Re: [PATCH dwarves] btf_encoder: fix and complete filtering out zero-sized per-CPU variables

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Em Thu, May 27, 2021 at 11:55:10AM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Mon, May 24, 2021 at 04:42:22PM -0700, Andrii Nakryiko escreveu:
> > btf_encoder is ignoring zero-sized per-CPU ELF symbols, but the same has to be
> > done for DWARF variables when matching them with ELF symbols. This is due to
> > zero-sized DWARF variables matching unrelated (non-zero-sized) variable that
> > happens to be allocated at the exact same address, leading to a lot of
> > confusion in BTF.
>  
> > See [0] for when this causes big problems.
>  
> >   [0] https://lore.kernel.org/bpf/CAEf4BzZ0-sihSL-UAm21JcaCCY92CqfNxycHRZYXcoj8OYb=wA@xxxxxxxxxxxxxx/

I also added this:

Reported-by: Michal Suchánek <msuchanek@xxxxxxx>

Michal, so you tested this patch and verified it fixed the problem? If
so please let me know so that I also add:

Tested-by: Michal Suchánek <msuchanek@xxxxxxx>

Thanks,

- Arnaldo
 
> > +++ b/btf_encoder.c
> > @@ -550,6 +551,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> >  
> >  		/* addr has to be recorded before we follow spec */
> >  		addr = var->ip.addr;
> > +		dwarf_name = variable__name(var, cu);
> >  
> >  		/* DWARF takes into account .data..percpu section offset
> >  		 * within its segment, which for vmlinux is 0, but for kernel
> > @@ -582,11 +584,9 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> >  		 *  modules per-CPU data section has non-zero offset so all
> >  		 *  per-CPU symbols have non-zero values.
> >  		 */
> > -		if (var->ip.addr == 0) {
> > -			dwarf_name = variable__name(var, cu);
> > +		if (var->ip.addr == 0)
> >  			if (!dwarf_name || strcmp(dwarf_name, name))
> >  				continue;
> > -		}
> >  
> >  		if (var->spec)
> >  			var = var->spec;
> > @@ -600,6 +600,13 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
> 
> I just changed the above hunk to be:
> 
> @@ -583,7 +585,6 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>                  *  per-CPU symbols have non-zero values.
>                  */
>                 if (var->ip.addr == 0) {
> -                       dwarf_name = variable__name(var, cu);
>                         if (!dwarf_name || strcmp(dwarf_name, name))
>                                 continue;
>                 }
> 
> 
> Which is shorter and keeps the {} around a multi line if block, ok?
> 
> Thanks, applied!
> 
> - Arnaldo

-- 

- Arnaldo



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