linux-kernel - Re: [PATCH v2] hex2bin: make the function hex_to

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Message-ID: <YnJQXr3igEMTqY3+@smile.fi.intel.com>
Date:   Wed, 4 May 2022 13:07:26 +0300
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Stafford Horne <shorne@...il.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andy@...nel.org>,
        device-mapper development <dm-devel@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Mike Snitzer <msnitzer@...hat.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Milan Broz <gmazyland@...il.com>
Subject: Re: [PATCH v2] hex2bin: make the function hex_to_bin constant-time

On Wed, May 04, 2022 at 11:57:29AM +0200, Jason A. Donenfeld wrote:
> On Wed, May 04, 2022 at 11:42:27AM +0200, Jason A. Donenfeld wrote:
> > So more likely is that this patch just helps unmask a real issue
> > elsewhere -- linker, compiler, or register restoration after preemption.
> > I don't think there's anything to do with regards to the patch of this
> > thread, as it's clearly fine. 
> 
> The problem even goes away if I just add a nop...

Alignment? Compiler bug? HW issue?

> diff --git a/lib/hexdump.c b/lib/hexdump.c
> index 06833d404398..ace74f9b3d5a 100644
> --- a/lib/hexdump.c
> +++ b/lib/hexdump.c
> @@ -46,6 +46,7 @@ EXPORT_SYMBOL(hex_asc_upper);
>  int hex_to_bin(unsigned char ch)
>  {
>  	unsigned char cu = ch & 0xdf;
> +	__asm__("l.nop 0");
>  	return -1 +
>  		((ch - '0' +  1) & (unsigned)((ch - '9' - 1) & ('0' - 1 - ch)) >> 8) +
>  		((cu - 'A' + 11) & (unsigned)((cu - 'F' - 1) & ('A' - 1 - cu)) >> 8);
> 

-- 
With Best Regards,
Andy Shevchenko