Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

[Denys Vlasenko]

On 03/24/2015 10:40 PM, Andy Lutomirski wrote:
> The syscall and sysenter stuff is IMO really nasty.  Here's how I'd
> like it to work:
> 
> When you do "call __kernel_vsyscall", I want the net effect to be that
> your eax, ebx, ecx, edx, esi, edi, and ebp at the time of the call end
> up *verbatim* in pt_regs.  Your eip and rsp should be such that, if we
> iret normally using pt_regs, we end up returning correctly to
> userspace.  I want this to be true *regardless* of whether we're doing
> a fast-path or slow-path system call.
> 
> This means that we have, literally (see below for why ret $4):
> 
> int $0x80
> ret $4  <-- regs->eip points here
> 
> Then we add an opportunistic return trampoline: if a special ti flag
> is set (which we set on entry here) and the return eip and regs are
> appropriate, then we change the return at the last minute to vdso code
> that looks like:
> 
> popl $ecx
> popl $edx
> ret

I don't fully understand your intent.

> The vdso code would be something like (so untested it's not even funny):
> 
> __kernel_vsyscall:
>   ALTERNATIVE_2(something or other)
> 
> __kernel_vsyscall_for_intel:
>   pushl $edx
>   pushl $ecx
>   sysenter
>   hlt  <-- just for clarity
> 
> __kernel_vsyscall_for_amd:
>   pushl $ecx
>   syscall
> __vsyscall_after_syscall_insn:
>  ret $4 <-- for binary tracers only

This ret would use former ecx value as return address?


> __kernel_vsyscall_for_int80:
>   int $0x80  <-- regs->eip points here during *all* vsyscalls
> 
> __kernel_vsyscall_slow_ret:
>   ret $4

After returning, this will pop an extra word from __kernel_vsyscall() caller.
They don't expect that.


> __kernel_vsyscall_sysretl_target:
>   popl $ecx
>   ret
> 
> There is no sysexit.  Take that, Intel.
> 
> On sysenter, we copy regs->cx and regs->dx from user memory and then
> we increment regs->sp by 4 and point regs->eip to
> __kernel_vsyscall_for_int80.  On syscall, we copy regs->cx from user
> memory and point regs->eip to __kernel_vsyscall_for_int80.
> 
> On opportunistic sysretl, we do:
> 
> *regs->sp = regs->cx;  /* put_user or whatever */
> regs->eip = __kernel_vsyscall_sysretl_target
> ...
> sysretl
> 
> We never do sysexit or sysretl in any other code path.  That is, there
> is no really fast path anymore.

I still don't understand the purpose those "ret 4" insns.
They don't look right.
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