linux-kernel - Re: [PATCH 0/7] use struct pt_regs based syscall calling for x86-64

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Date:   Fri, 30 Mar 2018 12:16:02 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     linux-kernel@...r.kernel.org, viro@...IV.linux.org.uk,
        torvalds@...ux-foundation.org, arnd@...db.de,
        linux-arch@...r.kernel.org, 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>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Subject: Re: [PATCH 0/7] use struct pt_regs based syscall calling for x86-64


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

> A few questions remain, from important stuff to bikeshedding:
> 
> 1) Is it acceptable to pass the existing struct pt_regs to the sys_*()
>    kernel functions in emulate_vsyscall(), or should it use a hand-crafted
>    struct pt_regs instead?

I think so: we already have task_pt_regs() which gives access to the real return 
registers on the kernel stack.

I think as long as we constify the pointer, we should pass in the real thing.

> 2) Is it the right approach to generate the __sys32_ia32_*() names to
>    include in the syscall table on-the-fly, or should they all be listed
>    in arch/x86/entry/syscalls/syscall_32.tbl ?

I think as a general principle all system call tables should point to the 
first-hop wrapper symbol name (i.e. __sys32_ia32_*() in this case), not to the 
generic symbol name - even though we could generate the former from the latter.

The more indirection in these tables, the harder to read they become I think.

> 3) I have chosen to name the default 64-bit syscall stub sys_*(), same as
>    the "normal" syscall, and the IA32_EMULATION compat syscall stub
>    compat_sys_*(), same as the "normal" compat syscall. Though this
>    might cause some confusion, as the "same" function uses a different
>    calling convention and different parameters on x86, it has the
>    advantages that
>         - the kernel *has* a function sys_*() implementing the syscall,
>           so those curious in stack traces etc. will find it in plain
>           sight,
>         - it is easier to handle in the syscall table generation, and
>         - error injection works the same.

I don't think there should be a symbol space overlap, that will only lead to 
confusion. The symbols can be _similar_, with a prefix, underscores or so, but 
they shouldn't match I think.

> The whole series is available at
> 
>         https://git.kernel.org/pub/scm/linux/kernel/git/brodo/linux.git syscalls-WIP

BTW., I'd like all these bits to go through the x86 tree.

What is the expected merge route of the generic preparatory bits?

Thanks,

	Ingo