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

Dominik Brodowski
2018-03-30T10:48:40Z

On Fri, Mar 30, 2018 at 12:16:02PM +0200, Ingo Molnar wrote:
>
> * Dominik Brodowski <linux@dominikbrodowski.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.

Good idea. I have updated the patchset accordingly.

> > 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.

OK, I'll wait for a few more opinions on these two related issues, and update
the code accordingly then.

> > 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?

My current plan is to push the 109 patch bomb to remove in-kernel calls to syscalls
directly to Linus once v4.16 is released.

For this series of seven patches, I am content with them going upstream through
the x86 tree (once that contains a backmerge of Linus' tree or the syscalls
tree, obviously). IMO, these seven patches should be kept together, and not routed
upstream through different channels.

Thanks,
Dominik