Re: [PATCH bpf v2] bpf: Add config for skipping BTF enum64s

Re: [PATCH bpf v2] bpf: Add config for skipping BTF enum64s

[Jiri Olsa]

On Sun, Aug 28, 2022 at 08:33:17PM -0300, Martin Rodriguez Reboredo wrote:
> After the release of pahole 1.24 some people in the dwarves mailing list
> notified issues related to building the kernel with the BTF_DEBUG_INFO
> option toggled. They seem to be happenning due to the kernel and
> resolve_btfids interpreting btf types erroneously. In the dwarves list
> I've proposed a change to the scripts that I've written while testing
> the Rust kernel, it simply passes the --skip_encoding_btf_enum64 to
> pahole if it has version 1.24.
> 
> v1 -> v2:
> - Switch to off by default and remove the config option.
> - Send it to stable instead.

hi,
we have change that needs to go to stable kernels but does not have the
equivalent fix in Linus tree

what would be the best way to submit it?

the issue is that new 'pahole' will generate BTF data that are not supported
by older kernels, so we need to add --skip_encoding_btf_enum64 option to
stable kernel's scripts/pahole-flags.sh to generate proper BTF data

we got complains that after upgrading to latest pahole the stable kernel
compilation fails

thanks,
jirka


> 
> Signed-off-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> ---
>  scripts/pahole-flags.sh | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/scripts/pahole-flags.sh b/scripts/pahole-flags.sh
> index 0d99ef17e4a5..0a48fd86bc68 100755
> --- a/scripts/pahole-flags.sh
> +++ b/scripts/pahole-flags.sh
> @@ -19,5 +19,8 @@ fi
>  if [ "${pahole_ver}" -ge "122" ]; then
>  	extra_paholeopt="${extra_paholeopt} -j"
>  fi
> +if [ "${pahole_ver}" -ge "124" ]; then
> +	extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_enum64"
> +fi
>  
>  echo ${extra_paholeopt}
> -- 
> 2.37.2
> 

Re: [PATCH bpf v2] bpf: Add config for skipping BTF enum64s

[Greg KH]

On Fri, Sep 02, 2022 at 06:51:00PM +0200, Jiri Olsa wrote:
> On Sun, Aug 28, 2022 at 08:33:17PM -0300, Martin Rodriguez Reboredo wrote:
> > After the release of pahole 1.24 some people in the dwarves mailing list
> > notified issues related to building the kernel with the BTF_DEBUG_INFO
> > option toggled. They seem to be happenning due to the kernel and
> > resolve_btfids interpreting btf types erroneously. In the dwarves list
> > I've proposed a change to the scripts that I've written while testing
> > the Rust kernel, it simply passes the --skip_encoding_btf_enum64 to
> > pahole if it has version 1.24.
> > 
> > v1 -> v2:
> > - Switch to off by default and remove the config option.
> > - Send it to stable instead.
> 
> hi,
> we have change that needs to go to stable kernels but does not have the
> equivalent fix in Linus tree

Why isn't it also relevant in Linus's tree?

> what would be the best way to submit it?

Submit it here and document the heck out of why this isn't in Linus's
tree, what changes instead fixed it there, and so on.  Look in the
archives for examples of how this is done, one recent one that I can
think of is here:
	https://lore.kernel.org/r/20220831191348.3388208-1-jannh@google.com

> the issue is that new 'pahole' will generate BTF data that are not supported
> by older kernels, so we need to add --skip_encoding_btf_enum64 option to
> stable kernel's scripts/pahole-flags.sh to generate proper BTF data
> 
> we got complains that after upgrading to latest pahole the stable kernel
> compilation fails

And what is happening in Linus's tree for this same issue?

thanks,

greg k-h

Re: [PATCH bpf v2] bpf: Add config for skipping BTF enum64s

[Arnaldo Carvalho de Melo]

Em Sat, Sep 03, 2022 at 07:26:58AM +0200, Greg KH escreveu:
> On Fri, Sep 02, 2022 at 06:51:00PM +0200, Jiri Olsa wrote:
> > On Sun, Aug 28, 2022 at 08:33:17PM -0300, Martin Rodriguez Reboredo wrote:
> > > After the release of pahole 1.24 some people in the dwarves mailing list
> > > notified issues related to building the kernel with the BTF_DEBUG_INFO
> > > option toggled. They seem to be happenning due to the kernel and
> > > resolve_btfids interpreting btf types erroneously. In the dwarves list
> > > I've proposed a change to the scripts that I've written while testing
> > > the Rust kernel, it simply passes the --skip_encoding_btf_enum64 to
> > > pahole if it has version 1.24.
> > > 
> > > v1 -> v2:
> > > - Switch to off by default and remove the config option.
> > > - Send it to stable instead.
> > 
> > hi,
> > we have change that needs to go to stable kernels but does not have the
> > equivalent fix in Linus tree
> 
> Why isn't it also relevant in Linus's tree?

See below.
 
> > what would be the best way to submit it?
> 
> Submit it here and document the heck out of why this isn't in Linus's
> tree, what changes instead fixed it there, and so on.  Look in the
> archives for examples of how this is done, one recent one that I can
> think of is here:
> 	https://lore.kernel.org/r/20220831191348.3388208-1-jannh@google.com
> 
> > the issue is that new 'pahole' will generate BTF data that are not supported
> > by older kernels, so we need to add --skip_encoding_btf_enum64 option to
> > stable kernel's scripts/pahole-flags.sh to generate proper BTF data
> > 
> > we got complains that after upgrading to latest pahole the stable kernel
> > compilation fails
> 
> And what is happening in Linus's tree for this same issue?

So, BTF_KIND_ENUM64 is a new BTF tag, one that is not accepted by older
kernels, but is accepted by the BPF verifier on Linus' tree.

Its about avoiding having a pahole command line with lots of
--enable-new-feature-foo for new stuff with the default producing the
most recent BTF spec.

One way to documenting it: if you update pahole, then please use
--skip_encoding_FOO for these new FOO features on kernels where those
aren't supported.

So this isn't a backport from a fix on Linus' tree, as both the older
pahole that doesn't encode BTF_KIND_ENUM64 and the new one, that encodes
it by default, work with Linus' tree.

Does this violates the stable@ rules?

- Arnaldo

Re: [PATCH bpf v2] bpf: Add config for skipping BTF enum64s

[Greg KH]

On Sat, Sep 03, 2022 at 11:13:45AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Sat, Sep 03, 2022 at 07:26:58AM +0200, Greg KH escreveu:
> > On Fri, Sep 02, 2022 at 06:51:00PM +0200, Jiri Olsa wrote:
> > > On Sun, Aug 28, 2022 at 08:33:17PM -0300, Martin Rodriguez Reboredo wrote:
> > > > After the release of pahole 1.24 some people in the dwarves mailing list
> > > > notified issues related to building the kernel with the BTF_DEBUG_INFO
> > > > option toggled. They seem to be happenning due to the kernel and
> > > > resolve_btfids interpreting btf types erroneously. In the dwarves list
> > > > I've proposed a change to the scripts that I've written while testing
> > > > the Rust kernel, it simply passes the --skip_encoding_btf_enum64 to
> > > > pahole if it has version 1.24.
> > > > 
> > > > v1 -> v2:
> > > > - Switch to off by default and remove the config option.
> > > > - Send it to stable instead.
> > > 
> > > hi,
> > > we have change that needs to go to stable kernels but does not have the
> > > equivalent fix in Linus tree
> > 
> > Why isn't it also relevant in Linus's tree?
> 
> See below.
>  
> > > what would be the best way to submit it?
> > 
> > Submit it here and document the heck out of why this isn't in Linus's
> > tree, what changes instead fixed it there, and so on.  Look in the
> > archives for examples of how this is done, one recent one that I can
> > think of is here:
> > 	https://lore.kernel.org/r/20220831191348.3388208-1-jannh@google.com
> > 
> > > the issue is that new 'pahole' will generate BTF data that are not supported
> > > by older kernels, so we need to add --skip_encoding_btf_enum64 option to
> > > stable kernel's scripts/pahole-flags.sh to generate proper BTF data
> > > 
> > > we got complains that after upgrading to latest pahole the stable kernel
> > > compilation fails
> > 
> > And what is happening in Linus's tree for this same issue?
> 
> So, BTF_KIND_ENUM64 is a new BTF tag, one that is not accepted by older
> kernels, but is accepted by the BPF verifier on Linus' tree.
> 
> Its about avoiding having a pahole command line with lots of
> --enable-new-feature-foo for new stuff with the default producing the
> most recent BTF spec.
> 
> One way to documenting it: if you update pahole, then please use
> --skip_encoding_FOO for these new FOO features on kernels where those
> aren't supported.
> 
> So this isn't a backport from a fix on Linus' tree, as both the older
> pahole that doesn't encode BTF_KIND_ENUM64 and the new one, that encodes
> it by default, work with Linus' tree.
> 
> Does this violates the stable@ rules?

Not really, if it fixes an issue for those kernels when using newer
tools, that's fine.  Just document it well like you did here.

thanks,

greg k-h