Re: [PATCH] linux: mips: Fix getdents64 fallback on mips64-n32

[Andreas Schwab <schwab@linux-m68k.org>]

On Nov 17 2020, Adhemerval Zanella wrote:

> diff --git a/sysdeps/unix/sysv/linux/mips/mips64/getdents64.c b/sysdeps/unix/sysv/linux/mips/mips64/getdents64.c
> index d18a5297dc..368128244e 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips64/getdents64.c
> +++ b/sysdeps/unix/sysv/linux/mips/mips64/getdents64.c
> @@ -90,17 +90,14 @@ __getdents64 (int fd, void *buf, size_t nbytes)
>  
>    while ((char *) kdp < (char *) skdp + r)
>      {
> -      /* This macro is used to avoid aliasing violation.  */
> -#define KDP_MEMBER(src, member)			     			\
> -    (__typeof__((struct kernel_dirent){0}.member) *)			\
> -      memcpy (&((__typeof__((struct kernel_dirent){0}.member)){0}),	\
> -	      ((char *)(src) + offsetof (struct kernel_dirent, member)),\
> -	      sizeof ((struct kernel_dirent){0}.member))
> -
>        /* This is a conservative approximation, since some of size_diff might
>  	 fit into the existing padding for alignment.  */
> -      unsigned short int k_reclen = *KDP_MEMBER (kdp, d_reclen);
> -      unsigned short int new_reclen = ALIGN_UP (k_reclen + size_diff,
> +
> +      /* Obtain the d_ino, d_off, and d_reclen from kernel filled buffer.  */
> +      struct kernel_dirent kdirent;
> +      memcpy (&kdirent, kdp, sizeof (struct kernel_dirent));

How good is the compiler at eliding the memcpy?  What would change if
kdp would just be void *?

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."