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Re: [Qemu-trivial] [Qemu-devel] [PATCH v1 5/5] linux-user/elfload: Fix G


From: Eric Blake
Subject: Re: [Qemu-trivial] [Qemu-devel] [PATCH v1 5/5] linux-user/elfload: Fix GCC 9 build warnings
Date: Tue, 30 Apr 2019 16:10:07 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 4/30/19 4:01 PM, Alistair Francis wrote:
> On Tue, Apr 30, 2019 at 1:36 PM Laurent Vivier <address@hidden> wrote:
>>
>> Le 30/04/2019 à 22:09, Alistair Francis a écrit :
>>> Fix this warning when building with GCC9 on Fedora 30:
>>> In function ‘strncpy’,
>>>     inlined from ‘fill_psinfo’ at 
>>> /home/alistair/qemu/linux-user/elfload.c:3208:12,
>>>     inlined from ‘fill_note_info’ at 
>>> /home/alistair/qemu/linux-user/elfload.c:3390:5,
>>>     inlined from ‘elf_core_dump’ at 
>>> /home/alistair/qemu/linux-user/elfload.c:3539:9:
>>> /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ 
>>> specified bound 16 equals destination size [-Werror=stringop-truncation]
>>>   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos 
>>> (__dest));
>>>       |          
>>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>

>>        /*
>>         * Using strncpy here is fine: at max-length,
>>>       * this field is not NUL-terminated.
>>>       */>      (void) strncpy(psinfo->pr_fname, base_filename,
>>> -                   sizeof(psinfo->pr_fname));
>>> +                   sizeof(psinfo->pr_fname) - 1);
>>
>> Read the comment above :)
> 
> GCC can't read the comment though. The only other option I can think
> of is using a pragma, which I avoided using unless I had to. Would you
> prefer a pragma here? Or do you have a better solution?

psinfo is struct target_elf_prpsinfo, which we declare.  Why not just
use the QEMU_NONSTRING attribute in the declaration, to tell the
compiler our exact intents (untested, but something like this):

diff --git i/linux-user/elfload.c w/linux-user/elfload.c
index c1a26021f8d..6ebb2eeb957 100644
--- i/linux-user/elfload.c
+++ w/linux-user/elfload.c
@@ -2872,7 +2872,7 @@ struct target_elf_prpsinfo {
     target_gid_t pr_gid;
     target_pid_t pr_pid, pr_ppid, pr_pgrp, pr_sid;
     /* Lots missing */
-    char    pr_fname[16];           /* filename of executable */
+    char    pr_fname[16] QEMU_NONSTRING;           /* filename of
executable */
     char    pr_psargs[ELF_PRARGSZ]; /* initial part of arg list */
 };




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

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