Re: [PATCH v5] x86: use builtins to read eflags

["Andy Lutomirski" <luto@kernel.org>]

On Fri, Mar 18, 2022, at 4:42 PM, Segher Boessenkool wrote:
> On Fri, Mar 18, 2022 at 04:10:55PM -0700, Linus Torvalds wrote:
>> It would be lovely to have some explicit model for "I want the frame
>> to have been set up for backtraces", but here we are.
>
> So please define exactly what that *means*?  Preferably portably, but I
> reckon at least some of it will have to be machine-specific (and ABI-
> specific).  But it needs to be well-defined, clearly defined, defined at
> all, and *documented* :-)
>
>> Marking '%rsp
>> used makes the compiler understand it's not a leaf function.
>
> As I said before, this is explicitly incorrect code.  Always was, but
> it is documented since a while (since GCC 9).  Clobbering the stack
> pointer can never be correct, the stack pointer after an asm has to be
> identical to the one before that asm!
>
>> And while we have other uses for it that then use the actual value,
>> those don't care about the exact value of the stack pointer register,
>> they just want "give me a pointer that is contained within the current
>> stack", because we control the stack allocation and do funky things
>> there. So "any random stack pointer value in this function" is
>> perfectly fine and expected.
>
> You can use %rsp as *input* operand just fine, which is all you need for
> that.
>
>> But for user mode, it would probably be a great idea to also have a "I
>> cannot use a redzone in this function" thing. The kernel can't use it
>> because we have nested exceptions, but maybe some day even the kernel
>> could make use of (controlled) red-zoning.
>
> Yes.  We just have to figure out what the exact semantics we want is,
> and how to express that in a target-independent way, and then relatedly
> what a good name for it would be ("redzone" in the clobber list is the
> best I can come up with right now, but that may have to change).

Here’s the semantics I want:

I want to tell the compiler that an asm statement makes a function call. I want to specify the stack alignment and offset I need.  I want the compiler to make it work. Something like this, but preferably with better syntax:

asm("asm here" ::: "call" (align=16, offset=0));

This means that the asm in question wants rsp to be 0 more than a multiple of 16 and that it wants precisely the setup needed for a call to be done. If frame pointers are enabled, a frame should be set up. If there is a redzone then either the compiler needs to not use it or needs to advance rsp past it.

>
>
> Segher