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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 12 Jun 2015 16:28:06 -0700
Message-ID: <CALCETrWp79ASjZ1f28_u6bVO4b0Mh4=n6H8VhbA=Y6qcQLG45w@mail.gmail.com>
Subject: Re: [PATCH 3/5] x86/asm/entry/32: Shorten __audit_syscall_entry args preparation
To: Ingo Molnar <mingo@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Kees Cook <keescook@chromium.org>, Borislav Petkov <bp@alien8.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@plumgrid.com>, Oleg Nesterov <oleg@redhat.com>,
        Will Drewry <wad@chromium.org>, Steven Rostedt <rostedt@goodmis.org>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Jun 9, 2015 11:21 PM, "Ingo Molnar" <mingo@kernel.org> wrote:
>
>
> * Denys Vlasenko <dvlasenk@redhat.com> wrote:
>
> > We use three MOVs to swap edx and ecx. We can use one XCHG instead.
> >
> > Expand the comments. It's difficult to keep track which arg# every register
> > corresponds to, so spell it out.
>
> > +     /*
> > +      * At this point, registers hold syscall args in 32-bit ABI:
> > +      * eax is syscall#, args are in ebx,ecx,edx,esi,edi,ebp.
> > +      * Shuffle them to match what __audit_syscall_entry() wants.
> > +      */
> > +     movl    %esi, %r8d              /* arg5 (r8): 4th syscall arg */
> > +     xchg    %ecx, %edx              /* arg4 (rcx): 3rd syscall arg (edx) */
> > +                                     /* arg3 (rdx): 2nd syscall arg (ecx) */
> > +     movl    %ebx, %esi              /* arg2 (rsi): 1st syscall arg */
> > +     movl    %eax, %edi              /* arg1 (rdi): syscall number */
> >       call    __audit_syscall_entry
>
> So while we are at it I improved this a bit more, to:
>
>         /*
>          * At this point, registers hold syscall args in 32-bit syscall ABI:
>          *   eax is syscall#, args are in ebx,ecx,edx,esi,edi,ebp.
>          *
>          * We want to pass them to __audit_syscall_entry(), which is a 64-bit
>          * C function with 5 parameters, so shuffle them to match what
>          * __audit_syscall_entry() expects: rdi,rsi,rdx,rcx,r8.
>          */
>         movl    %esi, %r8d              /* arg5 (r8 ) <= 4th syscall arg (esi) */
>         xchg    %ecx, %edx              /* arg4 (rcx) <= 3rd syscall arg (edx) */
>                                         /* arg3 (rdx) <= 2nd syscall arg (ecx) */
>         movl    %ebx, %esi              /* arg2 (rsi) <= 1st syscall arg (ebx) */
>         movl    %eax, %edi              /* arg1 (rdi) <= syscall number  (eax) */
>         call    __audit_syscall_entry
>
> Btw., syscall auditing is not auditing syscall arguments #5 and #6?

Indeed.  That's the least of its problems.  Don't ever read that code
or you might accidentally git rm it.

--Andy