Re: [Qemu-devel] [PATCH v3 3/5] hw/acpi: Use QEMU_NONSTRING for non NUL-terminated arrays

[Andrew Jones]

On Wed, Dec 19, 2018 at 01:43:40PM +0100, Philippe Mathieu-Daudé wrote:
> Hi Drew,
> 
> On 12/19/18 11:10 AM, Andrew Jones wrote:
> > On Tue, Dec 18, 2018 at 06:51:20PM +0100, Philippe Mathieu-Daudé wrote:
> >> GCC 8 added a -Wstringop-truncation warning:
> >>
> >>   The -Wstringop-truncation warning added in GCC 8.0 via r254630 for
> >>   bug 81117 is specifically intended to highlight likely unintended
> >>   uses of the strncpy function that truncate the terminating NUL
> >>   character from the source string.
> >>
> >> This new warning leads to compilation failures:
> >>
> >>     CC      hw/acpi/core.o
> >>   In function 'acpi_table_install', inlined from 'acpi_table_add' at 
> >> qemu/hw/acpi/core.c:296:5:
> >>   qemu/hw/acpi/core.c:184:9: error: 'strncpy' specified bound 4 equals 
> >> destination size [-Werror=stringop-truncation]
> >>            strncpy(ext_hdr->sig, hdrs->sig, sizeof ext_hdr->sig);
> >>            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>   make: *** [qemu/rules.mak:69: hw/acpi/core.o] Error 1
> >>
> >> Use the QEMU_NONSTRING attribute, since ACPI tables don't require the
> >> strings to be NUL-terminated.
> > 
> > Aren't we always starting with zero-initialized structures in ACPI code?
> > If so, then we should be able to change the strncpy's to memcpy's.
> 
> The first call zero-initializes, but then we call realloc():
> 
>     /* We won't fail from here on. Initialize / extend the globals. */
>     if (acpi_tables == NULL) {
>         acpi_tables_len = sizeof(uint16_t);
>         acpi_tables = g_malloc0(acpi_tables_len);
>     }
> 
>     acpi_tables = g_realloc(acpi_tables, acpi_tables_len +
>                                          ACPI_TABLE_PFX_SIZE +
>                                          sizeof dfl_hdr + body_size);
> 
>     ext_hdr = (struct acpi_table_header *)(acpi_tables +
>                                            acpi_tables_len);
> 
> So memcpy() isn't enough.

Ah, thanks.

> 
> I can resend the previous patch which uses strpadcpy() if you prefer,
> Igor already reviewed it:
> 
> https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg04406.html
>

I do like strpadcpy() better, but I'm not going to lose sleep about
this either way it goes.

Thanks,
drew