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

[Philippe Mathieu-Daudé]

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 <address@hidden>
> > ---
> > Cc: Kevin Wolf <address@hidden>
> > Cc: Max Reitz <address@hidden>
> > Cc: address@hidden
> > ---
> >   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");