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SubjectRe: [RFC PATCH 01/13] list: introduce speculative safe list_for_each_entry()
On Thu, Feb 17, 2022 at 07:48:17PM +0100, Jakob Koschel wrote:
> list_for_each_entry() selects either the correct value (pos) or a safe
> value for the additional mispredicted iteration (NULL) for the list
> iterator.
> list_for_each_entry() calls select_nospec(), which performs
> a branch-less select.
>
> On x86, this select is performed via a cmov. Otherwise, it's performed
> via various shift/mask/etc. operations.
>
> Kasper Acknowledgements: Jakob Koschel, Brian Johannesmeyer, Kaveh
> Razavi, Herbert Bos, Cristiano Giuffrida from the VUSec group at VU
> Amsterdam.
>
> Co-developed-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
> Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
> ---
> arch/x86/include/asm/barrier.h | 12 ++++++++++++
> include/linux/list.h | 3 ++-
> include/linux/nospec.h | 16 ++++++++++++++++
> 3 files changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
> index 35389b2af88e..722797ad74e2 100644
> --- a/arch/x86/include/asm/barrier.h
> +++ b/arch/x86/include/asm/barrier.h
> @@ -48,6 +48,18 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
> /* Override the default implementation from linux/nospec.h. */
> #define array_index_mask_nospec array_index_mask_nospec
>
> +/* Override the default implementation from linux/nospec.h. */
> +#define select_nospec(cond, exptrue, expfalse) \
> +({ \
> + typeof(exptrue) _out = (exptrue); \
> + \
> + asm volatile("test %1, %1\n\t" \
> + "cmove %2, %0" \
> + : "+r" (_out) \
> + : "r" (cond), "r" (expfalse)); \
> + _out; \
> +})
> +
> /* Prevent speculative execution past this barrier. */
> #define barrier_nospec() alternative("", "lfence", X86_FEATURE_LFENCE_RDTSC)
>
> diff --git a/include/linux/list.h b/include/linux/list.h
> index dd6c2041d09c..1a1b39fdd122 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -636,7 +636,8 @@ static inline void list_splice_tail_init(struct list_head *list,
> */
> #define list_for_each_entry(pos, head, member) \
> for (pos = list_first_entry(head, typeof(*pos), member); \
> - !list_entry_is_head(pos, head, member); \
> + ({ bool _cond = !list_entry_is_head(pos, head, member); \
> + pos = select_nospec(_cond, pos, NULL); _cond; }); \
> pos = list_next_entry(pos, member))
>

You are not "introducing" a new macro for this, you are modifying the
existing one such that all users of it now have the select_nospec() call
in it.

Is that intentional? This is going to hit a _lot_ of existing entries
that probably do not need it at all.

Why not just create list_for_each_entry_nospec()?

thanks,

greg k-h

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