Re: [PATCH] vfprintf: Allocate the user argument buffer on the heap

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 19/06/2017 13:20, Florian Weimer wrote:
> 2017-06-19  Florian Weimer  <fweimer@redhat.com>
> 
> 	* stdio-common/vfprintf.c (allocate_user_args_buffer): New
> 	function.
> 	(printf_positional): Call it.
> 
> diff --git a/stdio-common/vfprintf.c b/stdio-common/vfprintf.c
> index e0c6edf..b15c5d0 100644
> --- a/stdio-common/vfprintf.c
> +++ b/stdio-common/vfprintf.c
> @@ -1618,6 +1618,26 @@ do_positional:
>    return done;
>  }
>  
> +
> +
> +/* Called from printf_positional to determine the size of the user
> +   argument area and allocate it, after discovering that one is
> +   needed.  This function returns NULL on allocation failure.  */
> +static void *
> +allocate_user_args_buffer (size_t nargs, const int *args_size,
> +			   const int *args_type)
> +{
> +  assert (__printf_va_arg_table != NULL);
> +  size_t size = 0;
> +  for (size_t i = 0; i < nargs; ++i)
> +    if ((args_type[i] & PA_FLAG_MASK) == 0
> +	&& args_type[i] >= PA_LAST
> +	&& __printf_va_arg_table[args_type[i] - PA_LAST] != NULL)
> +      size += roundup (args_size[i], _Alignof (max_align_t));
> +  assert (size > 0);
> +  return malloc (size);
> +}
> +
>  static int
>  printf_positional (_IO_FILE *s, const CHAR_T *format, int readonly_format,
>  		   va_list ap, va_list *ap_savep, int done, int nspecs_done,
> @@ -1636,6 +1656,12 @@ printf_positional (_IO_FILE *s, const CHAR_T *format, int readonly_format,
>    struct scratch_buffer argsbuf;
>    scratch_buffer_init (&argsbuf);
>  
> +  /* Allocation area for user argument data.  Lazily allocated by
> +     allocate_user_args_buffer.  */
> +  void *user_args_buffer = NULL;
> +  /* Upcoming allocation within user_args_buffer.  */
> +  void *user_args_buffer_next = NULL;
> +
>    /* Array with information about the needed arguments.  This has to
>       be dynamically extensible.  */
>    size_t nspecs = 0;
> @@ -1796,7 +1822,34 @@ printf_positional (_IO_FILE *s, const CHAR_T *format, int readonly_format,
>  	else if (__glibc_unlikely (__printf_va_arg_table != NULL)
>  		 && __printf_va_arg_table[args_type[cnt] - PA_LAST] != NULL)
>  	  {
> -	    args_value[cnt].pa_user = alloca (args_size[cnt]);
> +	    /* Allocate from user_args_buffer.  */
> +	    size_t allocation_size = args_size[cnt];
> +	    void *allocation;
> +	    if (allocation_size == 0)
> +	      /* Nothing to allocate.  */
> +	      allocation = NULL;
> +	    else
> +	      {
> +		if (user_args_buffer == NULL)
> +		  {
> +		    /* First user argument.  Allocate the complete
> +		       buffer.  */
> +		    user_args_buffer = allocate_user_args_buffer
> +		      (nargs, args_size, args_type);
> +		    if (user_args_buffer == NULL)
> +		      {
> +			done = -1;
> +			goto all_done;
> +		      }
> +		    user_args_buffer_next = user_args_buffer;
> +		  }
> +		allocation = user_args_buffer_next;
> +		user_args_buffer_next
> +		  += roundup (allocation_size, _Alignof (max_align_t));
> +	      }
> +	    /* Install the allocated pointer and use the callback to
> +	       extract the argument.  */
> +	    args_value[cnt].pa_user = allocation;
>  	    (*__printf_va_arg_table[args_type[cnt] - PA_LAST])
>  	      (args_value[cnt].pa_user, ap_savep);

I am trying to convince myself it is worth to add all this complexity
to allocate for user defined types, but I am failing to understand why
can we just simplify it to a malloc using 'args_size[cnt]' (as the alloca
is already using it).  And why do we need to keep track of the buffer
allocation after the callback track?  Could we just free it after the
call?

>  	  }
> @@ -1953,6 +2006,7 @@ printf_positional (_IO_FILE *s, const CHAR_T *format, int readonly_format,
>  		 - specs[nspecs_done].end_of_fmt);
>      }
>   all_done:
> +  free (user_args_buffer);
>    scratch_buffer_free (&argsbuf);
>    scratch_buffer_free (&specsbuf);
>    return done;
>