Re: [Qemu-block] [Qemu-devel] Fwd: [BUG] Failed to compile using gcc7.1

[Philippe Mathieu-Daudé]

Hi Tsung-en,

On 06/11/2017 04:08 PM, Tsung-en Hsiao wrote:
> Hi all,
> I encountered the same problem on gcc 7.1.1 and found Qu's mail in
> this list from google search.
>
> Temporarily fix it by specifying the string length in snprintf
> directive. Hope this is helpful to other people encountered the same
> problem.

Thank your for sharing this.

> @@ -1,9 +1,7 @@
> ---
> --- a/block/blkdebug.c
> -                 "blkdebug:%s:%s", s->config_file ?: "",
> --- a/block/blkverify.c
> -                 "blkverify:%s:%s",
> --- a/hw/usb/bus.c
> -        snprintf(downstream->path, sizeof(downstream->path), "%s.%d",
> -        snprintf(downstream->path, sizeof(downstream->path), "%d", portnr);
> --
> +++ b/block/blkdebug.c
> +                 "blkdebug:%.2037s:%.2037s", s->config_file ?: "",

It is a rather funny way to silent this warning :) Truncating the 
filename until it fits.

However I don't think it is the correct way since there is indeed an 
overflow of bs->exact_filename.

Apparently exact_filename from "block/block_int.h" is defined to hold a 
pathname:
    char exact_filename[PATH_MAX];

but is used for more than that (for example in blkdebug.c it might use 
until 10+2*PATH_MAX chars).

I suppose it started as a buffer to hold a pathname then more block 
drivers were added and this buffer ended used differently.

If it is a multi-purpose buffer one safer option might be to declare it 
as a GString* and use g_string_printf().

I CC'ed the block folks to have their feedback.

Regards,

Phil.

> +++ b/block/blkverify.c
> +                 "blkverify:%.2038s:%.2038s",
> +++ b/hw/usb/bus.c
> +        snprintf(downstream->path, sizeof(downstream->path), "%.12s.%d",
> +        snprintf(downstream->path, sizeof(downstream->path), "%.12d", portnr);
>
> Tsung-en Hsiao
>
> > Qu Wenruo Wrote:
> >
> > Hi all,
> >
> > After upgrading gcc from 6.3.1 to 7.1.1, qemu can't be compiled with gcc.
> >
> > The error is:
> >
> > ------
> >  CC      block/blkdebug.o
> > block/blkdebug.c: In function 'blkdebug_refresh_filename':
> >
> > block/blkdebug.c:693:31: error: '%s' directive output may be truncated writing 
> > up to 4095 bytes into a region of size 4086 [-Werror=format-truncation=]
> >
> >                  "blkdebug:%s:%s", s->config_file ?: "",
> >                               ^~
> > In file included from /usr/include/stdio.h:939:0,
> >                 from /home/adam/qemu/include/qemu/osdep.h:68,
> >                 from block/blkdebug.c:25:
> >
> > /usr/include/bits/stdio2.h:64:10: note: '__builtin___snprintf_chk' output 11 or 
> > more bytes (assuming 4106) into a destination of size 4096
> >
> >   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
> >          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >        __bos (__s), __fmt, __va_arg_pack ());
> >        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > cc1: all warnings being treated as errors
> > make: *** [/home/adam/qemu/rules.mak:69: block/blkdebug.o] Error 1
> > ------
> >
> > It seems that gcc 7 is introducing more restrict check for printf.
> >
> > If using clang, although there are some extra warning, it can at least pass the 
> > compile.
> >
> > Thanks,
> > Qu