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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
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Subject: Re: Is adding an argument to an existing syscall okay?
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* 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.