Re: [PATCH v2 5.4 regression fix] x86/boot: Provide memzero_explicit

[Arvind Sankar <nivedita@alum.mit.edu>]

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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.

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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