
> using assembler if necessary), but it's another obscure step that libc
> implementers would have to take.