Re: [patch 06/10] selftests/rseq: Make registration flexible for legacy and optimized mode

[Dmitry Vyukov]

On Wed, 29 Apr 2026 at 01:34, Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
>
> rseq_register_current_thread() either uses the glibc registered RSEQ region
> or registers it's own region with the legacy size of 32 bytes.
>
> That worked so far, but becomes a problem when the kernel implements a
> distinction between legacy and performance optimized behavior based on the
> registration size as that does not allow to test both modes with the self
> test suite.
>
> Add two arguments to the function. One to enforce that the registration is
> not using libc provided mode and one to tell the registration to use the
> legacy size and not the kernel advertised size.
>
> Rename it and make the original one a inline wrapper which preserves the
> existing behavior.
>
> Fixes: 566d8015f7ee ("rseq: Avoid CPU/MM CID updates when no event pending")
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

> ---
>  tools/testing/selftests/rseq/rseq-abi.h |    7 ++++-
>  tools/testing/selftests/rseq/rseq.c     |   39 ++++++++++++++------------------
>  tools/testing/selftests/rseq/rseq.h     |    8 +++++-
>  3 files changed, 31 insertions(+), 23 deletions(-)
>
> --- a/tools/testing/selftests/rseq/rseq-abi.h
> +++ b/tools/testing/selftests/rseq/rseq-abi.h
> @@ -192,9 +192,14 @@ struct rseq_abi {
>         struct rseq_abi_slice_ctrl slice_ctrl;
>
>         /*
> +        * Place holder to push the size above 32 bytes.
> +        */
> +       __u8 __reserved;
> +
> +       /*
>          * Flexible array member at end of structure, after last feature field.
>          */
>         char end[];
> -} __attribute__((aligned(4 * sizeof(__u64))));
> +} __attribute__((aligned(256)));
>
>  #endif /* _RSEQ_ABI_H */
> --- a/tools/testing/selftests/rseq/rseq.c
> +++ b/tools/testing/selftests/rseq/rseq.c
> @@ -56,6 +56,7 @@ ptrdiff_t rseq_offset;
>   * unsuccessful.
>   */
>  unsigned int rseq_size = -1U;
> +static unsigned int rseq_alloc_size;
>
>  /* Flags used during rseq registration.  */
>  unsigned int rseq_flags;
> @@ -115,29 +116,17 @@ bool rseq_available(void)
>         }
>  }
>
> -/* The rseq areas need to be at least 32 bytes. */
> -static
> -unsigned int get_rseq_min_alloc_size(void)
> -{
> -       unsigned int alloc_size = rseq_size;
> -
> -       if (alloc_size < ORIG_RSEQ_ALLOC_SIZE)
> -               alloc_size = ORIG_RSEQ_ALLOC_SIZE;
> -       return alloc_size;
> -}
> -
>  /*
>   * Return the feature size supported by the kernel.
>   *
>   * Depending on the value returned by getauxval(AT_RSEQ_FEATURE_SIZE):
>   *
> - * 0:   Return ORIG_RSEQ_FEATURE_SIZE (20)
> + *   0: Return ORIG_RSEQ_FEATURE_SIZE (20)
>   * > 0: Return the value from getauxval(AT_RSEQ_FEATURE_SIZE).
>   *
>   * It should never return a value below ORIG_RSEQ_FEATURE_SIZE.
>   */
> -static
> -unsigned int get_rseq_kernel_feature_size(void)
> +static unsigned int get_rseq_kernel_feature_size(void)
>  {
>         unsigned long auxv_rseq_feature_size, auxv_rseq_align;
>
> @@ -152,15 +141,24 @@ unsigned int get_rseq_kernel_feature_siz
>                 return ORIG_RSEQ_FEATURE_SIZE;
>  }
>
> -int rseq_register_current_thread(void)
> +int __rseq_register_current_thread(bool nolibc, bool legacy)
>  {
> +       unsigned int size;
>         int rc;
>
>         if (!rseq_ownership) {
>                 /* Treat libc's ownership as a successful registration. */
> -               return 0;
> +               return nolibc ? -EBUSY : 0;
>         }
> -       rc = sys_rseq(&__rseq.abi, get_rseq_min_alloc_size(), 0, RSEQ_SIG);
> +
> +       /* The minimal allocation size is 32, which is the legacy allocation size */
> +       size = get_rseq_kernel_feature_size();
> +       if (legacy || size < ORIG_RSEQ_ALLOC_SIZE)
> +               rseq_alloc_size = ORIG_RSEQ_ALLOC_SIZE;
> +       else
> +               rseq_alloc_size = size;
> +
> +       rc = sys_rseq(&__rseq.abi, rseq_alloc_size, 0, RSEQ_SIG);
>         if (rc) {
>                 /*
>                  * After at least one thread has registered successfully
> @@ -179,9 +177,8 @@ int rseq_register_current_thread(void)
>          * The first thread to register sets the rseq_size to mimic the libc
>          * behavior.
>          */
> -       if (RSEQ_READ_ONCE(rseq_size) == 0) {
> -               RSEQ_WRITE_ONCE(rseq_size, get_rseq_kernel_feature_size());
> -       }
> +       if (RSEQ_READ_ONCE(rseq_size) == 0)
> +               RSEQ_WRITE_ONCE(rseq_size, size);
>
>         return 0;
>  }
> @@ -194,7 +191,7 @@ int rseq_unregister_current_thread(void)
>                 /* Treat libc's ownership as a successful unregistration. */
>                 return 0;
>         }
> -       rc = sys_rseq(&__rseq.abi, get_rseq_min_alloc_size(), RSEQ_ABI_FLAG_UNREGISTER, RSEQ_SIG);
> +       rc = sys_rseq(&__rseq.abi, rseq_alloc_size, RSEQ_ABI_FLAG_UNREGISTER, RSEQ_SIG);
>         if (rc)
>                 return -1;
>         return 0;
> --- a/tools/testing/selftests/rseq/rseq.h
> +++ b/tools/testing/selftests/rseq/rseq.h
> @@ -8,6 +8,7 @@
>  #ifndef RSEQ_H
>  #define RSEQ_H
>
> +#include <assert.h>
>  #include <stdint.h>
>  #include <stdbool.h>
>  #include <pthread.h>
> @@ -142,7 +143,12 @@ static inline struct rseq_abi *rseq_get_
>   * succeed. A restartable sequence executed from a non-registered
>   * thread will always fail.
>   */
> -int rseq_register_current_thread(void);
> +int __rseq_register_current_thread(bool nolibc, bool legacy);
> +
> +static inline int rseq_register_current_thread(void)
> +{
> +       return __rseq_register_current_thread(false, false);
> +}
>
>  /*
>   * Unregister rseq for current thread.
>