On Sat, Oct 12, 2019 at 4:59 PM Bill Wendling <morbo@xxxxxxxxxx> wrote:
>
> It's fragile to try to retrieve the stack pointer by taking the address
> of a variable on the stack. For instance, clang reserves more stack
> space than gcc here, indicating that the variable may not be at the
> start of the stack. Instead of relying upon this to work, retrieve the
> "%rbp" value, which contains the value of "%rsp" before stack
> allocation.
>
> Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
> ---
> x86/realmode.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/x86/realmode.c b/x86/realmode.c
> index cf45fd6..7c89dd1 100644
> --- a/x86/realmode.c
> +++ b/x86/realmode.c
> @@ -518,11 +518,12 @@ extern void retf_imm(void);
>
> static void test_call(void)
> {
> - u32 esp[16];
> u32 addr;
>
> inregs = (struct regs){ 0 };
> - inregs.esp = (u32)esp;
> +
> + // At this point the original stack pointer is in %ebp.
> + asm volatile ("mov %%ebp, %0" : "=rm"(inregs.esp));
I don't think we should assume the existence of frame pointers.
Moreover, I think %esp is actually the value that should be saved
here, regardless of how large the current frame is.
> MK_INSN(call1, "mov $test_function, %eax \n\t"
> "call *%eax\n\t");
> --
> 2.23.0.700.g56cf767bdb-goog
>
