linux-kernel - Re: [PATCH] x86/entry: Improve system call entry comments

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Date:	Tue, 8 Mar 2016 11:30:04 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/entry: Improve system call entry comments


* H. Peter Anvin <hpa@...or.com> wrote:

> On March 7, 2016 12:22:28 AM PST, Ingo Molnar <mingo@...nel.org> wrote:
> >
> >* Andy Lutomirski <luto@...nel.org> wrote:
> >
> >> Ingo suggested that the comments should explain when the various
> >> entries are used.  This adds these explanations and improves other
> >> parts of the comments.
> >
> >Thanks for doing this, this is really useful!
> >
> >One very small detail I noticed:
> >
> >> +/*
> >> + * 32-bit legacy system call entry.
> >> + *
> >> + * 32-bit x86 Linux system calls traditionally used the INT $0x80
> >> + * instruction.  INT $0x80 lands here.
> >> + *
> >> + * This entry point can be used by 32-bit and 64-bit programs to
> >perform
> >> + * 32-bit system calls.  Instances of INT $0x80 can be found inline
> >in
> >> + * various programs and libraries.  It is also used by the vDSO's
> >> + * __kernel_vsyscall fallback for hardware that doesn't support a
> >faster
> >> + * entry method.  Restarted 32-bit system calls also fall back to
> >INT
> >> + * $0x80 regardless of what instruction was originally used to do
> >the
> >> + * system call.
> >> + *
> >> + * This is considered a slow path.  It is not used by modern libc
> >> + * implementations on modern hardware except during process startup.
> >> + *
> >> + * Arguments:
> >> + * eax  system call number
> >> + * ebx  arg1
> >> + * ecx  arg2
> >> + * edx  arg3
> >> + * esi  arg4
> >> + * edi  arg5
> >> + * ebp  arg6
> >> + */
> >>  ENTRY(entry_INT80_32)
> >
> >entry_INT80_32() is only used on pure 32-bit kernels, 64-bit kernels
> >use 
> >entry_INT80_compat(). So the above text should not talk about 64-bit
> >programs, as 
> >they can never trigger this specific entry point, right?
> >
> >So I'd change the explanation to something like:
> >
> >> + * This entry point is active on 32-bit kernels and can thus be used
> >by 32-bit 
> >> + * programs to perform 32-bit system calls. (Programs running on
> >64-bit
> >> + * kernels executing INT $0x80 will land on another entry point: 
> >> + * entry_INT80_compat. The ABI is identical.)
> >
> >Agreed?
> >
> >Thanks,
> >
> >	Ingo
> 
> Sadly I believe Android still uses int $0x80 in the upstream version.

I don't see how that fact conflicts with my statement: on 64-bit kernels INT $0x80 
will (of course) work, but will land on another entry point: entry_INT80_compat(), 
not entry_INT80_32().

On 32-bit kernels the INT $0x80 entry point is entry_INT80_32().

Thanks,

	Ingo