linux-kernel - Re: [PATCH v2 2/7] kdb: Use format-strings rather than '\0' injection in kdb

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Date: Tue, 23 Apr 2024 11:37:14 +0100
From: Daniel Thompson <daniel.thompson@...aro.org>
To: Doug Anderson <dianders@...omium.org>
Cc: Jason Wessel <jason.wessel@...driver.com>,
	kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v2 2/7] kdb: Use format-strings rather than '\0'
 injection in kdb_read()

On Mon, Apr 22, 2024 at 04:52:04PM -0700, Doug Anderson wrote:
> Hi,
>
> On Mon, Apr 22, 2024 at 9:37 AM Daniel Thompson
> <daniel.thompson@...aro.org> wrote:
> >
> > Currently when kdb_read() needs to reposition the cursor it uses copy and
> > paste code that works by injecting an '\0' at the cursor position before
> > delivering a carriage-return and reprinting the line (which stops at the
> > '\0').
> >
> > Tidy up the code by hoisting the copy and paste code into an appropriately
> > named function. Additionally let's replace the '\0' injection with a
> > proper field width parameter so that the string will be abridged during
> > formatting instead.
> >
> > Cc: stable@...r.kernel.org # Not a bug fix but it is needed for later bug fixes
> > Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> > ---
> >  kernel/debug/kdb/kdb_io.c | 34 ++++++++++++++--------------------
> >  1 file changed, 14 insertions(+), 20 deletions(-)
>
> Looks like a nice fix, but I think this'll create a compile warning on
> some compilers. The variable "tmp" is no longer used, I think.
>
> Once the "tmp" variable is deleted, feel free to add my Reviewed-by.

Good spot. I'll fix that.


> NOTE: patch #7 in your series re-adds a user of "tmp", but since this
> one is "Cc: stable" you will need to delete it here and then re-add it
> in patch #7.
>
>
> > diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> > index 06dfbccb10336..a42607e4d1aba 100644
> > --- a/kernel/debug/kdb/kdb_io.c
> > +++ b/kernel/debug/kdb/kdb_io.c
> > @@ -184,6 +184,13 @@ char kdb_getchar(void)
> >         unreachable();
> >  }
> >
> > +static void kdb_position_cursor(char *prompt, char *buffer, char *cp)
> > +{
> > +       kdb_printf("\r%s", kdb_prompt_str);
> > +       if (cp > buffer)
> > +               kdb_printf("%.*s", (int)(cp - buffer), buffer);
>
> nit: personally, I'd take the "if" statement out. I'm nearly certain
> that kdb_printf() can handle zero-length for the width argument and
> "buffer" can never be _after_ cp (so you can't get negative).

The kernel will correctly format zero-width fields... but kdb_printf()
will also inject the empty string into the log if running with
LOGGING=1. It is true the dmesg output with LOGGING=1 is pretty nasty
when doing line editing but I still didn't want to make it worse!

Oh... and we can't combine into one call to kdb_printf() since that
renders into a fixed length static buffer and we could get unwanted
truncation. I might just add a comment about that.


Daniel.