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    SubjectRe: [PATCH v7 11/22] x86/decompressor: Avoid the need for a stack in the 32-bit trampoline
    On Fri, Jul 28, 2023 at 11:09:05AM +0200, Ard Biesheuvel wrote:
    > The 32-bit trampoline no longer uses the stack for anything except
    > performing a far return back to long mode. Currently, this stack is
    > placed in the same page that carries the trampoline code, which means
    > this page must be mapped writable and executable, and the stack is
    > therefore executable as well.
    >
    > Replace the far return with a far jump, so that the return address can
    > be pre-calculated and patched into the code before it is called. This
    > removes the need for a stack entirely, and in a later patch, this will
    > be taken advantage of by removing writable permissions from (and adding
    > executable permissions to) this code page explicitly when booting via
    > the EFI stub.
    >
    > Not touching the stack pointer also makes it more straight-forward to
    > call the trampoline code as an ordinary 64-bit function from C code.
    >
    > Note that we need to preserve the value of RSP across the switch into
    ^^

    Passive voice pls.

    > compatibility mode: the stack pointer may get truncated to 32 bits.
    >
    > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
    > ---
    > arch/x86/boot/compressed/head_64.S | 64 ++++++++++----------
    > arch/x86/boot/compressed/pgtable.h | 4 +-
    > arch/x86/boot/compressed/pgtable_64.c | 12 +++-
    > 3 files changed, 44 insertions(+), 36 deletions(-)
    >
    > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
    > index 491d985be75fd5b0..1b0c61d1b389fd37 100644
    > --- a/arch/x86/boot/compressed/head_64.S
    > +++ b/arch/x86/boot/compressed/head_64.S
    > @@ -449,9 +449,6 @@ SYM_CODE_START(startup_64)
    > leaq TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax
    > call *%rax
    >
    > - /* Restore the stack, the 32-bit trampoline uses its own stack */
    > - leaq rva(boot_stack_end)(%rbx), %rsp
    > -
    > /*
    > * cleanup_trampoline() would restore trampoline memory.
    > *
    > @@ -537,32 +534,37 @@ SYM_FUNC_END(.Lrelocated)
    > * EDI contains the base address of the trampoline memory.
    > * Non-zero ESI means trampoline needs to enable 5-level paging.
    > */
    > + .section ".rodata", "a", @progbits
    > SYM_CODE_START(trampoline_32bit_src)
    > - /* Grab return address */
    > - movq (%rsp), %rax
    > -
    > - /* Set up 32-bit addressable stack */
    > - leaq TRAMPOLINE_32BIT_STACK_END(%rdi), %rsp
    > -
    > - /* Preserve return address and other live 64-bit registers */
    > - pushq %rax
    > + /* Preserve live 64-bit registers */
    > pushq %r15
    > pushq %rbp
    > pushq %rbx
    >
    > + /* Preserve top half of RSP in a legacy mode GPR to avoid truncation */
    > + movq %rsp, %rbx
    > + shrq $32, %rbx
    > +
    > /* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */
    > pushq $__KERNEL32_CS
    > leaq 0f(%rip), %rax
    > pushq %rax
    > lretq
    >
    > + /*
    > + * The 32-bit code below will do a far jump back to long mode and end
    > + * up here after reconfiguring the number of paging levels.
    > + */
    > +.Lret: shlq $32, %rbx // Reconstruct stack pointer

    No side comments pls.

    --
    Regards/Gruss,
    Boris.

    https://people.kernel.org/tglx/notes-about-netiquette

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