Re: Proposal for finishing the 64-bit x86 syscall cleanup

From: Brian Gerst
Date: Wed Aug 26 2015 - 23:13:28 EST

On Wed, Aug 26, 2015 at 1:10 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, Aug 25, 2015 at 10:20 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>>>>> Thing 2: vdso compilation with binutils that doesn't support .cfi directives
>>>>> Userspace debuggers really like having the vdso properly
>>>>> CFI-annotated, and the 32-bit fast syscall entries are annotatied
>>>>> manually in hexidecimal. AFAIK Jan Beulich is the only person who
>>>>> understands it.
>>>>> I want to be able to change the entries a little bit to clean them up
>>>>> (and possibly rework the SYSCALL32 and SYSENTER register tricks, which
>>>>> currently suck), but it's really, really messy right now because of
>>>>> the hex CFI stuff. Could we just drop the CFI annotations if the
>>>>> binutils version is too old or even just require new enough binutils
>>>>> to build 32-bit and compat kernels?
>>>> One thing I want to do is rework the 32-bit VDSO into a single image,
>>>> using alternatives to handle the selection of entry method. The
>>>> open-coded CFI crap has made that near impossible to do.
>>> Yes please!
>>> But please don't change the actual instruction ordering at all yet,
>>> since the SYSCALL case seems to be buggy right now.
>>> (If you want to be really fancy, don't use alternatives. Instead
>>> teach vdso2c to annotate the actual dynamic table function pointers so
>>> we can rewrite the pointers at boot time. That will save a cycle or
>>> two.)
>> The easiest way to select the right entry code is by changing the ELF
>> AUX vector. That gets the normal usage, but there are two additional
>> cases that need addressing.
>> 1) Some code could possibly lookup the __kernel_vsyscall symbol
>> directly and call it, but that's non-standard. If there is code out
>> there that does this, we could update the ELF symbol table to point
>> __kernel_vsyscall to the chosen entry point, or just remove the symbol
>> and let the caller fall back to INT80.
> Here's an alternate proposal, which is mostly copied from what I
> posted here yesterday afternoon:
> I think we should consider doing this:
> __kernel_vsyscall:
> push %ecx
> push %edx
> movl %esp, %edx
> ALTERNATIVE (Intel with SEP):
> sysenter
> ALTERNATIVE (AMD with SYSCALL32 on 64-bit kernels):
> syscall
> hlt /* keep weird binary tracers from utterly screwing up */
> ALTERNATIVE (if neither of the other cases apply):
> nops
> movl %edx, %esp
> movl (%esp), %edx
> movl 8(%esp), %ecx
> int $0x80
> vsyscall_after_int80:
> popl %edx
> popl %ecx
> ret

This could interfere with sigreturn/ptrace, since if EDX or ECX are
changed those would get overwritten by the pops from the stack.
That's a problem with the current code too.

> First, in the case where we have neither SEP nor SYSCALL32, I claim
> that this Just Works. We push a couple regs, pointlessly shuffle esp,
> restore the regs, do int $0x80 with the same regs we started with, and
> then (again, pointlessly) pop the regs we pushed.

If neither SYSENTER or SYSCALL are supported then it should just be
"int $0x80; ret;", nothing more. You can't assume all int80 calls
come from the VDSO. In fact, fork/clone cannot use the VDSO (the
saved values on the stack are not copied to the child) and always uses
int80 directly.

> Now we make the semantics of *both* syscall32 and sysenter that they
> load edx into regs->sp, fetch regs->dx and regs->cx from memory, and
> set regs->ip to vsyscall_after_int80. (This is a wee bit slower than
> the current sysenter code because it does two memory fetches instead
> of one.) Then they proceed just like int80. In particular, anything
> that does "ip -= 2" works exactly like int80 because it points at an
> actual int80 instruction.
> Note that sysenter's slow path already sort of works like this.
> Now we've fixed the correctness issues but we've killed performance,
> as we'll use IRET instead of SYSRET to get back out. We can fix that
> using opportunstic sysret. If we're returning from a compat syscall
> that entered via sysenter or syscall32, if regs->ip ==
> vsyscall_after_int80, regs->r11 == regs->flags, regs->ss == __USER_DS,
> regs->cs == __USER32_CS, and flags are sane, then return using
> SYSRETL. (The r11 check is probably unnecessary.)
> This is not quite as elegant as 64-bit opportunistic sysret, since
> we're zapping ecx. This should be unobservable except by debuggers,
> since we already know that we're returning to a 'pop ecx' instruction.
> NB: I don't think we can enable SYSCALL on 32-bit kernels. IIRC
> there's no MSR_SYSCALL_MASK support, which makes the whole thing
> basically useless, since we can't mask TF and we don't control ESP.

The original implementation of SYSCALL on the K6 had a bad side
effect. You could only return to userspace with SYSRET. It set some
internal state that caused IRET to fault, which meant you couldn't
context switch to another task that had taken a page fault for
example. That made it unusable without some ugly hacks. It may work
differently on modern AMD processors in legacy mode, but it's probably
not worth trying to support it.

Brian Gerst
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