Re: [PATCH next] ext4: Fix diagnostic printf formats

[Andy Shevchenko]

On Fri, Mar 27, 2026 at 12:54:12PM +0000, David Laight wrote:
> On Fri, 27 Mar 2026 12:48:56 +0200
> Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > On Thu, Mar 26, 2026 at 08:18:04PM +0000, david.laight.linux@xxxxxxxxx wrote:
> > 
> > > The formats for non-terminated names should be "%.*s" not "%*.s".
> > > The kernel currently treats "%*.s" as equivalent to "%*s" whereas
> > > userspace requires it be equivalent to "%*.0s".
> > > Neither is correct here.  
> > 
> > This entire code seems was never tested properly and it's a dead code
> > until one defines manually DX_DEBUG. It also has tons of plain printk()
> > calls that may behave differently if the first character is not printable
> > but maps to the level of printk().
> > 
> > I'm not sure how your patch helps with all that, but apparently the
> > printed data has to be NUL-terminated, otherwise I have no idea how
> > it was ever working without crashes.
> 
> I noticed that as well.
> I suspect it way have worked for the person that wrote it because the
> name strings all happened to be NUL terminated.
> There is certainly likely to be a '\0' before you 'fall off' mapped
> memory and crash - so maybe they just ignored the extra characters.
> 
> Clearly the other option is to delete it all.

I would go for the history of the change and if it's old enough and not
mentioned in any Documentation or not-so-old email thread, kill all that
for good. But better to hear the ext4 maintainers first.

> But regardless the format string is wrong.

P.S. A bit of off-topic, have you seen this?
https://elixir.bootlin.com/linux/v7.0-rc5/source/kernel/stacktrace.c#L33
Is it correct use of %c?

-- 
With Best Regards,
Andy Shevchenko