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Date: Wed, 9 Sep 2015 12:42:28 -0400
From: Rich Felker <dalias@...c.org>
To: Zack Weinberg <zackw@...ix.com>
Cc: gcc@....gnu.org, llvmdev@...uiuc.edu,
GNU C Library <libc-alpha@...rceware.org>, musl@...ts.openwall.com
Subject: Re: Compiler support for erasure of sensitive data
On Wed, Sep 09, 2015 at 12:36:01PM -0400, Zack Weinberg wrote:
> The first, simpler problem is strictly optimization. explicit_bzero
> can be optimized to memset followed by a vacuous use of the memory
> region (generating no machine instructions, but preventing the stores
> from being deleted as dead); this is valuable because the sensitive
> data is often small and fixed-size, so the memset can in turn be
> replaced by inline code. (This also facilitates implementation of
> -D_FORTIFY_SOURCE checks for explicit_bzero.) Again looking at
> libressl, 92 of those 152 uses are improved by a crude version of this
> optimization:
>
> void explicit_bzero(void *, size_t);
> extern inline __attribute__((gnu_inline, always_inline))
> void explicit_bzero_constn(void *ptr, size_t len)
> {
> typedef struct {char x[len];} memblk;
> memset(ptr, 0, len);
> asm("" : : "m" (*(memblk __attribute__((may_alias)) *)ptr));
> }
> #define explicit_bzero(s, n) \
> (__extension__(__builtin_constant_p(n) && (n) > 0 \
> ? explicit_bzero_constn(s, n) \
> : explicit_bzero(s, n)))
>
> I call this "crude" because it only works in GCC, when compiling C,
> and when the length parameter is compile-time constant. GCC issues no
> error for this code when 'len' is not compile-time constant, but it is
> not documented to work reliably. When compiling C++, GCC does not
> accept a structure containing an array whose size is not *lexically*
> constant; even if the body of explicit_bzero_constn is moved into the
> macro so that the whole thing is guarded by __builtin_constant_p,
> using explicit_bzero with a non-constant size will cause a compile
> error. The same is true for Clang whether compiling C or C++.
>
> This problem could be solved with a very simple feature addition:
>
> extern inline __attribute__((gnu_inline, always_inline))
> void explicit_bzero(void *ptr, size_t len)
> {
> memset(ptr, 0, len);
> __builtin_use_memory(ptr, len);
> }
You're making this harder than it needs to be. The "m" constraint is
the wrong thing to use here. Simply use:
__asm__(""::"r"(ptr):"memory");
The memory constraint implies that the asm can read or write any
memory that's reachable by it. The lack of output constraints implies
__volatile__ which is also needed.
Rich
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