Re: [PATCH v2 7/9] x86/vdso: abstract out vdso system call internals

["H. Peter Anvin" <hpa@zytor.com>]

On 2025-11-12 02:31, Uros Bizjak wrote:
> 
> Unfortunately, %ebp is still special with -fno-omit-frame-pointer, so
> using "ebp" as _sys_arg6 on 32-bit targets will result in:
> 
> error: bp cannot be used in ‘asm’ here
> 
> Please see how %ebp register is handled in
> arch/x86/include/asm/vmware.h, vmware_hypercall_hb_out() and
> vmware_hypercall_hb_in().
> 

#ifdef CONFIG_X86_64
#define VMW_BP_CONSTRAINT "r"
#else
#define VMW_BP_CONSTRAINT "m"
#endif

        asm_inline volatile (
                UNWIND_HINT_SAVE
                "push %%" _ASM_BP "\n\t"
                UNWIND_HINT_UNDEFINED
                "mov %[in6], %%" _ASM_BP "\n\t"
                "rep outsb\n\t"
                "pop %%" _ASM_BP "\n\t"
                UNWIND_HINT_RESTORE
                : "=a" (out0), "=b" (*out1)
                : "a" (VMWARE_HYPERVISOR_MAGIC),
                  "b" (cmd),
                  "c" (in2),
                  "d" (in3 | VMWARE_HYPERVISOR_PORT_HB),
                  "S" (in4),
                  "D" (in5),
                  [in6] VMW_BP_CONSTRAINT (in6)
                : "cc", "memory");
        return out0;

That code is actually incorrect, in at least two ways:


1. It should be conditioned on frame pointers enabled, not x86-64 vs i386.
2. The compiler is perfectly within its right to emit an %esp-relative
   reference for the "m"-constrained [in6]. This is particularly likely
   when *not* compiled with frame pointers, see #1.

A better sequence might be:

	pushl %[in6]
	push %ebp
	mov 4(%esp),%ebp
	<stuff>
	pop %ebp
	pop %[junk]

Then %[in6] can even safely be a "g" constraint (hence pushl).

	-hpa