[PATCH] smbios: Fix table when no string is present

[Simon Glass]

Hi Matthias,

On Thu, 25 Mar 2021 at 16:38, Matthias Brugger <matthias.bgg at gmail.com> wrote:
>
>
>
> On 25/03/2021 01:38, Simon Glass wrote:
> > Hi Matthias,
> >
> > On Thu, 18 Mar 2021 at 00:30, <matthias.bgg at kernel.org> wrote:
> >>
> >> From: Matthias Brugger <mbrugger at suse.com>
> >>
> >> When no string is present in a table, next_ptr points to the same
> >> location as eos. When calculating the string table length, we would only
> >> reserve one \0. By spec a SMBIOS table has to end with two \0\0 when no
> >> strings a present.
> >>
> >> Signed-off-by: Matthias Brugger <mbrugger at suse.com>
> >>
> >> ---
> >>
> >>  lib/smbios.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > There is a bug here but I don't think this is right fix. I remember
> > worrying about this, making the same change as you did, reverting it
> > and then forgetting about it :-( It has no tests.
> >
> > You are effectively changing the definition of next_ptr here:
> >
> >  * @next_ptr: pointer to the start of the next string to be added. When the
> >  * table is not empty, this points to the byte after the \0 of the
> >  * previous string.
> >
>
> That's true.
>
> > (there is a typo in that in the code)
> >
> > I think that breaks adding new strings.
> >
>
> Well it doesn't because when adding a new string, ctx->next_ptr gets overwritten
> [1]. It is only used to calculate the lenght of the string in
> smbios_string_table_len() to calculate the size of the table [2]. But yes it's
> not the obvious way to handle the case when no string is present.
>
> > Can you instead change smbios_string_table_len() to add 2?
> >
>
> Do you mean returning 2 when no string is present (ctx->next_ptr == ctx->eos)?
>
> Adding 2 will break the case when we have a string present, as a string already
> finishes with '\0' and we only need a second '\0'.

Oh dear, this is complicated. I think we need to add a few unit tests.
Do you have time to try that?

Regards,
Simon

>
> Regards,
> Matthias
>
> [1] https://source.denx.de/u-boot/u-boot/-/blob/master/lib/smbios.c#L95
> [2] https://source.denx.de/u-boot/u-boot/-/blob/master/lib/smbios.c#L196
>
> >>
> >> diff --git a/lib/smbios.c b/lib/smbios.c
> >> index 7d463c84a9..d21d37cdac 100644
> >> --- a/lib/smbios.c
> >> +++ b/lib/smbios.c
> >> @@ -153,7 +153,7 @@ static int smbios_add_prop(struct smbios_ctx *ctx, const char *prop)
> >>  static void smbios_set_eos(struct smbios_ctx *ctx, char *eos)
> >>  {
> >>         ctx->eos = eos;
> >> -       ctx->next_ptr = eos;
> >> +       ctx->next_ptr = eos + 1;
> >>         ctx->last_str = NULL;
> >>  }
> >>
> >> --
> >> 2.30.2
> >>
> >
> > Regards,
> > Simon
> >