linux-kernel - Re: [PATCH 0/3] syscalls: clean up stub naming convention

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Date:   Mon, 9 Apr 2018 08:39:21 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Maninder Singh <maninder1.s@...sung.com>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 0/3] syscalls: clean up stub naming convention

* Dominik Brodowski <linux@...inikbrodowski.net> wrote:

> > One more fundamental question: why do we have the __do_sys_waitid() and 
> > __inline_sys_waitid() distinction - aren't the function call signatures the same 
> > with no conversion done?
> > 
> > I.e. couldn't we just do a single, static __do_sys_waitid(), where the compiler 
> > would decide to what extent inlining is justified? This would allow the compiler 
> > to inline all the intermediate code into the stubs themselves.
> > 
> > Or is this a side effect of the error injection feature, which needs to add extra 
> > logic at this intermediate level? That too should be able to use the 
> > __do_sys_waitid() variant though.
> 
> Error injection is unrelated. It seems to be for three reasons, if I read
> the code (include/linux/syscalls.h) correctly:
> 
> 	asmlinkage long __do_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__))
> 
> 1)  This takes arguments of type long (to protect against CVE-2009-0029);
>     see https://lwn.net/Articles/604287/ : "Digging into the history of
>     this, it turns out that the long version ensures that 32-bit values
>     are correctly sign-extended for some 64-bit kernel platforms,
>     preventing a historical vulnerability."
> 
> 	{
> 		long ret = __in_sys##name(__MAP(x,__SC_CAST,__VA_ARGS__));
> 		__MAP(x,__SC_TEST,__VA_ARGS__);

I see - so it's _not_ the same function call signature, but a wrapper with a 
sign-extended version, which is fair and useful. So on architectures where this 
matters there's type conversion and active code generated.

Thanks,

	Ingo