Re: [PATCH 6/6] qemu-io-cmds: Silent GCC9 format-overflow warning

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 12/18/19 5:15 AM, Richard Henderson wrote:
> On 12/17/19 7:34 AM, Philippe Mathieu-Daudé wrote:
>> GCC9 is confused when building with CFLAG -O3:
>>
>>    In function ‘help_oneline’,
>>        inlined from ‘help_all’ at qemu-io-cmds.c:2414:9,
>>        inlined from ‘help_f’ at qemu-io-cmds.c:2424:9:
>>    qemu-io-cmds.c:2389:9: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
>>     2389 |         printf("%s ", ct->name);
>>          |         ^~~~~~~~~~~~~~~~~~~~~~~
>>
>> Audit shows this can't happen. Give a hint to GCC adding an
>> assert() call.
> 
> This deserves more investigation.  From my glance it appears you are right --
> and moreover impossible for gcc to have come to this conclusion.  Which begs
> the question of how that is.

New entries are added to cmdtab[] in qemuio_add_command() which is 
public (also called in qemu-io.c). So you can insert a cmdinfo_t with a 
name being NULL. This is why I thought GCC was correct first, and I tried:

-- >8 --
--- a/qemu-io-cmds.c
+++ b/qemu-io-cmds.c
@@ -42,6 +42,7 @@ void qemuio_add_command(const cmdinfo_t *ci)
       * Catch it now rather than letting it manifest as a crash if a
       * particular set of command line options are used.
       */
+    assert(ci->name);
      assert(ci->perm == 0 ||
             (ci->flags & (CMD_FLAG_GLOBAL | CMD_NOFILE_OK)) == 0);
      cmdtab = g_renew(cmdinfo_t, cmdtab, ++ncmds);
---

But this didn't fix the warning... So I moved the assert() in the other 
location.

> 
> Did you file a gcc bug report?

No because I'm not sure this is a bug, but if you confirm I'll file one :)

>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>> Cc: Kevin Wolf <kwolf@redhat.com>
>> Cc: Max Reitz <mreitz@redhat.com>
>> Cc: qemu-block@nongnu.org
>> ---
>>   qemu-io-cmds.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
>> index 1b7e700020..9e956a5dd4 100644
>> --- a/qemu-io-cmds.c
>> +++ b/qemu-io-cmds.c
>> @@ -2411,6 +2411,7 @@ static void help_all(void)
>>       const cmdinfo_t *ct;
>>   
>>       for (ct = cmdtab; ct < &cmdtab[ncmds]; ct++) {
>> +        assert(ct->name);
>>           help_oneline(ct->name, ct);
>>       }
>>       printf("\nUse 'help commandname' for extended help.\n");
>>
>