RE: [PATCH v19 1/2] eal: add __rte_may_alias and __rte_aligned to unaligned typedefs

["Morten Brørup" <mb@smartsharesystems.com>]

> From: David Marchand [mailto:david.marchand@redhat.com]
> Sent: Monday, 16 February 2026 15.29
> 
> Hello Scott, Andre, Tyler,
> 
> On Mon, 2 Feb 2026 at 05:48, <scott.k.mitch1@gmail.com> wrote:
> >
> > From: Scott Mitchell <scott.k.mitch1@gmail.com>
> >
> > Add __rte_may_alias attribute to unaligned_uint{16,32,64}_t typedefs
> > to prevent GCC strict-aliasing optimization bugs. GCC has a bug where
> > it incorrectly elides struct initialization when strict aliasing is
> > enabled, causing reads from uninitialized memory.
> >
> > Add __rte_aligned(1) attribute to unaligned_uint{16,32,64}_t typedefs
> > which allows for safe access at any alignment. Without this,
> accessing
> > a uint16_t at an odd address is undefined behavior. Without this
> > UBSan detects `UndefinedBehaviorSanitizer: undefined-behavior`.
> >
> > Fixes: 7621d6a8d0bd ("eal: add and use unaligned integer types")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Scott Mitchell <scott.k.mitch1@gmail.com>
> 
> [snip]
> 
> > diff --git a/lib/eal/include/rte_common.h
> b/lib/eal/include/rte_common.h
> > index 573bf4f2ce..7b36966019 100644
> > --- a/lib/eal/include/rte_common.h
> > +++ b/lib/eal/include/rte_common.h
> > @@ -121,16 +121,42 @@ extern "C" {
> >  #define __rte_aligned(a) __attribute__((__aligned__(a)))
> >  #endif
> >
> > -#ifdef RTE_ARCH_STRICT_ALIGN
> > -typedef uint64_t unaligned_uint64_t __rte_aligned(1);
> > -typedef uint32_t unaligned_uint32_t __rte_aligned(1);
> > -typedef uint16_t unaligned_uint16_t __rte_aligned(1);
> > +/**
> > + * Macro to mark a type that is not subject to type-based aliasing
> rules
> > + */
> > +#ifdef RTE_TOOLCHAIN_MSVC
> > +#define __rte_may_alias
> >  #else
> > -typedef uint64_t unaligned_uint64_t;
> > -typedef uint32_t unaligned_uint32_t;
> > -typedef uint16_t unaligned_uint16_t;
> > +#define __rte_may_alias __attribute__((__may_alias__))
> >  #endif
> >
> > +/* Unaligned types implementation notes:
> > + * __rte_aligned(1) - Reduces alignment requirement to 1 byte,
> allowing
> > + *                    these types to safely access memory at any
> address.
> > + *                    Without this, accessing a uint16_t at an odd
> address
> > + *                    is undefined behavior (even on x86 where
> hardware
> > + *                    handles it).
> > + *
> > + * __rte_may_alias  - Prevents strict-aliasing optimization bugs
> where
> > + *                    compilers may incorrectly elide memory
> operations
> > + *                    when casting between pointer types.
> > + */
> > +
> > +/**
> > + * Type for safe unaligned u64 access.
> > + */
> > +typedef __rte_may_alias __rte_aligned(1) uint64_t
> unaligned_uint64_t;
> > +
> > +/**
> > + * Type for safe unaligned u32 access.
> > + */
> > +typedef __rte_may_alias __rte_aligned(1) uint32_t
> unaligned_uint32_t;
> > +
> > +/**
> > + * Type for safe unaligned u16 access.
> > + */
> > +typedef __rte_may_alias __rte_aligned(1) uint16_t
> unaligned_uint16_t;
> > +
> >  /**
> >   * @deprecated
> >   * @see __rte_packed_begin
> > @@ -159,15 +185,6 @@ typedef uint16_t unaligned_uint16_t;
> >  #define __rte_packed_end __attribute__((__packed__))
> >  #endif
> >
> > -/**
> > - * Macro to mark a type that is not subject to type-based aliasing
> rules
> > - */
> > -#ifdef RTE_TOOLCHAIN_MSVC
> > -#define __rte_may_alias
> > -#else
> > -#define __rte_may_alias __attribute__((__may_alias__))
> > -#endif
> > -
> >  /******* Macro to mark functions and fields scheduled for removal
> *****/
> >  #ifdef RTE_TOOLCHAIN_MSVC
> >  #define __rte_deprecated
> 
> This change raises a warning in checkpatch.
> https://mails.dpdk.org/archives/test-report/2026-February/955237.html
> 
> IIRC, we added this check for MSVC support, making sure no
> __rte_aligned() would be added in unsupported locations.

It looks like MSVC can use alignment for type definitions too:
https://learn.microsoft.com/en-us/cpp/cpp/align-cpp?view=msvc-170#vclrf_declspecaligntypedef

It is applied on a structure, though, so may not be viable for scalar types. IDK.

But it looks like MSVC can only increase alignment:
https://learn.microsoft.com/en-us/cpp/cpp/align-cpp?view=msvc-170#:~:text=__declspec(align(%23))%20can%20only%20increase%20alignment%20restrictions.

So packing may be needed too.

> 
> @Microsoft guys, do you have a suggestion?
> 
> 
> --
> David Marchand