Re: [PATCH v2] nptl: Unconditionally use a 32-byte rseq area

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 7/20/23 07:46, Florian Weimer wrote:
> If the kernel headers provide a larger struct rseq, we used that
> size as the argument to the rseq system call.  As a result,
> rseq registration would fail on older kernels which only accept
> size 32.

Yes, this change appears to be needed considering the way the upstream 
kernel uapi has evolved.

Thanks,

Mathieu

> 
> Tested on x86_64-linux-gnu.  Built with build-many-glibcs.py.
> This needs to be backported all the way to glibc 2.35.
> 
> ---
> v2: Drop further struct rseq reference on ia64.  Remove #include <sys/rseq.h>.
>   nptl/descr.h | 18 ++++++++++++++----
>   1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/nptl/descr.h b/nptl/descr.h
> index d06abd6ad9..0171576c23 100644
> --- a/nptl/descr.h
> +++ b/nptl/descr.h
> @@ -34,7 +34,6 @@
>   #include <bits/types/res_state.h>
>   #include <kernel-features.h>
>   #include <tls-internal-struct.h>
> -#include <sys/rseq.h>
>   #include <internal-sigset.h>
>   
>   #ifndef TCB_ALIGNMENT
> @@ -405,14 +404,25 @@ struct pthread
>     /* Used on strsignal.  */
>     struct tls_internal_t tls_state;
>   
> -  /* rseq area registered with the kernel.  */
> -  struct rseq rseq_area;
> +  /* rseq area registered with the kernel.  Use a custom definition
> +     here to isolate from kernel struct rseq changes.  The
> +     implementation of sched_getcpu needs acccess to the cpu_id field;
> +     the other fields are unused and not included here.  */
> +  union
> +  {
> +    struct
> +    {
> +      uint32_t cpu_id_start;
> +      uint32_t cpu_id;
> +    };
> +    char pad[32];		/* Original rseq area size.  */
> +  } rseq_area __attribute__ ((aligned (32)));
>   
>     /* Amount of end padding, if any, in this structure.
>        This definition relies on rseq_area being last.  */
>   #define PTHREAD_STRUCT_END_PADDING \
>     (sizeof (struct pthread) - offsetof (struct pthread, rseq_area) \
> -   + sizeof (struct rseq))
> +   + sizeof ((struct pthread) {}.rseq_area))
>   } __attribute ((aligned (TCB_ALIGNMENT)));
>   
>   static inline bool
> 
> base-commit: 3edc4ff2ceff4a59587ebecb94148d3bcfa1df62
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com