Re: [Qemu-trivial] [Qemu-devel] [PATCH v1 4/5] linux-user/uname: Fix GCC 9 build warnings

[Eric Blake]

On 4/30/19 3:09 PM, Alistair Francis wrote:
> Fix this warning when building with GCC9 on Fedora 30:
> In function ‘strncpy’,
>     inlined from ‘sys_uname’ at /home/alistair/qemu/linux-user/uname.c:94:3:
> /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ 
> output may be truncated copying 64 bytes from a string of length 64 
> [-Werror=stringop-truncation]
>   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos 
> (__dest));
>       |          
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Signed-off-by: Alistair Francis <address@hidden>
> ---
>  linux-user/uname.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/linux-user/uname.c b/linux-user/uname.c
> index 313b79dbad..293b2238f2 100644
> --- a/linux-user/uname.c
> +++ b/linux-user/uname.c
> @@ -90,6 +90,11 @@ int sys_uname(struct new_utsname *buf)
>     * struct linux kernel uses).
>     */
>  
> +#if defined(CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE) && QEMU_GNUC_PREREQ(9, 0)
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wstringop-truncation"
> +#endif

Why do we need the pragma?

> +
>    memset(buf, 0, sizeof(*buf));

We are prezeroing the entire field, at which point...

>    COPY_UTSNAME_FIELD(buf->sysname, uts_buf.sysname);
>    COPY_UTSNAME_FIELD(buf->nodename, uts_buf.nodename);

...using strncpy() for a shorter string is wasteful (we're writing the
tail end twice), and for a long string is warning-prone.  Why not
rewrite COPY_UTSNAME_FIELD() to use memcpy() for the MIN(strlen(src),
__NEW_UTS_LEN) and drop the write of the trailing NUL, since it will
already be NUL from your memset()?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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