Re: [PATCH RESEND v5 1/3] x86/asm, x86/boot: expose inline memcmp

[Borislav Petkov <bp@xxxxxxxxx>]

On Tue, Jun 30, 2026 at 02:21:46PM -0300, Mauricio Faria de Oliveira wrote:
> Move the inline memcmp function currently only available in 'boot/string.c'
> into the shared string function header <asm/shared/string.h> to be reused.
> 
> This is not done through <asm/string.h> to avoid pulling unnecessary code
> in 'boot/string.c' that causes build errors in 'boot/compressed/string.c'
> and 'purgatory/purgatory.ro'.
> 
> Note that the inline memcmp() returns 0/1, not -1/0/1 as regular memcmp()
> (reported by David Laight <david.laight.linux@xxxxxxxxx>).

This belongs ...


> 
> Signed-off-by: Mauricio Faria de Oliveira <mfo@xxxxxxxxxx>
> ---

... here, if anywhere.

>  arch/x86/boot/string.c               |  6 ++----
>  arch/x86/include/asm/shared/string.h | 16 ++++++++++++++++
>  arch/x86/include/asm/string.h        |  1 +
>  3 files changed, 19 insertions(+), 4 deletions(-)

Also, PeterZ has a point - we should group all __inline_ functions into
shared/string.h and say why that is the case at the top of the header.

> diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
> index 
> ac0f900ebc47efa81c92e1bb2010ea41677899c4..be454a6864225f3a972c3e81826b77ed4e8a57fe
>  100644
> --- a/arch/x86/boot/string.c
> +++ b/arch/x86/boot/string.c
> @@ -15,6 +15,7 @@
>  #include <linux/errno.h>
>  #include <linux/limits.h>
>  #include <asm/asm.h>
> +#include <asm/shared/string.h>
>  #include "ctype.h"
>  #include "string.h"
>  
> @@ -31,10 +32,7 @@
>  
>  int memcmp(const void *s1, const void *s2, size_t len)
>  {
> -     bool diff;
> -     asm("repe cmpsb"
> -         : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
> -     return diff;
> +     return __inline_memcmp(s1, s2, len);
>  }
>  
>  /*
> diff --git a/arch/x86/include/asm/shared/string.h 
> b/arch/x86/include/asm/shared/string.h
> new file mode 100644
> index 
> 0000000000000000000000000000000000000000..422952152f533ad75b98f3873297b39c4f5e2477
> --- /dev/null
> +++ b/arch/x86/include/asm/shared/string.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_SHARED_STRING_H
> +#define _ASM_X86_SHARED_STRING_H
> +
> +/* Note: this memcmp() returns 0/1, not -1/0/1 as regular memcmp(). */

Please document it properly - what does 0 mean as a return value and what 1.

Thx.

-- 
Regards/Gruss,
    Boris.

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