Re: [PATCH v1 1/2] vdso: Add VDSO_UNALIGNED_PACKED config option for ARMv5

[David Laight <david.laight.linux@gmail.com>]

On Mon, 19 Oct 2026 13:12:56 -0700
Ian Rogers <irogers@google.com> wrote:

> Commit a339671db64b ("vdso: Switch get/put_unaligned() from packed struct
> to memcpy()") switched __get_unaligned_t() and __put_unaligned_t() from
> using a __packed struct to __builtin_memcpy() to avoid strict aliasing
> undefined behavior.

Looking at it the vdso only uses it in the getrandom code for copy+zero.
Even then it only actually uses when misaligned transfers are supported
and the buffer itself is u8[] - so no strict aliasing issues.
So that code could very easily do something else and then the horrid defines
in vdso/unaligned.h could probably go away.

Then the kernel could go back to using packed structs (or something else
since that itself is possibly dubious).

David

> 
> On ARMv5 with GCC, __builtin_memcpy() may not be inlined and instead emit
> a much slower out-of-line memcpy() call. Add CONFIG_VDSO_UNALIGNED_PACKED,
> enabled on ARMv5 (CPU_32v5), to use the __packed struct implementation on
> ARMv5 while keeping __builtin_memcpy() elsewhere.
> 
> Fixes: a339671db64b ("vdso: Switch get/put_unaligned() from packed struct to memcpy()")
> Reported-by: Stefan Kerkmann <s.kerkmann@pengutronix.de>
> Closes: https://lore.kernel.org/lkml/47f8c30b-e326-4ff0-a2bc-5683209a9e2d@pengutronix.de/
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  include/vdso/unaligned.h | 16 ++++++++++++++++
>  lib/vdso/Kconfig         |  8 ++++++++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
> index 9076483c9fbb..726fd15efc82 100644
> --- a/include/vdso/unaligned.h
> +++ b/include/vdso/unaligned.h
> @@ -4,6 +4,20 @@
>  
>  #include <linux/compiler_types.h>
>  
> +#ifdef CONFIG_VDSO_UNALIGNED_PACKED
> +
> +#define __get_unaligned_t(type, ptr) ({							\
> +	const struct { type x; } __packed * __get_pptr = (typeof(__get_pptr))(ptr);	\
> +	__get_pptr->x;									\
> +})
> +
> +#define __put_unaligned_t(type, val, ptr) do {						\
> +	struct { type x; } __packed * __put_pptr = (typeof(__put_pptr))(ptr);		\
> +	__put_pptr->x = (val);								\
> +} while (0)
> +
> +#else /* !CONFIG_VDSO_UNALIGNED_PACKED */
> +
>  /**
>   * __get_unaligned_t - read an unaligned value from memory.
>   * @type:	the type to load from the pointer.
> @@ -41,4 +55,6 @@
>  			 sizeof(__put_unaligned_val));			\
>  } while (0)
>  
> +#endif /* CONFIG_VDSO_UNALIGNED_PACKED */
> +
>  #endif /* __VDSO_UNALIGNED_H */
> diff --git a/lib/vdso/Kconfig b/lib/vdso/Kconfig
> index 597f5f0f9681..709eb193999f 100644
> --- a/lib/vdso/Kconfig
> +++ b/lib/vdso/Kconfig
> @@ -23,3 +23,11 @@ config VDSO_GETRANDOM
>  	select VDSO_DATASTORE
>  	help
>  	  Selected by architectures that support vDSO getrandom().
> +
> +config VDSO_UNALIGNED_PACKED
> +	bool
> +	default CPU_32v5
> +	help
> +	  Use packed structs rather than memcpy() in include/vdso/unaligned.h
> +	  for unaligned memory accesses. Enabled on ARMv5 where GCC may emit
> +	  out-of-line memcpy() calls instead of inlining byte accesses.