Re: [PATCH 1/2] kcsan: Instrument memcpy/memset/memmove with newer Clang

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, Sep 07, 2022 at 07:39:02PM +0200, Marco Elver wrote:
> With Clang version 16+, -fsanitize=thread will turn
> memcpy/memset/memmove calls in instrumented functions into
> __tsan_memcpy/__tsan_memset/__tsan_memmove calls respectively.
> 
> Add these functions to the core KCSAN runtime, so that we (a) catch data
> races with mem* functions, and (b) won't run into linker errors with
> such newer compilers.
> 
> Cc: stable@vger.kernel.org # v5.10+

For (b) I think this is Ok, but for (a), what the atomic guarantee of
our mem* functions? Per-byte atomic or something more complicated (for
example, providing best effort atomic if a memory location in the range
is naturally-aligned to a machine word)?

If it's a per-byte atomicity, then maybe another KCSAN_ACCESS_* flags is
needed, otherwise memset(0x8, 0, 0x2) is considered as atomic if
ASSUME_PLAIN_WRITES_ATOMIC=y. Unless I'm missing something.

Anyway, this may be worth another patch and some discussion/doc, because
it just improve the accuracy of the tool. In other words, this patch and
the "stable" tag look good to me.

Regards,
Boqun

> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  kernel/kcsan/core.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
> index fe12dfe254ec..66ef48aa86e0 100644
> --- a/kernel/kcsan/core.c
> +++ b/kernel/kcsan/core.c
> @@ -18,6 +18,7 @@
>  #include <linux/percpu.h>
>  #include <linux/preempt.h>
>  #include <linux/sched.h>
> +#include <linux/string.h>
>  #include <linux/uaccess.h>
>  
>  #include "encoding.h"
> @@ -1308,3 +1309,29 @@ noinline void __tsan_atomic_signal_fence(int memorder)
>  	}
>  }
>  EXPORT_SYMBOL(__tsan_atomic_signal_fence);
> +
> +void *__tsan_memset(void *s, int c, size_t count);
> +noinline void *__tsan_memset(void *s, int c, size_t count)
> +{
> +	check_access(s, count, KCSAN_ACCESS_WRITE, _RET_IP_);
> +	return __memset(s, c, count);
> +}
> +EXPORT_SYMBOL(__tsan_memset);
> +
> +void *__tsan_memmove(void *dst, const void *src, size_t len);
> +noinline void *__tsan_memmove(void *dst, const void *src, size_t len)
> +{
> +	check_access(dst, len, KCSAN_ACCESS_WRITE, _RET_IP_);
> +	check_access(src, len, 0, _RET_IP_);
> +	return __memmove(dst, src, len);
> +}
> +EXPORT_SYMBOL(__tsan_memmove);
> +
> +void *__tsan_memcpy(void *dst, const void *src, size_t len);
> +noinline void *__tsan_memcpy(void *dst, const void *src, size_t len)
> +{
> +	check_access(dst, len, KCSAN_ACCESS_WRITE, _RET_IP_);
> +	check_access(src, len, 0, _RET_IP_);
> +	return __memcpy(dst, src, len);
> +}
> +EXPORT_SYMBOL(__tsan_memcpy);
> -- 
> 2.37.2.789.g6183377224-goog
>