Re: [Patch, Newlib-nano]Improve printf to support non nul-terminated string

[Corinna Vinschen <vinschen at redhat dot com>]

Hi Bin,

since that's your code, can you please take a look into this patch?


Thanks,
Corinna


On Nov  1 11:16, Terry Guo wrote:
> Hi there,
> 
> When use printf to print string that isn't nul-terminated like below:
> 
> char cp[16];
> printf("%.*s", sizeof(cp), moo);
> 
> current printf implementation in newlib-nano has a subtle bug that is the
> call to strlen(cp). Suppose all 16 elements in cp are non-nul characters and
> the storage next to cp is filled with value 0xFF (which is quite common for
> flash of an embedded device), the strlen function will travel the memory
> until trigger an error.
> 
> The Newlib implementation already realized this issue. The attached patch
> intends to reuse Newlib implementation here for Newlib-nano to avoid such
> issue.
> 
> BR,
> Terry
> 
> 2014-11-01  Terry Guo  <terry.guo@arm.com>
> 
>      * libc/stdio/nano-vfprintf_i.c (_printf_i): Use Newlib approach to
>      handle string that might be not nul-terminated.
>      * testsuite/newlib.stdio/nulprintf.c: New test.

> diff --git a/newlib/libc/stdio/nano-vfprintf_i.c b/newlib/libc/stdio/nano-vfprintf_i.c
> index b1b0d1d..2bc459d 100644
> --- a/newlib/libc/stdio/nano-vfprintf_i.c
> +++ b/newlib/libc/stdio/nano-vfprintf_i.c
> @@ -211,8 +211,14 @@ number:
>      case 's':
>        cp = GET_ARG (N, *ap, char_ptr_t);
>        /* Precision gives the maximum number of chars to be written from a
> -	 string, and take prec == -1 into consideration.  */
> -      if ((u_int)(pdata->size = strlen (cp)) > (u_int)(pdata->prec))
> +	 string, and take prec == -1 into consideration.
> +	 Use normal Newlib approach here to support case where cp is not
> +	 nul-terminated.  */
> +      char *p = memchr (cp, 0, pdata->prec);
> +
> +      if (p != NULL)
> +	pdata->size = p - cp;
> +      else
>  	pdata->size = pdata->prec;
>        /* Below code is kept for reading.  The check is redundant because
>  	 pdata->prec will be set to pdata->size if it is -1 previously.  */
> diff --git a/newlib/testsuite/newlib.stdio/nulprintf.c b/newlib/testsuite/newlib.stdio/nulprintf.c
> new file mode 100644
> index 0000000..5e4131b
> --- /dev/null
> +++ b/newlib/testsuite/newlib.stdio/nulprintf.c
> @@ -0,0 +1,17 @@
> +/*
> + * Copyright (C) 2014 by ARM Ltd. All rights reserved.
> + *
> + * Permission to use, copy, modify, and distribute this software
> + * is freely granted, provided that this notice is preserved.
> + */
> +
> +#include <stdio.h>
> +#include "check.h"
> +
> +const char m[8] = {'M','M','M','M','M','M','M','M'};
> +
> +int main()
> +{
> +  printf ("%.*s\n", 8, m);
> +  exit (0);
> +}


-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat

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