Re: [PATCH v2] x86/hyperv: Use __naked attribute to fix stackless C function

[Uros Bizjak]

On Fri, Feb 27, 2026 at 11:40 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> hv_crash_c_entry() is a C function that is entered without a stack,
> and this is only allowed for functions that have the __naked attribute,
> which informs the compiler that it must not emit the usual prologue and
> epilogue or emit any other kind of instrumentation that relies on a
> stack frame.
>
> So split up the function, and set the __naked attribute on the initial
> part that sets up the stack, GDT, IDT and other pieces that are needed
> for ordinary C execution. Given that function calls are not permitted
> either, use the existing long return coded in an asm() block to call the
> second part of the function, which is an ordinary function that is
> permitted to call other functions as usual.
>
> Cc: Mukesh Rathor <mrathor@xxxxxxxxxxxxxxxxxxx>
> Cc: Wei Liu <wei.liu@xxxxxxxxxx>
> Cc: Uros Bizjak <ubizjak@xxxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: linux-hyperv@xxxxxxxxxxxxxxx
> Fixes: 94212d34618c ("x86/hyperv: Implement hypervisor RAM collection into vmcore")
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
> v2: apply some asm tweaks suggested by Uros and Andrew

Acked by: Uros Bizjak <ubizjak@xxxxxxxxx>

FYI: GCC by design inserts ud2 at the end of x86 naked functions. This
is intended to help debugging in case someone forgets ret/jmp, so
program execution does not wander into whatever follows the function.
IIRC, when ud2 follows ret, code analyzers may report "unreachable
code" warnings. I don't know if this is still the case, but
nevertheless this should be considered an important "safety net"
feature of the compiler.

Uros.