[PATCH v3] libio: do not unbuffer legacy standard files in compatibility mode [BZ #24228]

[Florian Weimer]

* Dmitry V. Levin:

> diff --git a/libio/genops.c b/libio/genops.c
> index 2a0d9b81df..aa92d61b6b 100644
> --- a/libio/genops.c
> +++ b/libio/genops.c
> @@ -789,6 +789,10 @@ _IO_unbuffer_all (void)
>  
>    for (fp = (FILE *) _IO_list_all; fp; fp = fp->_chain)
>      {
> +#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
> +      if (__glibc_unlikely (&_IO_stdin_used == NULL) && _IO_legacy_file (fp))
> +	continue;
> +#endif
>        if (! (fp->_flags & _IO_UNBUFFERED)
>  	  /* Iff stream is un-orientated, it wasn't used. */
>  	  && fp->_mode != 0)

I believe a better fix would be this, in case an old-style file showed
up for a different reason:

#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
          bool potentially_wide_stream = _IO_vtable_offset (fp) != 0;
#else
          bool potentially_wide_stream = true;
#endif
	  if (potentially_wide_stream && fp->_mode > 0)
	    _IO_wsetb (fp, NULL, NULL, 0);

This is _IO_new_fclose handles this situation.

I fear the test is unreliable because it depends on what fp->_mode
evaluates to (which is not actually present in the struct with old
files).  But the test is definitely better than nothing.

Thanks,
Florian