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From: Arvind Sankar <nivedita@alum.mit.edu>
To: Ingo Molnar <mingo@kernel.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>,
	Hans de Goede <hdegoede@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	linux-crypto@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	Stephan Mueller <smueller@chronox.de>,
	linux-s390@vger.kernel.org
Subject: Re: [PATCH v2 5.4 regression fix] x86/boot: Provide memzero_explicit
Date: Mon, 7 Oct 2019 14:42:37 -0400
Message-ID: <20191007184237.GB13589@rani.riverdale.lan> (raw)
In-Reply-To: <20191007154007.GA96929@gmail.com>


[-- Attachment #1: Type: text/plain, Size: 1055 bytes --]

On Mon, Oct 07, 2019 at 05:40:07PM +0200, Ingo Molnar wrote:
> 
> * Arvind Sankar <nivedita@alum.mit.edu> wrote:
> 
> > With the barrier in there, is there any reason to *not* inline the
> > function? barrier_data() is an asm statement that tells the compiler
> > that the asm uses the memory that was set to zero, thus preventing it
> > from removing the memset even if nothing else uses that memory later. A
> > more detailed comment is there in compiler-gcc.h. I can't see why it
> > wouldn't work even if it were inlined.
> > 
> > If the function can indeed be inlined, we could just make the common
> > implementation a macro and avoid duplicating it? As mentioned in another
> > mail, we otherwise will likely need another duplicate implementation for
> > arch/s390/purgatory as well.
> 
> I suspect macro would be justified in this case. Mind sending a v3 patch 
> to demonstrate how it would all look like?
> 
> I'll zap v2 if the macro solution looks better.
> 
> Thanks,
> 
> 	Ingo

Patch attached to turn memzero_explicit into inline function.

[-- Attachment #2: 0001-lib-string-make-memzero_explicit-inline-instead-of-e.patch --]
[-- Type: text/x-diff, Size: 2880 bytes --]

From 25834b8040eff72478489be0bd8a2ff549af7f94 Mon Sep 17 00:00:00 2001
From: Arvind Sankar <nivedita@alum.mit.edu>
Date: Mon, 7 Oct 2019 14:34:24 -0400
Subject: [PATCH] lib/string: make memzero_explicit inline instead of external

With the use of the barrier implied by barrier_data(), there is no need
for memzero_explicit to be extern. Making it inline saves the overhead
of a function call, and allows the code to be reused in arch/*/purgatory
without having to duplicate the implementation.

Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get input, memzero_explicit")
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
 include/linux/string.h | 21 ++++++++++++++++++++-
 lib/string.c           | 21 ---------------------
 2 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/include/linux/string.h b/include/linux/string.h
index b2f9df7f0761..b6ccdc2c7f02 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -227,7 +227,26 @@ static inline bool strstarts(const char *str, const char *prefix)
 }
 
 size_t memweight(const void *ptr, size_t bytes);
-void memzero_explicit(void *s, size_t count);
+
+/**
+ * memzero_explicit - Fill a region of memory (e.g. sensitive
+ *		      keying data) with 0s.
+ * @s: Pointer to the start of the area.
+ * @count: The size of the area.
+ *
+ * Note: usually using memset() is just fine (!), but in cases
+ * where clearing out _local_ data at the end of a scope is
+ * necessary, memzero_explicit() should be used instead in
+ * order to prevent the compiler from optimising away zeroing.
+ *
+ * memzero_explicit() doesn't need an arch-specific version as
+ * it just invokes the one of memset() implicitly.
+ */
+static inline void memzero_explicit(void *s, size_t count)
+{
+	memset(s, 0, count);
+	barrier_data(s);
+}
 
 /**
  * kbasename - return the last part of a pathname.
diff --git a/lib/string.c b/lib/string.c
index cd7a10c19210..08ec58cc673b 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -748,27 +748,6 @@ void *memset(void *s, int c, size_t count)
 EXPORT_SYMBOL(memset);
 #endif
 
-/**
- * memzero_explicit - Fill a region of memory (e.g. sensitive
- *		      keying data) with 0s.
- * @s: Pointer to the start of the area.
- * @count: The size of the area.
- *
- * Note: usually using memset() is just fine (!), but in cases
- * where clearing out _local_ data at the end of a scope is
- * necessary, memzero_explicit() should be used instead in
- * order to prevent the compiler from optimising away zeroing.
- *
- * memzero_explicit() doesn't need an arch-specific version as
- * it just invokes the one of memset() implicitly.
- */
-void memzero_explicit(void *s, size_t count)
-{
-	memset(s, 0, count);
-	barrier_data(s);
-}
-EXPORT_SYMBOL(memzero_explicit);
-
 #ifndef __HAVE_ARCH_MEMSET16
 /**
  * memset16() - Fill a memory area with a uint16_t
-- 
2.21.0


  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-07 13:47 Hans de Goede
2019-10-07 14:00 ` Ingo Molnar
2019-10-07 14:11   ` Hans de Goede
2019-10-07 14:22     ` Ingo Molnar
2019-10-07 14:29       ` Hans de Goede
2019-10-07 14:46         ` Ingo Molnar
2019-10-07 15:20           ` Arvind Sankar
2019-10-07 15:40             ` Ingo Molnar
2019-10-07 18:42               ` Arvind Sankar [this message]
2019-10-07 19:36                 ` Hans de Goede
2019-10-07 22:00                   ` [PATCH] lib/string: make memzero_explicit inline instead of external Arvind Sankar
2019-10-08 11:33                     ` [tip: x86/urgent] lib/string: Make memzero_explicit() " tip-bot2 for Arvind Sankar
2019-10-08 11:33                     ` tip-bot2 for Arvind Sankar
2019-10-10  2:52                     ` [PATCH] lib/string: make memzero_explicit " Dave Young
2019-10-10  6:56                       ` Dave Young
2019-10-07 14:49 ` [tip: x86/urgent] x86/boot: Provide memzero_explicit() tip-bot2 for Hans de Goede
2019-10-07 14:49 ` tip-bot2 for Hans de Goede

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