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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Nov 17, 2020, at 1:58 PM, Peter Oskolkov posk@google.com wrote:

> 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.

I think what Florian refers to here is if there would be a glibc library
wrapper exposing the system call to applications. There, the number of
arguments would matter. But it does not exist today.

In some sense, it's a good thing that there isn't such wrapper exposed
yet. It also makes me wonder whether exposing system calls directly as a
library ABI is a good thing. It appears that library ABIs have stronger
restrictions with respect to number and types of parameters than system
calls.

Thanks,

Mathieu

> 
> 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.

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com