New read etc. checking macros

[Jakub Jelinek]

On Fri, Feb 25, 2005 at 04:46:42PM -0800, Roland McGrath wrote:
> > Yeah, I think we can only protect read etc. with _FORTIFY_SOURCE >= 2
> > which has some limitations against POSIX and not allowing bigger len
> > than the buffer that is provided is reasonable limitation.
> 
> Does the gcc magic handle cases like:
> 
> struct {
>   uint32_t magic;
>   char foo[3];
>   char bar[27];
> } record;
> 
> if (read(fd, &record.magic, sizeof record.magic) == sizeof record.magic &&
>     record.magic == 0x1234567)
>   n = read(fd, &record.foo, sizeof record - (&record.foo - (char*)&record));
> 
> i.e., so __bos yields 30 rather than 3?

__bos (&record.foo) is 3 for -D_FORTIFY_SOURCE=2, and 30 for
-D_FORTIFY_SOURCE=1.  So read etc. probably wants to use __bos0
(&record.foo) which is always 30.

	Jakub