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

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Message-ID: <CAFULd4aMOueG5y3K7F57ryLtt-JUhE-yfsDODC=5+bgjF_gtSw@mail.gmail.com>
Date: Wed, 12 Nov 2025 11:31:05 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, "Theodore Ts'o" <tytso@....edu>, 
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>, 
	Xin Li <xin@...or.com>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Andy Lutomirski <luto@...nel.org>, Ard Biesheuvel <ardb@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	Brian Gerst <brgerst@...il.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Ingo Molnar <mingo@...hat.com>, James Morse <james.morse@....com>, 
	Jarkko Sakkinen <jarkko@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, Kees Cook <kees@...nel.org>, 
	Nam Cao <namcao@...utronix.de>, Oleg Nesterov <oleg@...hat.com>, Perry Yuan <perry.yuan@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, Thomas Huth <thuth@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, linux-sgx@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 7/9] x86/vdso: abstract out vdso system call internals

On Wed, Nov 12, 2025 at 5:38 AM H. Peter Anvin <hpa@...or.com> wrote:
>
> Abstract out the calling of true system calls from the vdso into
> macros.
>
> It has been a very long time since gcc did not allow %ebx or %ebp in
> inline asm in 32-bit PIC mode; remove the corresponding hacks.
>
> Remove the use of memory output constraints in gettimeofday.h in favor
> of "memory" clobbers. The resulting code is identical for the current
> use cases, as the system call is usually a terminal fallback anyway,
> and it merely complicates the macroization.
>
> This patch adds only a handful of more lines of code than it removes,
> and in fact could be made substantially smaller by removing the macros
> for the argument counts that aren't currently used, however, it seems
> better to be general from the start.
>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>

[...]

> diff --git a/arch/x86/include/asm/vdso/sys_call.h b/arch/x86/include/asm/vdso/sys_call.h
> new file mode 100644
> index 000000000000..6b1fbcdcbd5c
> --- /dev/null
> +++ b/arch/x86/include/asm/vdso/sys_call.h
> @@ -0,0 +1,119 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Macros for issuing an inline system call from the vDSO.
> + */
> +
> +#ifndef X86_ASM_VDSO_SYS_CALL_H
> +#define X86_ASM_VDSO_SYS_CALL_H
> +
> +#include <linux/compiler.h>
> +#include <asm/cpufeatures.h>
> +#include <asm/alternative.h>
> +
> +/*
> + * Note: only three arguments are currently supported,
> + * because there are no constraint letters for r10, r8, r9.

The above comment does not apply when using local variables associated
with a register.

> + */
> +#ifdef CONFIG_X86_64
> +/* Using dummy output registers instead of clobbers avoids messing up
> +   user-specified clobbers. */
> +#define __sys_instr    "syscall"
> +#define __sys_clobber  "rcx", "r11", "memory"
> +#define __sys_nr(x,y)  __NR_ ## x
> +#define __sys_reg1     "rdi"
> +#define __sys_reg2     "rsi"
> +#define __sys_reg3     "rdx"
> +#define __sys_reg4     "r10"
> +#define __sys_reg5     "r8"
> +#define __sys_reg6     "r9"
> +#else
> +#define __sys_instr    "call __kernel_vsyscall"
> +#define __sys_clobber  "memory"
> +#define __sys_nr(x,y)  __NR_ ## x ## y
> +#define __sys_reg1     "ebx"
> +#define __sys_reg2     "ecx"
> +#define __sys_reg3     "edx"
> +#define __sys_reg4     "esi"
> +#define __sys_reg5     "edi"
> +#define __sys_reg6     "ebp"
> +#endif

[...]

> +#define VDSO_SYSCALL6(name,suf32,a1,a2,a3,a4,a5,a6)                    \
> +       ({                                                              \
> +               register long _sys_arg1 asm(__sys_reg1) = (long)(a1);   \
> +               register long _sys_arg2 asm(__sys_reg2) = (long)(a2);   \
> +               register long _sys_arg3 asm(__sys_reg3) = (long)(a3);   \
> +               register long _sys_arg4 asm(__sys_reg4) = (long)(a4);   \
> +               register long _sys_arg5 asm(__sys_reg5) = (long)(a5);   \
> +               register long _sys_arg6 asm(__sys_reg6) = (long)(a6);   \
> +               _VDSO_SYSCALL(name,suf32,                               \
> +                             "r" (_sys_arg1), "r" (_sys_arg2),         \
> +                             "r" (_sys_arg3), "r" (_sys_arg4),         \
> +                             "r" (_sys_arg5), "r" (_sys_arg6));        \
> +       })

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().

Uros.