Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

[Andy Lutomirski]

On Thu, Nov 8, 2018 at 1:31 PM Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
>
> On Thu, Nov 08, 2018 at 01:22:54PM -0800, Andy Lutomirski wrote:
> > > >
> > > > Why are these __packed?  It seems like it'll generate bad code for no
> > > > obvious purpose.
> > >
> > > That prevents any possibility that the compiler will insert padding, although in
> > > 64-bit kernel this should not happen to either struct.  Also all xstate
> > > components here are packed.
> > >
> >
> > They both seem like bugs, perhaps.  As I understand it, __packed
> > removes padding, but it also forces the compiler to expect the fields
> > to be unaligned even if they are actually aligned.
>
> How is that? Andy, mind to point where you get that this
> attribute forces compiler to make such assumption?

It's from memory.  But gcc seems to agree with me I compiled this:

struct foo {
    int x;
} __attribute__((packed));

int read_foo(struct foo *f)
{
    return f->x;
}

int align_of_foo_x(struct foo *f)
{
    return __alignof__(f->x);
}

Compiling with -O2 gives:

    .globl    read_foo
    .type    read_foo, @function
read_foo:
    movl    (%rdi), %eax
    ret
    .size    read_foo, .-read_foo

    .p2align 4,,15
    .globl    align_of_foo_x
    .type    align_of_foo_x, @function
align_of_foo_x:
    movl    $1, %eax
    ret
    .size    align_of_foo_x, .-align_of_foo_x

So gcc thinks that the x field is one-byte-aligned, but the code is
okay (at least in this instance) on x86.
Building for armv5 gives:

    .type    read_foo, %function
read_foo:
    @ args = 0, pretend = 0, frame = 0
    @ frame_needed = 0, uses_anonymous_args = 0
    @ link register save eliminated.
    ldrb    r3, [r0]    @ zero_extendqisi2
    ldrb    r1, [r0, #1]    @ zero_extendqisi2
    ldrb    r2, [r0, #2]    @ zero_extendqisi2
    orr    r3, r3, r1, lsl #8
    ldrb    r0, [r0, #3]    @ zero_extendqisi2
    orr    r3, r3, r2, lsl #16
    orr    r0, r3, r0, lsl #24
    bx    lr
    .size    read_foo, .-read_foo
    .align    2
    .global    align_of_foo_x
    .syntax unified
    .arm
    .fpu vfpv3-d16
    .type    align_of_foo_x, %function

So I'm pretty sure I'm right.