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

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Date:   Wed, 4 May 2022 11:57:29 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Stafford Horne <shorne@...il.com>
Cc:     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 11:42:27AM +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...

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);