Re: [PATCH v3 09/15] libfdt: Handle unknown tags in fdt_next_tag()

[Herve Codina <herve.codina@bootlin.com>]

Hi David,

On Thu, 17 Sep 2026 19:36:06 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Sep 17, 2026 at 10:34:10AM +0200, Herve Codina wrote:
> > Hi David,
> > 
> > On Wed, 16 Sep 2026 19:10:27 +1000
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Wed, Aug 26, 2026 at 10:31:40AM +0200, Herve Codina wrote:  
> > > > The structured tag value definition introduced recently gives the
> > > > ability to ignore unknown tags without any error when they are read.
> > > > 
> > > > libfdt uses fdt_next_tag() to get a tag.
> > > > 
> > > > Filtering out tags that should be ignored in fdt_next_tag() allows to
> > > > have the filtering done globally and allows, in future releases, to have
> > > > a central place to add new known tags that should not be filtered out.
> > > > 
> > > > An already known tag exists with the meaning of "just ignore". This tag
> > > > is FDT_NOP. fdt_next_tag() callers already handle the FDT_NOP tag.
> > > > 
> > > > Avoid unneeded modification at callers side and use a fake FDT_NOP tag
> > > > when an unknown tag that should be ignored is encountered.
> > > > 
> > > > Add also fdt_next_tag_() internal function for callers who need to know
> > > > if the FDT_NOP tag returned is a real FDT_NOP or a fake FDT_NOP due to
> > > > an unknown tag.    
> > > 
> > > I'm a bit unsure about this one.  If we were designing the libfdt
> > > interface from scratch, I'd definitely say that fdt_next_tag() - as a
> > > low level function - should return every tag, and it's up to the
> > > caller to skip unknown ones.  fdt_next_tag() already provides
> > > nextoffset making it fairly easy to do so.
> > > 
> > > But, of course, we're not building from scratch so we do need to
> > > consider backwards compatibility.  Supplying a skipping and
> > > non-skipping version as you have here is the obvious approach.
> > > 
> > > But I'm not entirely convinced it's the only or best approach.  A user
> > > which calls fdt_next_tag() is, in a sense, already opting in to
> > > low-level handling of the dtb.  They could well already be broken by
> > > new tags (depending on how they handle the default case).  Arguably
> > > it's reasonable to require them to be updated (once only) for this new
> > > family of tags.  
> > 
> > I can go in that way. We will have same kind of sequence in almost all
> > callers.
> > 
> >    do {
> >       tag = fdt_next_tag()
> >    } while (!fdt_tag_is_unknown(tag));  
> 
> Not really - users of fdt_next_tag() will typically already have a
> switch on the tag type, so the unknown tag handling would be an
> addition to the 'default' case, not requiring an extra loop.

Ok, those are to be update to use a switch on tag but that's not big deal
 - fdt_check_node_offset_()
 - fdt_check_prop_offset_()
 - fdt_add_subnode_namelen()
 - fdt_finish()

Some others like nextprop_() or fdt_next_node() mix a while loop and a
switch/case. Some rework to be planned.

> 
> > We have to care about those "unknown_and_skippable" tags only when
> > we modify a dtb. This is handled by a few functions.
> > 
> > The vast majority of callers of fdt_next_tag() just want to skip
> > unknown tags.  
> 
> I'm not sure that's true of the (few) callers outside libfdt itself.

Well, today there is no "unknown" tags and existing callers works correctly.

Unknown tags are reported as FDT_NOP. It should be ok with existing callers.
They should handle FDT_NOP.

> 
> > > Another potential approach would be to actually use the symbol
> > > versioning we have, but have som far only minimally exploited.  The
> > > LIBFDT_1.2 version of fdt_next_tag() would skip tags that postdate it,
> > > but we'd introduce a new version tag and the new default version of
> > > fdt_next_tag() would return all tags.  That would of course require
> > > figuring out how to do that with the verison script.  
> > 
> > Hum, I really don't want to have versioning in loop for fdt_next_tag().
> 
> I'm not sure what you mean by "in loop" here.

I don't want to add the versioning extra complexity in this context.
I am sure we can find a solution for this "all tags vs only known tags"
topic without versioning involved.

> 
> > Also, this could work in the dynamic lib (.so) case but not for static
> > link and even less for libfdt code copy compiled in bootloaders or Linux
> > kernel code.  
> 
> Are symbol versions entirely unsupported with static libraries?

I would say yes but maybe I am wrong.

https://sourceware.org/binutils/docs/ld/VERSION.html
https://sourceware.org/binutils/docs/ld.html#index-_002d_002dversion_002dscript_003dversion_002dscriptfile

> Obviously they'd have to be resolved at (static) link time, not
> runtime, but I don't see that it's impossible.
> 
> > fdt_next_tag() is a low level function accessible out of libfdt (API).
> > If it doesn't skip unknown tags even for other libfdt internal function it
> > will be a too lower function.  
> 
> I don't really follow what you're saying here.

Skipping unknown tag is a low level feature.
I think fdt_next_tag() can handle this low level feature.

Upper layer using fdt_next_tag() should not handle this tag skipping the
general case and let fdt_next_tag() do this job.

Got the feeling that we are going to add complexity at upper layer to handle
a feature that should be handled at low level.

> 
> > Ok, we can have a wrapper on top of
> > fdt_next_tag() to skip unknown tags.
> > 
> > fdt_next_tag() is a low level function but the rule related to skipping
> > unknown tags is also a low level rule.
> > 
> > We have to choose:
> > a) fdt_next_tag() as I proposed
> >    Of course ok for me
> > 
> > b) fdt_next_tag() returns all tags -> Full handling at caller side
> >    Ok, we can go in that direction but it will bring quite a lot of
> >    code duplication.  
> 
> I'm not sure it's that bad, since fdt_next_tag() loops will typically
> already have a 'default' case for irrelevant tags, which will
> generally be handled the same as unknown tags.

Well as we did for "offset 0 vs real root node offset", I think the best way
to known if it's bad or not is to try.
 
I will not have time this week to propose something and I have a training
next week.

If you're ok for a try, I will do it but not as quickly as I did for the
"offset 0 vs real root node offset" topic.

> 
> > I am not fully convinced by this option.
> > 
> > c) lib version
> >    I disagree on my side  
> 
> Yeah, I'm not really convinced by this one either.

We are on the same page.

> 
> > d) Other idea
> >    Of course, I am still open.  
> 
> I haven't thought of any more ideas yet.
> 

Best regards,
Hervé